Triangle Universities Nuclear Laboratory

Transcription

TUNL XLVII
PROGRESS REPORT
1 SEPTEMBER 2007 – 31 AUGUST 2008
Duke University
North Carolina State University
University of North Carolina at Chapel Hill
Box 90308, Durham, North Carolina 27708-0308, USA
Work described in this Progress Report is supported by the United States Department of Energy,
Office of Nuclear Physics, under:
Grant No. DE-FG02-97ER41033 and DE-FG02-03ER41231 (Duke University),
Grant No. DE-FG02-97ER41042 (North Carolina State University), and
Grant No. DE-FG02-97ER41041 (University of North Carolina).
Contents
Introduction
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Personnel
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1 Fundamental Symmetries in the Nucleus
1.1 Time-Reversal Violation: The Neutron Electric Dipole Moment . . . . . . . . . . .
1.1.1 Search for the Neutron Electric Dipole Moment . . . . . . . . . . . . . . . . .
1.1.2 Search for the Neutron Electric Dipole Moment: Geometric Phase Effects . .
1.1.3 Research and Development for the nEDM Project: Purification of 4 He . . . .
1.1.4 Engineering of Cryovessel and Refrigerators for the Measurement of the Electric Dipole Moment of the Neutron . . . . . . . . . . . . . . . . . . . . . . . .
1.1.5 Polarized 3 He Relaxation Studies at Low Temperatures . . . . . . . . . . . .
1.1.6 Simulation of 3 He Transport in 4 He using the Heat-Flush Technique . . . . .
1.1.7 Preparation of The 3 He Injection Test . . . . . . . . . . . . . . . . . . . . . .
1.2 Fundamental Coupling Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 New Precision Measurement of the 19 Ne Lifetime Measurement . . . . . . . .
1.2.2 The UCNA Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2 Neutrino Physics
2.1 ββ Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Double–Electron Capture on 112 Sn to the Excited 1871 keV State in 112 Cd –
A Possible Alternative to Double–Beta Decay . . . . . . . . . . . . . . . . . .
2.1.2 Progress at Kimballton Underground Research Facility . . . . . . . . . . . . .
2.1.3 Partial Cross Section for Neutron-Induced Reactions on Cu, Ge and Pb at E n
= 8 and 12 MeV for 0νββ Background Studies . . . . . . . . . . . . . . . . .
2.1.4 Development of a 1.4 kg, Segmented Ge Ionization Detector Enriched to 85%
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Ge for Underground Measurements of ββ Decay, Axions and Other Forms
of Dark Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Nuclear Astrophysics
3.1 Nucleosynthesis in AGB Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Folding Potential and R-matrix Calculations for 16 O(p,γ)17 F . . . . . . . . .
3.1.2 22 Ne+α Reaction Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3 Reaction Rate Uncertainties and 26 Al in AGB Silicon Carbide Stardust . . .
3.2 Explosive Nucleosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 New Simulations of the Classical Nova Outburst . . . . . . . . . . . . . . . .
3.2.2 X-Ray Burst Nucleosynthesis Sensitivity Studies . . . . . . . . . . . . . . . .
3.3 Cosmochronology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 The Astrophysical 187 Re/187 Os Ratio: Measurement of the 187 Re(n, 2nγ)186m Re
Destruction Cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Reaction Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 Matching of Experimental Thermonuclear Reaction Rates to Statistical Model
Results at High Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 A New Reaction-Rate Evaluation for Proton-Induced Reactions on A=16–40
Nuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Contents
TUNL XLVII 2007–08
4 Sub-Nucleonic Degrees of Freedom
4.1 Compton Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Compton@MaxLab Collaboration . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Nucleon Spin Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Measurement of Single Target-Spin Asymmetry in Semi-Inclusive Deep Inelastic Pion Electroproduction on a Transversely Polarized 3 He Target . . . . . .
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5 Few-Nucleon Interaction Dynamics
5.1 Nucleon-Nucleon Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 Direct Measurement of the nn-Scattering Length (The DIANNA Collaboration)
5.1.2 Neutron Capture Experiments . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 The A=3 System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Neutron-Deuteron Analyzing Power Ay (θ) at En = 21.0 MeV . . . . . . . . .
5.2.2 Measurements of the Neutron-Deuteron Breakup Cross Section for the Spaceand Coplanar-Star Configurations at 19 MeV . . . . . . . . . . . . . . . . . .
5.2.3 SCRE Experiments in the Breakup Reaction 2 H(p,pp)n . . . . . . . . . . . .
~ Elastic Scattering . .
5.2.4 Studies of the Spin-Correlation Coefficients for p
~+3 He
5.3 The A=4 System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 Neutron-3 He Analyzing Power between En = 1.60 and 5.54 MeV . . . . . . .
5.4 Reaction Dynamics of Light Nuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 Measurements of 11 B(~
p,α)8 Be . . . . . . . . . . . . . . . . . . . . . . . . . . .
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5.4.2 Measurements of B(α,α)11 B . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.3 Cross Section Measurements of the 10 B(d,n0 )11 C Reaction Below 160 keV . .
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6 The Many-Nucleon Problem
6.1 Random Matrix Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 Random Matrices and Chaos in Nuclear Physics . . . . . . . . . . . . .
6.2 Preequilibrium Nuclear Reactions . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1 Study of Preequilibrium Reactions at GEANIE/WNR . . . . . . . . . .
6.2.2 Preequilbrium Reaction Phenomenology . . . . . . . . . . . . . . . . . .
6.2.3 Preequilbrium Model Comparisons for 96 MeV (n,xn) Reactions . . . .
6.3 Nuclear Data Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3.1 Nuclear Data Evaluation Activities . . . . . . . . . . . . . . . . . . . . .
6.4 Neutron-Induced Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.1 Neutron-Induced Partial γ-Ray Cross-Section Measurements on 235,238 U
6.4.2 Neutron-Induced Reaction Cross-Section Measurements on GaAs . . . .
6.4.3 Mixed-Symmetry States and Anomalous Decays in 94 Zr . . . . . . . . .
6.5 γ-Ray-Induced Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5.1 Photodisintegration Cross Section Measurements for 142 Nd and 150 Nd
Low-energy E1 γ-ray Strength Functions . . . . . . . . . . . . . . . . . .
6.6 Radioactive Decays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.6.1 Attempt to Manipulate the Decay Rate of Radioactive Nuclei . . . . . .
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7 Photonuclear Reactions at HIγS
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7.1 Photodisintegration of the Deuteron . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
7.1.1 An Indirect Determination of the Gerasimov-Drell-Hearn (GDH) Sum Rule
and Forward Spin Polarizability (γ0 ) for the Deuteron at Low Energies . . . 98
7.1.2 Measurement of d(γ,n)p Reaction Cross Section . . . . . . . . . . . . . . . . . 100
7.2 γ-3 He Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7.2.1 Total Cross-Section Measurements of the Two-Body Photodisintegration of
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He at Low Energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
7.2.2 Gerasimov-Drell-Hearn Integral on 3 He Part I: 3-body Measurement for Eγ =
14.7 MeV and 11.4 MeV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
7.2.3 Photodisintegration of 3 He at Eγ = 12.8 MeV at HIγS . . . . . . . . . . . . . 106
7.3 Study of Many-Body Systems Using NRF Techniques . . . . . . . . . . . . . . . . . 108
7.3.1 Fine Structure of Nuclear Dipole Excitations below Particle Emission Threshold108
7.3.2 Population of h11/2 Isomers in N = 81 Isotones Using the (γ,n) Reaction . . 110
Contents
Nuclear Resonance Fluorescence Measurements on 142,150 Nd to Determine the
γ-Ray Strength Function for p-process Nucleosynthesis Calculations . . . . .
7.3.4 Fine Structure of the M1 Resonance in 90 Zr . . . . . . . . . . . . . . . . . . .
7.3.5 Multipole Mixing Ratios of Transitions in 11 B . . . . . . . . . . . . . . . . . .
7.4 Nuclear Astrophysics at HIγS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.1 First In-Beam Measurement of the 16 O(γ, α)12 C Reaction With the O-TPC
Detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4.2 Measurement of 9 Be(γ,n) at HIγS . . . . . . . . . . . . . . . . . . . . . . . .
7.4.3 Preliminary Measurements of Astrophysically Important States in 26 Mg with
HIγS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5 Interrogation of Special Nuclear Materials . . . . . . . . . . . . . . . . . . . . . . . .
7.5.1 Nuclear Resonance Fluorescence from 238 U . . . . . . . . . . . . . . . . . . .
7.5.2 Photodisintegration Cross Section of 241 Am . . . . . . . . . . . . . . . . . . .
7.6 HIγS Intrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.6.1 Precise Determination of Total Absolute γ-Ray Intensity at HIγS . . . . . . .
7.6.2 Beam Intensity Determination at HIγS Using the 2 H(γ,n)p Reaction . . . . .
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8 Applied Research
8.1 Plant Physiology Studies Using Nuclear Physics Techniques . . . . . . . . . . . . . .
8.1.1 Measurement of Carbon Allocation Responses of Plants to Changes in Nutrient
Availability Under Ambient and Elevated CO2 . . . . . . . . . . . . . . . . .
8.1.2 Radioactive Gas-Handling System for Supplying Pulses of 11 CO2 to Plants in
a Controlled-Environment Chamber . . . . . . . . . . . . . . . . . . . . . . .
8.2 Novel Techniques for Special Materials Identification . . . . . . . . . . . . . . . . . .
8.2.1 Detecting Specific Material with Nuclear Resonance Fluorescence . . . . . . .
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9 Nuclear Instrumentation and Methods
9.1 Accelerator Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1.1 Tandem Accelerator Operation . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2 Ion Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.2.1 Atomic Beam Polarized Ion Source . . . . . . . . . . . . . . . . . . . . . . . .
9.2.2 First ECRIS Beam to Target with Upgraded Accelerator System in the LENA
Laboratory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3 Development of the HIγS Target Room and Associated Equipment . . . . . . . . . .
9.3.1 The HIγS Frozen-Spin Target . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.3.2 Status of the HIγS Frozen-Spin Target Transverse Holding Coil . . . . . . . .
9.3.3 The HIγS NaI Detector Array: Construction and Testing . . . . . . . . . . .
9.3.4 GEANT4 Simulation Package for HINDA and HIFROST . . . . . . . . . . .
9.3.5 Commissioning the Optical Readout TPC (O-TPC) . . . . . . . . . . . . . .
9.3.6 A γ-Ray Beam Imaging System . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4 Polarized Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4.1 Pressure Shift and Broadening of Cesium D1 and D2 Lines in the Presence of
N2 , He, and Xe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.4.2 Production of K-Rb Hybrid Optical Pumping Cells for the Production of Hyperpolarized 3 He . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5 Beamlines, Targets, and Facility Development . . . . . . . . . . . . . . . . . . . . . .
9.5.1 Sensitivity of the KURF Low-Background Counting Facility . . . . . . . . . .
9.5.2 Development of an Ultracold Neutron Source at the NC State PULSTAR
Reactor Facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.3 DEAP/CLEAN Activities . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.4 Advanced Detector Array for Sensitive γ-Ray Measurement . . . . . . . . . .
9.5.5 Cosmic-Ray Angular Measurement Employing a Reconstructed APEX for
Muon Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.5.6 Absolute Efficiency Characterization of a High-Efficiency Neutron Counter
with a 252 Cf Source and Ionization Chamber . . . . . . . . . . . . . . . . . .
9.5.7 GEANT4 Simulation of SEGA Geometry . . . . . . . . . . . . . . . . . . . .
9.5.8 GEANT4 Simulation of a Highly Segmented n-Type Ge Array. . . . . . . . .
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Contents
9.5.9 New Differential Pumping System for Use with a Gas Scattering Chamber . . 177
9.5.10 Precise Determination of Detector Solid Angles . . . . . . . . . . . . . . . . . 178
A Appendices
A.1 Graduate Degrees Awarded . . . . . .
A.2 Publications . . . . . . . . . . . . . . .
A.3 Invited Talks, Seminars, and Colloquia
A.4 Professional Service Activities . . . . .
Index
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Introduction
The Triangle Universities Nuclear Laboratory (TUNL) is a center for low-energy nuclear physics
that is staffed by faculty from three consortium universities: Duke University, North Carolina State
University, and the University of North Carolina at Chapel Hill. TUNL faculty supervise about
40 graduate students conducting Ph.D. thesis projects on a wide variety of topics that include nuclear astophysics, fundamental symmetries, weak interactions, few-nucleon and subnucleon systems,
many-body nuclear systems, neutrino physics, and applications of nuclear physics to address current
needs of society. This document is a compilation of status reports of research projects conducted
by TUNL faculty, postdoctoral fellows, and students during the period of September 1, 2007 to
August 31, 2008, which covers parts of the second and third years of the current three-year grant
period (March 1, 2006 – February 28, 2009) between the U.S. Department of Energy and the three
collaborating universities.
Research highlights during the current report period include:
• Commissioning of the experimental program underway at the upgraded HIγS Facility:
All design capabilities and baseline performance specifications for γ-ray beam production have
been demonstrated at the upgraded High Intensity Gamma-ray Source (HIγS). The current
γ-ray beam capabilities are: reliable delivery of a nearly mono-energetic linearly or circularly
polarized γ-ray beam to target with polarization greater than 90% in the energy range from 2
to 60 MeV. The total beam flux before collimation is greater than 108 γ/s. Facility commissioning experiments are underway and will continue through December 2008. Initial results
indicate that the signal-to-background ratio in nuclear resonance fluorescence experiments at
HIγS is about a factor of 30 larger than similar experiments conducted with a γ-ray beam produced with a bremsstrahlung source. For examples of commissioning experiments, see Chapter
7.
• Electric Dipole Character of Pygmy Resonance in 138 Ba Verified at HIγS:
The illusive nature of the so-called “pygmy” dipole mode of nuclear excitation was experimentally verified to be electric by nuclear resonance fluorescence measurements on 138 Ba made at
HIγS. For details see the article by Tonchev et al. in Sect. 7.3.1.
• New Dipole Excited States Discovered in 138 Ba at HIγS:
Recent nuclear resonance fluorescence measurements at HIγS reveal eight new 1 + states in
138
Ba at excitation energies below the particle separation energy. These observed M1 transitions account for part of the distributed strength of the “pygmy” dipole resonance. The
observation of these previously unobserved states is made possible by the factor of about 30
enhancement in the signal-to-background ratio at HIγS over what is acheived at bremsstrahlung
γ-ray sources. For details see the article by Tonchev et al. in Sect. 7.3.1.
• Optical Readout Time Projection Chamber Ready to Start 16 O(γ,α) Measurements:
The optical readout time projection chamber (O-TPC) being developed for the 16 O(γ,α) reaction rate measurement at HIγS was commissioned this year. All major performance milestones
were demonstrated during in-beam engineering runs performed at HIγS this year. The system
is now ready to make photodisintegration measurements. For details see the articles by Gai et
al. in Sections 7.4.1 and 9.3.5.
• ECR Ion Source at LENA Ready for Production Runs:
A new electron-cyclotron resonance ion source (ECRIS) has been constructed and commis-
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Introduction
sioned as the primary beam ion source at the Laboratory for Experimental Nuclear Astrophysics (LENA). All critical performance milestones have been demonstrated, and the source
and laboratory shielding are being configured for production running. For details see the article
by Cesaratto et al. in Sect. 9.2.2.
• TUNL Deploys Facility for Underground Research:
This year a remote laboratory equipped for low-background counting measurements was deployed by TUNL in the Kimballton Underground Research Facility in Kimballton, VA. The
initial two types of experiments being conducted at this new facility are: (1) materials assay,
and (2) searches for two neutrino double beta (2νββ) decay and neutrinoless double electron capture (0νECEC). For details see the articles by Kidd et al. and Finnerty et al. in
Sections 2.1.2 and 9.5.1, respectively.
• 3 He Polarization Relaxation Time Sufficient for Neutron EDM Measurement:
Measurements of the 3 He spin relaxation time under conditions similar to those of the UCNbased neutron EDM experiment verify that the spin relaxation time of polarized 3 He in this
environment is sufficiently long that it should not be the limiting factor for the accuracy of the
experiment. These measurements were made at TUNL with polarized 3 He dissolved in a bath
of superfluid 4 He at 416 mK in an acrylic cell coated with dTPB-dPS, the same materials to be
used in the neutron EDM experiment. For details see the article by Gao et al. in Sect. 1.1.5.
The TUNL faculty members are conducting research on several of the physics frontiers identified
in the most recent Long Range Plan of the DOE/NSF Nuclear Science Advisory Committee. During
the current three-year grant cycle the TUNL research program is focused on the nine major areas
described below.
1. NUCLEAR ASTROPHYSICS
with emphasis on measurements which are important for solar-neutrino physics, stellar evolution and nucleosynthesis. Specific experiments address:
• astrophysical S-factors of (p, γ) reactions
• the abundance anomalies in globular clusters
• the explosive nucleosynthesis in novae
• the evolution of massive stars
• solar-neutrino physics
The accelerator facilities at TUNL where most of the nuclear astrophysics studies are conducted
include the Laboratory for Experimental Nuclear Astrophysics (LENA), which has a 200-keV
ECR ion source for high-intensity unpolarized beams and a 1-MeV Van de Graaff accelerator;
the upgraded HIγS facility, which has a highly polarized and nearly monoenergetic pulsed γray beam; and the Low-Energy Beam Accelerator Facility (LEBAF) in the tandem laboratory.
Off-site facilities used by TUNL faculty are located at Argonne National Laboratory (ANL)
and Oak Ridge National Laboratory (ORNL).
2. NEUTRINO PHYSICS
with the main emphasis on:
• anti-neutrino oscillation and geo-anti-neutrino studies at KamLAND
• measurements of data needed to correct the KamLAND data for neutron-induced background
• the study of double-beta decay and double electron capture to excited 0+ states
• preparation for a neutrinoless ββ decay search using
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3. FEW-NUCLEON SYSTEMS
with the aim of testing and refining the description of the nuclear force and its currents.
Specific experiments address:
Introduction
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• the neutron-neutron scattering length obtained from 2N and 3N systems
• the role of three-nucleon forces in the 3N and 4N continuum using hadronic and electromagnetic probes
• the analyzing power puzzle in 3N and 4N scattering
• light-nucleus reaction dynamics
4. FUNDAMENTAL-SYMMETRY STUDIES
with focus on:
• time-reversal invariance (neutron EDM)
• non-unitarity tests of the CKM matrix
• the neutron lifetime
• isospin-symmetry breaking
Off-site facilities used for this research include Kernfysisch Versneller Instituut (KVI), Los
Alamos Neutron Science Center (LANSCE), and the National Institute of Standards and
Technology (NIST). Preparations are under way for fundamental physics studies with neutron
beams at the Spallation Neutron Source (SNS).
5. QCD PHYSICS:
with focus on:
• nucleon structure
• the transition between non-pQCD and pQCD
The primary off-site facilities used for this work are Thomas Jefferson National Accelerator
Faclity and MAX-lab (Sweden).
6. MANY-BODY PHYSICS
with the main emphasis on:
• the determination of the parity of dipole states using polarized γ rays at HIγS
• measurements of (n, 2n) cross sections on actinide nuclei
• the excitation of isomeric states
• descriptions of nuclear reaction data with random matrix theory
• the study of preequilibrium nuclear reactions
The primary off-site facility used for this work is LANSCE.
7. APPLICATIONS
with focus on:
• plant physiology using accelerator produced
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• tomography with fast neutrons
• the transmutation of radioactive waste using accelerator-driven devices
8. R & D
Developments in technology and instrumentation are vital to our research and training program. We continued our innovative work in:
• the development of instrumentation for characterizing the γ-ray beam at HIγS
• polarized target development
• detector and scattering chamber development
• polarimeters for charged particles, neutrons and γ rays
9. NUCLEAR DATA PROGRAM
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• nuclear data evaluation for A = 3 − 20 for which TUNL is the international center
• web dissemination of nuclear structure information for A = 3 − 20
In addition to the above research areas, we are vigorously engaged in disseminating results, interacting with the broader nuclear physics community, and running outreach programs aimed at
undergraduate students. This year these activities include:
1. TUNL ran the NSF-supported Research Experience for Undergraduates (REU) program in
nuclear physics for the eighth consecutive year. Thirteen students participicated in this 10week summer program; eight of them were supported by the NSF REU grant and the other
five by grants to TUNL faculty.
2. The TUNL seminar program continues with characteristic vigor (32 invited speakers). It
is supplemented by the TUNL Informal Lunch Talks (TILT) where graduate students and
postdocs present their research projects, the TUNL Astrophysics Journal Club and a special
lecture series given by local speakers on Advances in Physics as part of the REU program
during the summer. A related talk series, the Triangle Nuclear Theory Colloquia, is also
beneficial to TUNL faculty and students.
The success of our research program is largely attributable to the talents and enthusiasm of the
19 faculty members, 18 post-doctoral associates and research staff and 44 graduate students from the
three Triangle universities. Also essential to our research program are long-term collaborations with
research groups from: Gettysburg College, North Carolina A&T State University, North Carolina
Central University, North Georgia College and State University, Penn State Altoona, Tennessee
Technological University, University of Conneticut at Avery Point, All-Russian Research Institute
of Technical Physics (Snezhinsk), Istituto Nazionale di Fisica Nucleare (Pisa), Jagiellonian University (Cracow), Joint Institute for Nuclear Research (Dubna), Technische Hochschule Darmstadt,
University of Cologne, and University of Mainz. In our applications and interdisciplinary research
program we colloborate with scientists from several national laboratories, including Lawrence Livermore National Laboratory and Los Alamos National Laboratory and with scientists from private
companies, such as, the Accelerator Driven Neutron Applications Corporation and the Physical
Optics Corporation.
Personnel
Department of Physics, Box 90308, Duke University,
Durham, NC 27708-0308
Department of Physics, Box 8202, North Carolina State University,
Raleigh, NC 27695-8202
Department of Physics and Astronomy, University of North Carolina,
Chapel Hill, NC 27599-3255
Faculty
Ahmed, M. W. (Assistant Research Professor)
Back, H. O. (Research Assistant Professor)
Bilpuch, E. G. (Professor Emeritus)
Champagne, A. E. (Associate Director, Professor)
Clegg, T. B. (Professor)
Gao, H. (Associate Professor)
Golub, R. (Research Professor)
Gould, C. R. (Professor)
Haase, D. G. (Professor)
Henning, R. (Assistant Professor)
Howell, C. R. (Director, Professor)
Huffman, P. R. (Associate Professor)
Iliadis, C. (Professor)
Karwowski, H. J. (Professor)
Kelley, J. H. (Research Assistant Professor)
Ludwig, E. J. (Professor Emeritus)
Merzbacher, E. (Professor Emeritus)
Mitchell, G. E. (Associate Director, Professor)
Roberson, N. R. (Professor Emeritus)
Tilley, D. R. (Professor Emeritus)
Tonchev, A. P. (Assistant Research Professor)
Tornow, W. (Professor)
Walter, R. L. (Professor Emeritus)
Weller, H. R. (Professor)
Young, A. R. (Professor)
Duke
NCSU
Duke
UNC
UNC
Duke
NCSU
NCSU
NCSU
UNC
Duke
NCSU
UNC
UNC
NCSU
UNC
UNC
NCSU
Duke
NCSU
Duke
Duke
Duke
Duke
NCSU
xii
Personnel
TUNL Advisory Committee
Balamuth, D. P.
Balantekin, A. B.
Beise, E. J.
Friar, J. L.
Garvey, G. T.
Vigdor, S. E.
Wiescher, M. C.
University of Pennsylvania
University of Wisconsin
University of Maryland
Los Alamos National Laboratory
Indiana University
Notre Dame University
Associated Faculty
Crawford, B. E.
Crowe, B. J.
Dutta, D.
Engel, J.
Gai, M.
Keeter, K. J.
Korobkina, E.
Markoff, D. M.
McLaughlin, G. C.
Norum, B.
Pedroni, R. S.
Prior, R. M.
Purcell, J.
Shriner, J. F.
Spraker, M. C.
Stephenson, S. L.
Weisel, G. J.
Gettysburg College
North Carolina Central University
Mississippi State University
University of North Carolina
University of Connecticut at Avery Point
Idaho State University
North Carolina Central University
University of Virginia
North Carolina A&T State University
North Georgia College and State University
Georgia State University
Tennessee Technological University
North Georgia College and State University
Gettysburg College
Penn State Altoona
Research Staff
Boswell, M.1 (Research Associate)
Chankova, R.2 (Research Associate)
Clinton, E.2 (Research Associate)
Crowell, A. S. (Research Scientist)
Dashdorj, D.3 (Research Associate)
Fallin, B. (Associate in Research)
Imig, A2 (Research Associate)
Kalbach Walker, C. (Senior Research Scientist)
Kwan, E.3 (Research Associate)
Mehta, V.4 (Research Associate, part time)
Qiang, Y. (Research Associate)
Ronquest, M.1 (Research Associate)
Rusev, G. (Research Associate)
Seo, P. (Research Associate)
Sheu, G.(Project Coordinator, Nuclear Data)
Stave, S. (Research Associate)
Teymurazyan, A.5 (Research Associate)
Ugalde, C.2 (Research Associate)
Zhu, X.F. (Research Associate)
1 As
of 1/08
7/08
3 Supported by NNSA/DOE
4 Supported by NSF/DOE
5 As of 12/07
2 Departed
UNC
NCSU
Duke
Duke
NCSU
Duke
UNC
Duke
Duke
NCSU
Duke
UNC
Duke
U. Conn/NCSU/U. Mass
Duke
Duke/NGC&SU
U. Mass
UNC
Duke
Personnel
xiii
Technical, Secretarial and Nuclear Data Support Staff for TUNL
Busch, M.
Carlin, B. P.
Carter, E. P.1
Dunham, J. D.
Emamian, M.2
Faircloth, J.2
Hoyt, D. T.3
Johnson, M.2
Li, J.2
Mikhailov, S.4
Mulkey, P. H.
Oakley, O.1,2
O’Quinn, R. M.
Pentico, M.
Popov, V.4
Rathbone, V.2
Swift, G.2,5
Wallace, P.2,5
Wang, P.2
Westerfeldt, C.
Mechanical Engineer
Electronics Supervisor
Accelerator Technician
Accelerator Technician
Mechanical Engineer (at HIγS)
Project and Building Maint. Coordinator (at HIγS)
Lab Apparatus Designer
Vacuum Technician (at HIγS)
Research Scientist (at HIγS))
Accelerator Scientist (at HIγS)
Electronics Technician
Electronics Technician (at HIγS)
Accelerator Supervisor
Accelerator Technician (at HIγS)
Accelerator Scientist (at HIγS)
Accelerator Technician (at HIγS)
Vacuum Engineer (at HIγS)
Electrical Engineer and Safety Manager (at HIγS)
RF Engineer (at HIγS)
Research Scientist, Radiation Safety Manager
Graduate Students
Angell, C.
Arnold, C.
Baramsai, B.
Bertone, P.
Boswell, M.6
Broussard, L.
Cesaratto, J.
Chen, W.
Chyzh, A.
Clasie B.
Couture, A.
Daigle, S.
Daniels, T.
Dubose, F.7
Esterline, J.
Finnerty, P.
Hammond, S.
Henshaw, S.
Holley, A.7
Huibregtse,C.
Hutcheson, A.6,8
Jockers, J.
Kephart, J.
UNC
UNC
NCSU
UNC
UNC
Duke
UNC
Duke
NCSU
MIT
UNC
UNC
UNC
NCSU
Duke
UNC
UNC
Duke
NCSU
NCSU
Duke
UNC
NCSU/PNNL
1 Part-time
2 As
of 8/08
4/08
4 Part-time through 6/08; full-time starting 7/08
5 Supported by DHS
6 Graduated with Ph.D. Degree between 9/07 and 8/08
7 Supported by NSF/DOE
8 Supported by NNSA
9 Graduated with Master’s Degree 5/07
3 Departed
Kidd, M.
Kiser, M.6,7
Longland, R.
MacMullin, S.
Martel, P.
Meier, N.
Mohr, W.9
Newton, J.
O’Shaugnessy, C.7
Palmquist, G.7
Pattie, R.7
Perdue, B.
Qian, X.
Strain, J.
Swank, C.
Swindell, A.
Ticehurst, D.
Tompkins, J.
Walker, C.
Wood, C.
Ye, Q.6
Zheng, W.
Zong, X.
Duke
Duke
UNC
UNC
U. Mass
NCSU
UNC
UNC
NCSU
NCSU
NCSU
Duke
Duke
UNC
NCSU
U. Conn
UNC
UNC
NCSU
UNC
Duke
Duke
Duke
xiv
Personnel
Visiting Scientists
Agvaanluvsan, U.
Algin, E.
Dashdorj, D.
France, R.
Fritzsche, M.
Hagmann, C.
Johnson, M.
Kriticka, M.
Lu, R.
3/08
3/08
4/08
6/08
9/07
2/08
2/08
6/08
9/07
- 4/08
- 4/08
McNabb, D.
Schwengner, R.
Sharapov, E.I.
von Witsch, W.
Wilhelmy, J.
Zweidinger, M.
2/08
1/08
3/08 - 4/08
11/07
4/08
9/07 - 10/07
-7/08
- 10/07
- 7/08
- 08/08
Lawrence Livermore National Laboratory
Eskisehir Osmangazi University, Meselik,Turkey
Georgia College and State University
Technische Universitat Darmstadt
Charles University, Prague, Czech Republic
Institute of Modern Physics of the
China Academy of Science, Lanzhou, China
Forschungszentrum Dresden-Rossendorf, Germany
JINR, Dubna, Russia
University of Bonn, Bonn, Germany
Technische Universitat Darmstadt
Administrative Support Personnel
Byrd, M.
Plesser D.
Pulis, T
West, B.
Temporary Staff
Temporary Staff
Temporary Staff
Staff Assisstant
Undergraduates
Andre, K.
Bennett, D.1
Cumberbatch, L.
Driessen, C.1
Eibin, S.1
Evans, L.
Finch, S.1
Johnson, B.1
Leviner, L.
Long, A.
Lynam, S.1
Macon, K.
Marquess, S.1
Metzler, D.
Pooser, E.1
Pronschinske, A.
Rich, G.
Sand, A.
Smith, A.
Threatt, L.
Yoast-Hull, T.1
1 Supported
by the TUNL NSF REU Program
UNC
DePauw University
NCCU
University of Wisconsin at Stevens Point
Elon University
UNC
NCSU
North Carolina A&T State University
NCSU
UNC
University of Wisconsin at Stevens Point
UNC
University of Maryland in Baltimore County
UNC
UNC
UNC
NCCU
Kenyon College
neutron beam
measurement
cells
C
magnetic
field coils
high voltage
electrode
ross sectional view of the nEDM apparatus. The approximate dimensions are
7.5 m long, 5.5 m tall, and 2.2 m wide. The neutron beam enters from the left and is
down-scattered in liquid helium to produce ultracold neutrons that are confined
within the measurement cells. The cells are positioned within a strong electric
and weak magnetic field. The cells are surrounded by a roughly 1,200 l volume of
liquid helium housed in a carbon-composite vessel. The apparatus is cooled to below
500 mK with a 3 He/4 He dilution refrigerator. (Inset) The entire nEDM experimental apparatus as envisioned at the external facility of the Fundamental Neutron
Physics Beamline facility at the Spallation Neutron Source.
Fundamental Symmetries in the
Nucleus
Chapter 1
•
•
Time-Reversal Violation: The Neutron Electric Dipole
Moment
Fundamental Coupling Constants
2
Fundamental Symmetries in the Nucleus
1.1
1.1.1
Time-Reversal Violation: The Neutron Electric Dipole
Moment
Search for the Neutron Electric Dipole Moment
M.W. Ahmed, M. Busch, F. DuBose, H. Gao, R. Golub, C.R. Gould, D.G. Haase, P.R.
Huffman, D. Kendellen, E. Korobkina, R.C. Lu, C.M. Swank, Q. Ye, A.R. Young, W.Z.
Zheng, X. Zong, X.F. Zhu, TUNL, the nedm collaboration
We describe an experimental program to develop a new technique to search for the neutron
electric dipole moment. This technique offers a factor of up to 100 increase in sensitivity over
existing measurements. The search for this moment has the potential to challenge calculations
that propose extensions to the Standard Model.
The possible existence of a nonzero electric
dipole moment (EDM) of the neutron is of great
fundamental interest in itself and directly impacts our understanding of the nature of electroweak and strong interactions. The experimental
search for this moment has the potential to reveal
new sources of time reversal (T) and charge conservation and parity (CP) violation and to challenge calculations that propose extensions to the
Standard Model.
The goal of the current experiment is to significantly improve the measurement sensitivity of
the neutron EDM over what is reported in the
literature. The experiment has the potential to
measure the magnitude of the neutron EDM or
lower the current experimental limit by one to
two orders of magnitude. Achieving these objectives will have a major impact on our understanding of the physics of both weak and strong
interactions. The physics goals of this experiment are timely and of unquestioned importance
to modern theories of electroweak and strong interactions.
The experiment is based on the magneticresonance technique of rotating a magnetic dipole
moment in a magnetic field. Polarized neutrons
and polarized 3 He atoms coexist in a bath of
superfluid 4 He at a temperature of ∼ 450 mK.
When placed in an external magnetic field, both
the neutron and 3 He magnetic dipoles precess in
the plane perpendicular to the magnetic field.
The measurement of the neutron EDM comes
from a measurement of the difference in the precession frequencies of the neutrons and the 3 He
atoms when a strong electric field parallel to the
magnetic field is reversed. In this comparison
measurement, the neutral 3 He atom is assumed
to have a negligible electric dipole moment. In
principle, this new type of EDM experiment in
conjunction with the Spallation Neutron Source
(SNS) can achieve more than two orders of magnitude improvement in the experimental limit for
the neutron EDM . This factor results from the
possibility of an increased electric field (a factor
of ∼ 5) due to the excellent dielectric properties
of superfluid 4 He, an increase in the total number
of ultracold neutrons (UCN) stored (∼ 100 fold
improvement that leads to factor of 10 gain in
sensitivity) and an increased storage time (∼ 5
times, which produces a factor of 2 in sensitivity) due to the low temperature of the walls.
The current experimental EDM bound, however,
is nearly limited by magnetic-field systematics.
With the proposed experiment, an EDM limit of
10−28 e·cm is possible; the use of 3 He as a volume
comagnetometer is crucial to the minimization of
the magnetic-field systematics.
TUNL is playing an active role in all aspects
of this experiment. Crucial to the initial development of this project is the research and development work of key components of the overall
experiment. Researchers at TUNL are presently
testing a number of outstanding issues related to
maximizing the sensitivity of the measurement.
These contributions include measuring the lifetime of polarized 3 He in a ∼ 450 mK deuteratedcoated acrylic cell filled with 4 He; testing the efficiency at which the 3 He injector system can infuse polarized 3 He into a bath of ∼ 450 mK liquid helium; investigating the use of the heat flush
technique to transport the polarized 3 He from the
injection system to the measurement cell and, af-
terwards, the depolarized 3 He from the measurement cell to the purifier; determining a method
by which one can remove the unpolarized 3 He
from the liquid 4 He; theoretical and experimental validation of the behavior of the geometric
phase effect in our experimental geometries; and
development of a prototype EPICS slow controls
system.
As the experiment works towards the CD2/3A milestone, TUNL researchers are designing the 4 He refrigeration system, dilution refrigeration system, and cryovessel. Working in collaboration with faculty from the NCSU mechanical engineering department and a NASA consultant, we have begun to design the 1,200 l carboncomposite vessel that will house the experimental
cells and high-voltage capacitor system. In addi-
3
tion, work has begun on the development and
characterization of a transient waveform digitization system for the light collection system. An
overview of the apparatus is shown in Fig. 1.1.
In addition to the research outlined above, details of which can be found in subsequent contributions, several TUNL faculty serve in leadership roles in the project: Gao, Golub, and Huffman serve on the project’s executive committee;
Haase and Huffman serve as subsystem managers
for the construction of the cryovessel and for the
assembly and commissioning of the subsystems,
respectively; and Huffman serves on the federal
project team as the project’s technical coordinator. Numerous others serve as work-package
managers for various components of the subsystems.
neutron beam
measurement
cells
magnetic
field coils
high voltage
electrode
Figure 1.1: (Color online) Cross sectional view of the nEDM apparatus. The approximate dimensions
are 7.5 m long, 5.5 m tall, and 2.2 m wide. The neutron beam enters from the left and is
down-scattered in liquid helium to produce ultracold neutrons that are confined within the
measurement cells. The cells are positioned within a strong electric and weak magnetic
field. The cells are surrounded by a roughly 1,200 l volume of liquid helium housed in
a carbon-composite vessel. The apparatus is cooled to below 500 mK with a 3 He/4 He
dilution refrigerator. (Inset) The entire nEDM experimental apparatus as envisioned at
the external facility of the Fundamental Neutron Physics Beamline facility at the Spallation
Neutron Source.
4
1.1.2
Search for the Neutron Electric Dipole Moment: Geometric Phase
Effects
H. Gao, R. Golub, P.R. Huffman, C.M. Swank, Q. Ye, TUNL; the nedm collaboration
In order to achieve the target precision of the nEDM experiment, it will be necessary to
deal with the systematic error associated with the interaction of the well known v × E field
with magnetic field gradients. This systematic has been studied theoretically for a number of
operating conditions, and is being investigated experimentally in newly constructed apparatus
at TUNL.
In the neutron electric dipole moment
(nEDM) apparatus, the interaction of the field
Bef f ∼ v × E with magnetic field gradients will
produce a frequency shift linear in the electric
field. Known as the geometric phase effect, and
mimicking a true EDM, this effect has recently
emerged as one of the primary systematic errors
limiting the precision of the next generation of
neutron EDM searches.
In the context of neutron EDM experiments,
the effect was first investigated experimentally
and theoretically by Pendelbury et al. [Pen04]
and later by Lamoreaux and Golub [Lam05]. In
our recently published work [Bar06], we have
introduced an analytic form for the correlation
function which determines the behavior of the
frequency shift, and we have shown in detail how
it depends on the operating conditions of the experiment. Using this analytic function for the
case of gas collisions, we have averaged over a
Maxwellian velocity distribution to calculate the
temperature dependence of the frequency shift
for 3 He diffusing in superfluid 4 He.
The results indicate that it may be possible
to fine tune the effect to high degree by an appropriate choice of operating temperature. The
present understanding of the effect is summarized
in Fig. 1.2. This is a plot of the normalized (linear in E) frequency shift δω vs. the normalized Larmor frequency ω0 for various values of
the collision mean free path λ and wall specularity, calculated for particles moving with fixed
velocity v in a cylindrical measurement cell of
radius R. The horizontal scale is fixed by the
frequency of motion around the cell, v/R. In
general the shift for ultracold neutrons (UCNs)
will be given by a value of ω0 /(v/R) > 4, and
for 3 He by ω0 /(v/R) 1. From an experimental
point of view it is very appealing to try to make
use of the zero crossings apparent in the figure to
reduce the systematic effects to zero. Our theoretical work indicates it is plausible to do this for
3
He, by tuning the temperature of the apparatus
to take advantage of the 1/T 7 dependence of the
diffusion constant for 3 He in superfluid 4 He.
Specifically for the nEDM experiment, we
have used our simulation results in combination
with 3 He transport calculations to set the nominal operating temperature for the nEDM experiment, 450 mK.
To study these geometric phase effects experimentally, we have constructed a cryogenic apparatus consisting of a dilution refrigerator that
we have incorporated into a non-magnetic Dewar.
The apparatus is shown in Fig. 1.3. The Dewar is
surrounded by a series of eight Helmholtz coils to
provide a uniform field. The initial experiments
have begun and are first aimed at verifying the
predicted depolarization rates of 3 He in 4 He at
T ≤ 500 mK in a deuterated polystyrene bottle.
5
Figure 1.2: (Color online) Fixed-velocity frequency shift induced by the geometric phase effect for
UCN’s and for 3 He [Bar06].
Figure 1.3: (Color online) TUNL dilution refrigerator setup for studying 3 He depolarization in superfluid 4 He and the temperature dependence of the geometric
phase effect.
The geometric-phase-effect measurements
rely on theoretical calculations which show that
instead of measuring the frequency shift arising
from this effect as the 3 He atoms move in an electric and magnetic field (i.e., basically repeating
the nEDM experiment with 3 He at a somewhat
less sensitive level), one can measure the T1 relaxation rate of the 3 He polarization in a magnetic
field gradient. If one applies a uniform magnetic
field gradient ∂Bz /∂z large enough so that it
dominates all other field gradients that may be
present, then the relaxation time, T1 can be determined by traditional NMR techniques in the
case when ∂Bz /∂z is large enough so that wall
relaxation can be neglected. This technique does
not require an electric field and thus significantly
simplifies the experiment.
The apparatus has been fully assembled, leak
tested and cooled to < 500 mK in a series of initial measurements.
Work is also underway to study the surface
quality of the deuterated polystyrene coatings.
The aim of these measurements is both to understand the surface roughness and to explore coating techniques to improve the surface quality. We
envision exploring how the surface quality affects
quantities such as 3 He depolarization.
[Bar06] A. L. Barabanov, R. Golub, and S. K.
Lamoreaux, Phys. Rev., A74, 052115
(2006).
[Lam05] S. K. Lamoreaux and R. Golub, Phys.
Rev., A71, 032104 (2005).
[Pen04] J. M. Pendlebury et al., Phys. Rev.,
A70, 032102 (2004).
6
1.1.3
Research and Development for the nEDM Project: Purification of
4
He
D.G. Haase, F. DuBose, R. Golub, P.R. Huffman, D. Kendellen, T. McCaw, TUNL
A cryogenic apparatus is being constructed to investigate the evaporative purification of liquid
4 He
for the nEDM measurement process. In the past year the dilution refrigerator has been
refurbished and tested with a new Dewar and vacuum can. The evaporative purification device
has been designed and is being mounted in the cryostat. We have investigated methods of
measuring the 3 He/4 He ratio using a helium spectrometer leak detector and a residual gas
analyzer.
Removal of 3 He from liquid 4 He
for the nEDM measurement
An SHE model 430 dilution refrigerator, constructed in 1981, has been refurbished for use in
these tests. The refrigerator was last used in sumThe objective of the 4 He Evaporative Purifica- mer 2000, at the Triangle Universities Nuclear
tion R&D task is to design and measure the effi- Laboratory. We acquired a new liquid-helium
ciency of a 4 He-purifier prototype for the neutron Dewar and constructed a new aluminum vacuum
electric dipole moment (nEDM) cryostat. This can for the system. The system has been installed
evaporator must reduce the 3 He concentration in in the laboratory in the Bureau of Mines building
the 4 He target from 10−10 to 10−12 in 100 to 200 on the NC State campus, where there is a floor
seconds. The research would answer the follow- pit suitable for the lowering and storage of the heing questions, which are important to the design lium Dewar. The dilution refrigerator had previand implementation of the final purifier:
ously operated at nearly 600 µmoles/second, but
3
the
• What is the purification efficiency of the 3 original Alcatel 2060H (60 m /hour) sealed
He recirculation pump was replaced by an Aldevice?
catel 2030H (42 m3 /hour). We have designed
• Does its operation correspond to our mod- and purchased a large bucket Dewar to enclose
els for the evaporation process?
the large (12 inch diameter) purifier. The new
aluminum
vacuum can has an inner diameter
• Do the 3 He surface states affect the evapo- of 17.5 inches and interior height of 38 inches.
rative process?
Unfortunately, considerable time was lost in the
Fall
semester in locating and repairing low tem• What is the optimum temperature for opperature
leaks in the vacuum can. These were
eration?
traced to weld problems and to differential con• What heat load will be transmitted to the tractions in the indium-sealed gasket between the
refrigerators?
aluminum flange and the stainless steel dilution
refrigerator flange.
• What is a suitable type of device for reducing the superfluid 4 He flow for this applicaIn March 2008, we cooled down the SHE 430
tion?
three times. In the first two cooldowns there were
1.1.3.1
problems with the liquid helium transfer and the
removal of the exchange gas. At the same time,
all of the relevant mechanical and electrical connections and parts of the pumping system were
checked and fixed. We consider these to be reaThese issues have been discussed in a paper sonable startup problems. In the third cooldown
by Hayden, Lamoreaux and Golub [Hay06], and a the cryostat was kept at liquid helium temperresource document by Haase [Haa06]. In the lat- ature for several days and operated below 0.1
ter, a prototype design for the 4 He purifier was K. The cryostat was precooled to liquid nitrogen
described.
(LN) temperature on March 24, and the liquid
• Can a reliable purification method be implemented that would remove the appropriate amounts of 3 He within the allotted time
in the measurement cycle?
7
helium (LHe) transfer began on March 25. The throughs to operate the valve and to open/close
helium exchange gas in the vacuum can was ad- a heat switch.
justed to cool the refrigerator to 4.2 K and then
pumped out before filling the dilution refrigerator. The refrigerator ran until March 28, completing all of the planned tests of cooling powers,
flow rates, and thermometry. The Dewar was
kept at 4.2 K until March 31. A total of 300 l
of LHe was used for the cooldown and operation.
Several calibrated thermometers will be used for
the test of the 3 He purifier. During the run, four
resistance thermometers (RuO and carbon glass)
were cross-calibrated against a calibrated germanium thermometer in the range of 1.0 K ≥ T ≥
0.1 K. The minimum temperature measured was
below 60 mK.
Because of the smaller Alcatel 2030H
pump, the average flow rate was near 300
µmoles/second, thus reducing the effective cooling power of the refrigerator. In Fig. 1.4 we show Figure 1.5: Schematic of the helium purifier showing the evaporator, valve and lifting
the measured cooling power as a function of temvolume.
perature.
1.1.3.3
Figure 1.4: Measurement of the cooling power of
the SHE 430 dilution refrigerator using the 9B3 oil booster pump with
an Alcatel 2030H mechanical backing
pump. The measured powers are compared to the operation of the same
machine at 422 µmol/sec with an Alcatel 2060H, and the theoretical performance at a 3 He flow rate of 300
µmol/sec.
1.1.3.2
Design of 4 He evaporator system
An almost-full-scale 4 He evaporator system, including gas adsorber, heat switches, a liquid helium reservoir and a mechanical lifter to raise and
lower the liquid helium level in the reservoir, has
been designed (see Fig. 1.5). The parts are being fitted for installation in the vacuum can. The
system requires two capacitance gauges to measure the liquid level, a linear feed-through to raise
and lower the lifting volume, and two rotary feed-
Measurement of
tions in 4 He
3
He concentra-
A diagnostic is needed to measure the effectiveness of the purifier. When the charcoal adsorber of the evaporator is heated, the desorbed gas
should have a concentration of 3 He orders of magnitude greater than that of the liquid. Normally
the relative 3 He/4 He ≡ X3 concentrations are
measured by special mass spectrometers or by
accelerator mass spectrometry. We have tested
a Leybold L200 mass spectrometer leak detector
and a Stanford Research Systems RGA 100 residual gas analyzer to determine X3 from natural helium gas samples and also from 3 He and 4 He calibrated leaks. It was found that the leak detector,
although it could be calibrated for both 3 He and
4
He signals, did not provide reproducible measurements of X3 . The RGA showed that much
of the problem was caused by large mass=2 and
mass=3 backgrounds. Still, the background limited the sensitivity of X3 to 10−3 . We have begun
testing a new RGA vacuum manifold that makes
use of LN cooled activated charcoal, a titanium
wire mesh to trap H2 and HD, and final LHe temperature trap [Dav90].
[Dav90] T. A. Davidson and D. E. Emerson,
9302, 1 (1990).
[Haa06] D. G. Haase, 2006, unpublished report
for NEDM Collaboration.
[Hay06] M. E. Hayden, S. K. Lamoreaux, and R.
Golub, In AIP Conference Proceedings,
volume 850, pp. 147–148, 2006.
8
1.1.4
Engineering of Cryovessel and Refrigerators for the Measurement
of the Electric Dipole Moment of the Neutron
D.G. Haase, P.R. Huffman, TUNL; J.W. Eischen, B.J. Angell, NC State University, Raleigh,
NC ; J. Boissevain, California Institute of Technology, Pasadena, CA; E. Ihloff, Massachusetts
Institute of Technology, Cambridge, MA
The proposed apparatus to measure the electric dipole moment of the neutron will be housed
in a large cryovessel and cooled by a high flow rate dilution refrigerator. A dedicated helium liquefier will support the cryogenic system. The target volume will be enclosed in a
fiber/epoxy composite insulation volume holding 1200 liters of liquid helium at 0.35 K. We
have conducted design studies for the cryovessel, liquefier, dilution refrigerator and insulation
volume in preparation for beginning the procurement process in early 2009.
1.1.4.1
Overview of nEDM Subtask 1.3
- Cryovessels, Refrigerators and
Related Equipment
The neutron electric dipole moment (nEDM)
cryogenic systems include three long lead time
items: the cryovessel which contains the experiment, the helium liquefier which cools it to 4.2 K,
and the dilution refrigerator which cools the target volume to 0.2 K - 0.5 K. Because of the
high costs (∼ $1 M each) and the long construction times (1- 1.5 years) for each component, the
nEDM collaboration is moving for early review
and procurement of these items.
The properties of the cryovessel, liquefier and
dilution refrigerator are determined by the cooling requirements of the experiment itself. The
design and selection of the components is therefore an iterative process, in which we have completed one iteration. There are two fundamental
experimental requirements:
• The target should be able to operate continuously for 90 days at 0.2 K - 0.5 K with
a total heat input of ≤ 80 mW at 0.5 K.
• It should be possible to cool the entire system to the operating temperature in a period of three to four weeks. This is necessary to complete the testing and experimental schedule in a reasonable time.
Four parts of the nEDM cryovessel shown in
Fig. 1.6 will be refrigerated. Each will be located
in the cryovessel, the large vacuum vessel that
encloses the helium target volume. These are:
(1) the intermediate temperature shields, (2) the
liquid helium (LHe) entrainment volume, (3) the
helium insulation volume and (4) the target volume. The two intermediate temperature shields
will be cooled by liquid nitrogen or by gas evaporated from the liquid helium entrainment volume.
The entrainment volume is thermally connected
to the 4.5 K thermal shields and the magnets.
The helium insulation volume (1000 l) will operate at 0.2 K–0.5 K and house the capacitors,
target volume and light guides. It will be cooled
by a heat exchanger to the dilution refrigerator.
Figure 1.6: (Color online) Cross section view
of the cryogenic components of the
nEDM cryovessel. The cryovessel is
directly connected to a liquid delivery
and gas return line from the helium
liquefier. The pumping lines from the
dilution refrigerator cryostat connect
to a vibration isolation block, three
roots blower pumping stacks and a
gas-handling/storage panel.
1.1.4.2
Cryovessel
We have modeled the heat inputs into the liquid nitrogen (LN) shield and the 4.5 K heliumcooled shield of the cryovessel. The shields will
be insulated by vacuum and multiple layers of
aluminized mylar superinsulation. The shields
will be constructed of aluminum alloy with multiple cooling tubes for LN or He gas welded to
the surfaces. Because of the overall heat load
to the cryovessel the 77 K LN shield was chosen in preference to a He gas-cooled intermediate
shield. The 4.5 K shield will be cooled through
a thermosyphon arrangement, where evaporating
helium from the entrainment volume is cycled
through the shield and then to the cold return
line of the helium liquefier. In the design process,
we have also calculated scenarios for cooling the
cryovessel and contents to 4.5 K and the final operating temperatures for the nEDM experiment.
Because of the large masses of the components
and the 1200 l volume of LHe in the insulation
volume, a cooldown will require 3–4 weeks. We
have also investigated safety requirements for the
cryovessel and other components and incorporated appropriate burst disks and pressure vents
into the design.
9
also a 1.5 K liquid helium evaporator to liquefy
the 3 He before it reaches the still of the dilution refrigerator. With Janis, we have developed
specifications for all of the dilution refrigerator
system, studied heat flows in the nEDM experiment, and designed heat exchangers for cooling
the LHe volumes in the experiment. The final
parameters of the designed system are consistent
with other such refrigerators, and we are ready
to prepare requests for quotes from vendors.
1.1.4.5
Insulation Volume
The neutrons will be confined in two 8 liter target volumes which are enclosed in a 1200 l insulation volume. The 1200 l volume is at electrical
ground; its dimensions are set by the need for
uniform electrical fields in the target. The insulation volume will be enclosed by the AC and
DC magnets and contain a set of 13 very sensitive
SQUID magnetometers. Therefore the insulation
volume will be constructed of a non-conductive,
non-magnetic fiber composite. This is similar to
the type of construction used for cryogenic fuel
1.1.4.3 Helium Liquefier
tanks for rockets. We have calculated stresses in
We hired an experienced cryogenic consultant to the proposed container. The end cap of the conhelp develop a helium refrigeration and liquefac- tainer will be a flat plate of composite, bolted and
tion budget for the nEDM cryogenic systems and sealed with an O-ring to a flange on the container.
to help specify an appropriate liquefier system. The end cap must include penetrations for winThe nEDM experiment will require about 13 l/hr dows for light guides, electrical feedthroughs and
of liquid helium and an additional 50 W of refrig- operators for bellows as shown in Fig. 1.7. These
eration through cold gas. The design is converg- penetrations cause large stresses on the end cap.
ing on a small turbine liquefier (e.g. Linde L70), The ANSYS stress models have been used to
with a cold gas return from the helium entrain- guide the design and placement of the penetrament volume and a warm gas return from the tions. We have contacted vendors to discuss the
dilution refrigerator separator and 1.5 K evap- parameters for construction and testing of the inorator. The fully closed system will include a sulation volume design.
compressor, liquefier engine, gas purifier, control
panel, liquid and gas transfer lines and a 2000 l
external liquid storage Dewar.
1.1.4.4
Dilution Refrigerator
We have studied configurations of several high
cooling power dilution refrigerators, many of
them used for polarized nuclear target systems.
We also contracted Janis Research to produce a
design study for the cryostat, pumping stacks and
gas-handling system for the nEDM dilution refrigerator. The final design is meant to operate
at a 3 He flow rate of 20 - 30 mmol/sec. This high
flow rate requires a gas heat exchanger – a separator – which uses helium vapor from the entrainment volume to cool the incoming 3 He. There is
Figure 1.7: (Color online) ANSYS stress analysis
of a preliminary configuration of the
end cap of the nEDM composite insulation volume
10
1.1.5
Polarized 3 He Relaxation Studies at Low Temperatures
H. Gao, R. Golub, F. Dubose, D. Dutta, P.R. Huffman, R.C. Lu, Q. Ye, X. Zong, W.Z.
Zheng, X.F. Zhu, TUNL
Over the past year, we have successfully carried out a first measurement of the 3 He
spin relaxation time in a dTPB-dPS (wavelength shifting material) coated acrylic cell filled
with superfluid 4 He at a temperature of 416 mK. Polarized 3 He atoms were dissolved in the
superfluid. Our preliminary results are promising for the planned nEDM experiment, and
new techniques for improving the dTPB-dPS coated acrylic surfaces are being developed.
The planned neutron electric dipole moment
(nEDM) experiment is based on the nuclear magnetic resonance technique. The overall experimental strategy is to form a three-component
fluid of ultracold neutrons and polarized 3 He
atoms dissolved in a bath of superfluid 4 He at
a temperature between 350-450 mK. Therefore,
understanding the relaxation mechanism of polarized 3 He nuclei in the storage cell under the
nEDM experimental conditions and maintaining
the 3 He polarization is crucial. In the nEDM experiment, the neutron storage cell will be made of
dTPB-dPS (wavelength shifting material) coated
acrylic and filled with superfluid 4 He. Therefore,
we are carrying out systematic studies of the polarized 3 He relaxation time in dTPB-dPS coated
acrylic cell at low temperatures at TUNL.
The relaxation times of polarized 3 He in a
cylindrical acrylic cell with the inner surface
coated with dTPB-dPS material were measured
at 1.9 K [Ye08] with the presence of superfluid
4
He at a magnetic holding field of 21 Gauss. Measurements below 500 mK (the nEDM experimental temperature) are being carried out using a dilution refrigerator (DR) in the TUNL assembly
hall in the FFSC building at Duke University.
room temperature, and the spin-exchange optical
pumping technique is used to polarize the 3 He.
To simplify the complicated system, we decided
to polarize 3 He on the polarizing station, which
is located in another building, then transfer it
to the cryogenic system using a portable magnetic field system. The d-TPB coated acrylic cell
(1.45 in. diameter and 2.0 in. long cylinder, attached to the glass capillary via a glass-to-copper
seal) is cooled by the DR mixing chamber to below 500 mK and filled with superfluid 4 He. The
glass valve separating the two cells is opened to
allow the polarized 3 He atoms to diffuse to the
bottom acrylic cell at low temperatures. A set
of RF and pick-up coils for the low temperature
NMR system are placed inside the inner vacuum
chamber (see Fig. 1.9) and a series of NMR-AFP
measurements are then performed to measure the
3
He longitudinal relaxation time (T1 ).
We obtained some very preliminary results
in August 2007 and also realized some issues in
the system which prevented the measurement cell
from being completely filled with superfluid 4 He
and also from reaching stable temperatures below
500 mK. A number of modifications have been
made to the cryogenic system to improve the
Figure 1.8 shows the experimental setup and cooling power, including adding a 0.25 mm hole
its schematics for the below-500-mK measure- in the 3 He transfer capillary to limit the 4 He film
ments. An eight-coil (33 in. diameter each, 16.5 flow; using gold plated 99.999% pure copper wires
in. separation) cylindrical magnet system is as- to connect the mixing chamber to the acrylic
sembled to produce a vertically uniform magnetic cell; and making many grooves on the acrylic
field of ∼7.5 Gauss along the dilution refrigera- cell to house the cooling copper wires. These
tor. As in the double-cell system in the 1.9 K methods have successfully increased the cooling
[Ye08] test, polarized 3 He atoms are introduced power transferred to the measurement cell. Most
into the bottom acrylic cell from a detachable recently we have finished a set of measurements
2 in. diameter glass cell sitting ∼ 86 inches of the 3 He relaxation time from the dTPB-dPS
above it outside of the DR. The two cells are coated acrylic cell at a temperature of ∼400 mK
connected via long Pyrex capillary tubing and with the cell filled with different amounts of suseparated by glass valves. Detachable Pyrex cells perfluid 4 He. The measurement with the cell full
are filled with ∼1.5 atm 3 He and ∼100 torr N2 at of superfluid 4 He at 416 mK gives a 3 He relax-
11
Figure 1.8: (Color online) Schematics of the below 500 mK test setup in FFSC at Duke University.
ation time of ∼1666 seconds, and the corresponding 3 He depolarization probability of the wall is
∼4.7×10−7 with a surface to volume ratio of ∼1.5
cm−1 . The total amount of 4 He (3 He) in the
cell is ∼ 2 moles (∼0.012 mole). This is a great
accomplishment after many setbacks associated
with such a challenging and complicated experimental apparatus. The neutron EDM experimental cell has dimensions of 7.6 cm × 10.2 cm
× 50.0 cm and its surface to volume ratio is
∼ 0.5 cm−1 . The scaled relaxation time of polarized 3 He for the nEDM conditions is therefore
∼5000 seconds. The next steps are improving the
dTPB-dPS coated acrylic surface to get a longer
3
He relaxation time, studying the temperature
dependence, and incorporating the B-field gradient in the z-direction to study the geometric
phase effect.
Figure 1.9: (Color online) Low temperature NMR
system and d-TPB coated acrylic cell.
[Ye08] Q. Ye et al., Phys. Rev., A77, 053408
(2008).
12
1.1.6
Simulation of 3 He Transport in 4 He using the Heat-Flush Technique
R. Golub, C.M. Swank, TUNL; G. Seidel, Brown University, Providence, RI ; the nedm collaboration
Polarized 3 He is used as a co-magnetometer in the neutron electric dipole moment experiment.
This helium, produced using an atomic beam source, is infused into a bath of ∼ 400 mK liquid
helium that resides above the measurement cells. This polarized 3 He must be moved from the
collection cell to the measurement cell in a short (∼ 100 s) amount of time while maintaining
a high degree of polarization. We have investigated using heat currents to move the 3 He both
into the measurement cells and from the measurement cells to the purification (removal)
region.
The isotopically pure helium used in the
nEDM measurement cells will be purified using
the heat flush technique. This technique uses the
mass flow of the normal fluid component of superfluid 4 He at a temperatures of ∼1.5 K to flush
away 3 He atoms. We are exploring the use of this
technique to move dilute concentrations of polarized 3 He throughout the nEDM apparatus.
The initial work was aimed at determining
the best physical model to describe the transport of 3 He via the phonon wind in the superfluid
4
He. While a number of models were considered
that involved either 1-D or 2-D heat transport
using an effective mean free path, the compressible Navier-Stokes model was determined to be
the most appropriate one to use for modeling the
flow of the normal fluid in the presence of the
heat flux.
In conjunction with the time-dependent
convection-diffusion equations,
∂c
+ u · ∇c = ∇ · (D∇c) ,
∂t
(1.1)
the Navier-Stokes formalism provides a complete
transport model. Here c is the concentration of
3
He, u is the velocity field, and D is the diffusion coefficient of 3 He due to phonon collisions.
The two-fluid model of superfluid helium has the
property that the phonon “density”, ρ, is described by the temperature. This normal fluid
density was taken to be the density of the fluid
in the Navier-Stokes equation,
h
T
ρu∇u = ∇ · −pI + η ∇u + (∇u)
− (2η/3) (∇ · u) I] + F,
dp = Sn (T )dT,
(1.4)
where
Sn (T ) =
4
2π 2 kB
T 3 = 9969T 3.
3
45h̄3 vthermal
(1.5)
Here vthermal is the average speed of the phonons,
kB is Boltzmann’s constant, and T is temperature.
Several geometries in the apparatus are being modeled. The first is the movement of the
3
He from the injection cell into a secondary volume. This secondary volume is required because
the injection region must operate at a temperature lower than the measurement cells. Next, the
helium must be transported from the secondary
volume into the measurement cells. After data
collection, the depolarized 3 He must be
(1.2)
and
∇ · (ρu) = 0.
Here, η is the viscosity of the normal fluid component in the two fluid model, I is the identity
matrix, F is the external force (never used in
the simulation), and p is the pressure. The pressure is determined by the temperature gradients
through the fountain pressure and is given by
(1.3)
Figure 1.10: (Color online) A block diagram of
the components of the 3 He transport
system.
transported from the measurement cells into a
holding volume. This volume is required to allow
the 3 He to be concentrated into a small extrac-
13
tion volume for removal. See Fig. 1.10 for a rough
schematic.
Figure 1.11: (Color online) A simple geometry showing the results of a Navier-Stokes simulation. The
two rectangular objects on the left-hand side are the two measurement cells. The cells are
connected to the intermediate volume at the right by tubing. The color scale represents
the distribution of temperatures.
Finite element analysis (FEM) using the ANSYS computer software is being used to simulate
the physical model. A simple test geometry can
be seen in Fig. 1.11.
A large parameter space has been narrowed
through the optimization of FEM models that
optimized the tube length and radius while
adapting the heat flux in accordance with normal fluid velocities within the tubes. The results
of one such calculation are shown in Table 1.1.6.
Design Parameter
Tube Length
Tube Radius
Maximum velocity
IV1 Temperature
Cell Temperature
Fraction in cell at 75 s
Polarization at 75 s
Value
300 cm
1.3 cm
44 cm/s
0.477 K
0.450 K
0.973
99.06 %
Table 1.1: Simulation parameters when moving
3 He from the intermediate volume to
the measurement cell using a heat flux
of 5 mW. The volume of IV1 is 1570 cc
and the total volume of the measurement cells is 8000 cc.
In addition to the transport, the depolarization was modeled using a lossy boundary condition. One thus wants to minimize the time spent
in pipes by minimizing the surface area to volume
ratio in the pipes, while staying below a minimum heat budget. Figure 1.12 shows that there
is a range of parameters such that the polarization conditions can be satisfied.
Figure 1.12: (Color online) Polarization conditions have been shown to be acceptable for a certain region of times
ranging from about 35 s to 75 s of
heat flush.
The heat-flush model, up to this point, is considered a success. We have shown that the 3 He
transport due to the heat flush is a viable option
for the nEDM experiment. We are now incorporating the heat flush into the current experimental design. Experimental tests of the heat flush
are being planned.
14
1.1.7
Preparation of The 3 He Injection Test
H. Gao, M. Busch, D. Dutta, R. Golub, D.G. Haase, P.R. Huffman, Q. Ye, W.Z. Zheng,
X.F. Zhu, TUNL; and others, Arizona State University, Brown University, California Institute of
Technology, Los Alamos National Laboratory, Massachusetts Institute of Technology, Simon Fraser
University
Over the past year, we have completed about 90% of the design of the
3 He
injection test
apparatus. We have also constructed a pulsed nuclear magnetic resonance system for the 3 He
measurement during the injection test. The entire apparatus is expected to be completed by
the end of the summer of 2008, and after that the test will take place at Los Alamos National
Laboratory.
The neutron electric dipole moment (nEDM) eter) designed for pNMR polarization measureexperiment requires ∼100% polarized 3 He atoms ments.
which are polarized by an atomic beam source
(ABS). The goal of the 3 He injection test is to
demonstrate that one can inject polarized 3 He
atoms from an ABS and collect them in a volume
filled with superfluid 4 He at the nEDM operating temperature of 300-400 mK with acceptable
polarization loss. Cryogenic problems associated
with the injection apparatus will be studied.
The experimental setup of the injection test
is shown in Fig. 1.13. The ABS provides a well
collimated ∼100% polarized 3 He beam with an
intensity of 1014 atoms/s and an average velocity of ∼100 m/s. At the ABS exit, the velocities and spins of 3 He atoms are along the ABS
downstream axis. After passing a vacuum transfer tube of ∼1.0 m in length, 3 He atoms enter
a collection reservoir, a Pyrex cell filled with superfluid 4 He at 0.35 K shown in Fig. 1.14. The
principles of the injection test, as well as a magnet system for the spin transport and for providing a uniform magnetic field for polarization
measurements, and a pulsed nuclear magnetic
resonance (pNMR) system for polarization measurements have all been discussed by H. Gao et
al. in the 2007 progress report. Except for the
ABS, all other parts of the apparatus are inside a
cryogenic Dewar of 2 layers of thermal radiation
shield with temperatures of 4 K and 50 K, respectively, cooled by a dilution refrigerator (DR).
Figure 1.13: (Color online) Experimental setup of
the injection test for the nEDM experiment.
A ductile thermal link between the DR and
the Pyrex cell is implemented through oxygen
free high-conductivity (OFHC, thermal conductivity λ ∼ 100 W/m ) copper foils. The outer
The Pyrex glass reservoir consists of an in- surface of the injection tube is covered with 20
jection tube and a measurement cell. The injec- layers of copper foil. GE Varnish will be applied
tion tube provides a liquid 4 He surface area large to glue the Pyrex and the copper foils to ensure
enough (∼4.0 cm in diameter) to accommodate good surface contact. When the mixing chamber
the 3 He beam profile. The measurement cell is a of the DR is operating at 0.25 K and the Pyrex
cylinder (2.0 cm in length and 2.0 cm in diam- cell is at 0.34 K, the cooling power through the
foils (20 × 0.025 cm in thickness, 5.0 cm in width,
and 90 cm in length) is 0.596 mW, determined
by thermal resistance. The thermal resistance is
dominated by the contribution from the wall of
the Pyrex cell (1.0 mm thickness).
Figure 1.14: (Color online) Cross-section view of
the injection test for the nEDM experiment.
The internal surface of the reservoir is coated
with cesium to suppress the depolarization of polarized 3 He from the wall. The depolarization
rate from the cesium-coated wall is believed to
represent a ≥100 fold improvement over the un-
15
coated Pyrex surface. Three Cs rings will be
made along the injection tube by chasing Cs vapor with a torch flame and then condensing the
vapor with dry ice attached to the outside of the
injection tube. Because of the no-wetting effect of
superfluid 4 He on the Cs surface, Cs will prevent
the superfluid film from creeping over the ring to
reach the warmer area along the injection tube
and vaporize. If the Cs ring works effectively to
block the superfluid film, it will not only reduce
the heat load from a superfluid film burner by
∼2.5 mW, but also limit the surface area seen
by the polarized 3 He atoms, thus increasing the
3
He spin-lattice relaxation time. Currently, our
R&D on the Cs ring test is continuing at TUNL.
The construction of the Cs ring will take place
in-situ before the entire apparatus is assembled
and cooled down.
A gas-handling system is being specially made
for filling a known amount of 3 He/4 He into the
collection volume for NMR calibration purposes
and for controlling the height of the liquid 4 He.
The gas-handling system and the rest of the injection system have been or will be shipped to
Los Alamos National Laboratory for the upcoming injection test in August 2008, and the goal is
to complete the entire test in time for the CD-2
review of the nEDM project.
16
1.2
1.2.1
Fundamental Coupling Constants
New Precision Measurement of the
19
Ne Lifetime Measurement
L. Broussard, H.O. Back, A.S. Crowell, C.R. Howell, R.W. Pattie, A.R. Young TUNL
High precision measurements of β-decay parameters in the
19 Ne
system allow for sensitive
probes of the weak interaction. A planned experiment at the Kernfysisch Versneller Instituut (KVI) in Groningen, the Netherlands, in spring 2009 will reduce the uncertainty of the
measured lifetime to below the 0.02% level, a factor of three improvement over previous measurements. We present results of a systematic study performed at KVI in February 2008, and
progress towards the final measurement.
A high precision measurement of the betaasymmetry and lifetime of the mixed, superallowed decay of 19 Ne provides valuable information about proposed extra-Standard Models and
can directly test our current model of the weak
interaction via the extraction of the CKM matrix
element Vud . The limiting uncertainties in this
system are experimental and are mainly due to
the measured lifetime. A factor of three improvement in the precision of the measured lifetime is
required for this system to become competitive
with the extraction of Vud from the superallowed
Fermi 0+ → 0+ decays.
When attempting such a high precision lifetime measurement, care must be taken to avoid
any time-dependent systematic effects. Contamination in the sample, diffusion from the detection area, time-varying backgrounds, and ratedependent detector effects can alter the measured
lifetime and must be eliminated or corrected below the level of our target precision. We can meet
these requirements in a planned experiment at
the TRIµP facility, at KVI. The TRIµP isotope
separator can produce very pure isotope beams
A
using two stages of Z
separation. We have developed a tape drive system to collect implanted
19
Ne and transport the sample to a shielded detection assembly. We then count the decays by
detecting the emitted positron and annihilation
gamma rays in coincidence.
We have demonstrated that our design of the
tape drive allows for smooth, fast motion while
not introducing any significant effects that can
affect our measurement (Fig. 1.15). We chose
an aluminized Mylar tape as a target for implantation of 19 Ne. Aluminized Mylar is known to
be an excellent barrier to diffusion, and Mylar
tape is used in tape drive systems for its dura-
bility. The tape performed very well in all stress
tests with the system; however, the tape guides
scratched through the aluminum layer, and examination of the data reveals that a small level
of diffusion could not be ruled out. New tape
designs using aluminum as the implantation target are now being explored. The tape drive used
three servomotors to drive the tape and tension
the supply and take-up reels. We were able to
consistently position the tape to within 2 mm
of the detector center, and we are designing a
photodiode-tracking system which can position
with sub-millimeter precision. Upgraded motors
and improvements to the tape guides will also
increase the reliability of the tape drive.
Figure 1.15: (Color online) The tape drive system
coupled to the end of the separator
beamline. The tape is stored on a
supply reel (bottom right), passes in
front of a vacuum window in the end
flange of the separator, and exits to
the left to the detector assembly (not
shown).
We have performed a detailed study of the
achievable purity of the sample using the isotope
separator. We used Silicon detectors in two locations in the separator to examine the effect of introducing energy degraders and varying the particle beam acceptance. Different isotopes can be
distinguished by their deposited energy and time
of flight through the separator. We have demonstrated that we can eliminate the effect of the
contamination to below our targeted precision.
Further analysis is ongoing to optimize the rate
of 19 Ne production.
Our detection system uses fast scintillators to
detect the positron and two high purity germanium (HPGe) segmented “clover” detectors to
detect the back-to-back 511 keV gamma radiation in coincidence. The detectors are mounted in
a layered aluminum, copper, and steel assembly
that also acts as graded shielding. The assembly
is surrounded by at least 10 cm of lead bricks.
During the high-current study, we found that we
could reduce the ambient and beam-generated
backgrounds to well below our sensitivity by requiring only a coincidence between the HPGe detectors, even with very broad energy cuts.
During the high-current runs we achieved
peak decay rates in our Ge detectors of about
3.5 kHz (with open slits in the TRIµP spectrometer), and we ultimately expect roughly the same
Ge detector rates with slits closed to the optimum for background rejection. We use CAEN’s
17
V1724 ADC to digitize the Ge detector signals.
Our plan is to directly digitize the output of each
clover’s pre-amp, but during our shakedown run
we put the pre-amp signals through shaping amplifiers, because baseline-restoration for the ADC
had not yet been implemented. The V1724 is a
relatively new module, and although preliminary
tests suggested we should experience virtually no
deadtime at our expected rates, the implementation for our shakedown run was not deadtime
free. We are working with CAEN’s engineers and
have already made progress in resolving these issues.
We are now upgrading our tape drive system
and implementing an alternative drive arrangement in parallel. Testing of these systems should
be complete by the end of the year, permitting
us to select the optimum drive arrangement for
our run. Development of a multiple computer
data acquisition system using the MIDAS software is now also underway, with one computer
acting as the data-taking “front end” and the
other as the analyzing “back end”, which allows
for online analysis. We have purchased a second digitizing ADC which will further increase
data rate capabilities. We expect to complete
all necessary improvements identified during the
systematic study before the high statistics run
early in 2009.
18
1.2.2
The UCNA Experiment
H.O. Back, L. Broussard, C.R. Cottrell, A.T. Holley, R.W. Pattie, Jr., A.R. Young,
TUNL
The UCNA collaboration has completed a preliminary measurement of the β-asymmetry in
neutron decay, the first measurement of β-decay angular correlations using ultracold neutrons.
Data obtained during the LANSCE 2007 run cycle should result in a measurement with uncertainties of less that 5% for the β-asymmetry. The experiment is in the process of obtaining
a more substantial data set, with the goal of reducing the experimental uncertainties to below
the 1% level by the end of 2008.
Measurements of neutron decay provide fundamental information on the parameters characterizing the weak interaction of the nucleon.
These parameters impact predictions of the solar neutrino flux, big bang nucleosynthesis, the
spin content of the nucleon, and tests of the
Goldberger-Treiman relation. In addition, they
place constraints on extensions to the Standard
Model such as supersymmetry and left-right symmetries. As we present below, the use of ultracold neutrons (UCNs) for angular correlation
measurements provides a different and powerful
approach to reducing the systematic errors (in
particular, associated with neutron polarization
and neutron-generated backgrounds) characteristic of traditional cold- or thermal-neutron-beam
experiments.
The UCNA experiment is located in the Los
Alamos Neutron Science Center (LANSCE) facility at Los Alamos National Laboratory, and utilizes a solid deuterium ultracold neutron source
developed for this project. The 800 MeV proton beam at LANSCE is directed at a tungsten
target, situated within an outer, bucket-shaped
shell of graphite and an inner shell of Be. Both
shells serve to reflect and partially moderate neutrons produced by the spallation process. The
neutrons are further moderated to temperatures
under 100K by a layer of polyethelene held at
room temperature and by a cold-moderator at
temperatures between 20 and 80K. A 2 liter volume of solid deuterium, held at 5K inside a UCN
guide, serves as the UCN converter. The solid
deuterium is isolated from the rest of the UCN
guide system by a valve. This valve is opened
for a brief interval around the instant the proton
beam is incident on the target, permitting produced UCNs to enter the guide system. The rest
of the time, this valve is closed to minimize UCN
loss due to absorption in the deuterium. UCNs
then move through a 15 m guide system incorporating a 7 T polarizer magnet, then through a
spin-flipper system, and into the bore of a 1 T
spectrometer magnet, where decays are observed
(see Fig. 1.16).
The µ·B interaction of the neutron with magnetic fields produces an energy change of ±60
neV/T, depending on whether the neutron spin
is parallel or antiparallel to the applied field. The
extremely low kinetic energy of UCNs (below 180
neV for our experiment) makes possible a method
unique to UCNs for producing highly polarized
neutron samples. The µ · B interaction produces
a 420 neV barrier to neutrons with their spin
parallel to the field (the neutron’s magnetic moment is negative) in our 7 T polarizer magnet,
completely blocking the transmission of this spin
state through the high field system. There is
therefore an essentially 100% polarized sample
of UCNs after traversing this region. The UCNs
then pass through a high-power, radio-frequency
adiabatic spin-flipper, which permits the preparation of a UCN sample polarized either parallel
or antiparallel to the field. The spin-flipper region is then coupled by our guide system to a
decay volume within the uniform 1T field of the
β-spectrometer.
During the 2007 run period, we achieved an
average decay rate of 6.5 Hz for the 36 hour, preliminary measurement of the β-asymmetry. The
run cycle included 60 min. of β-decay measurement, followed by an approximately 4 min. measurement of the depolarized UCN fraction that
accumulated in our decay volume during the βdecay measurement, and about a 12 min. measurement of the background. The state of the
19
1.0 T Superconducting Cu decay MWPC Plastic
scintillator
solenoidal magnet volume
PMT
Be-coated
Cu guide
mylar foil
DLC-coated
quartz guide
UCN from
SD 2 source
7T Polarizer/AFP
Light guide
Spectrometer
Polarizer/AFP
Switcher
Gatevalve
PPM
Source
Figure 1.16: A schematic diagram of the UCNA polarizer and β-spectrometer. The inset depicts the
layout of the source and guides coupled to the experiment.
permit us to push our uncertainties below the 1%
level by the end of the 2008 run period, making
us competitive with the most precise published
measurements. In addition, we are now exploring the introduction of Si detectors in a geometry
permitting the detection of β-particles and recoil
protons in coincidence. Such a scheme should
permit the reduction of the polarization systematic error and the measurement of additional angular correlations, some of which are particularly
interesting in providing constraints on supersymmetric extensions to the standard model.
a)
0
0.5
200
400
b)
600
800
1000
Signal
Background
0.4
Rate (Hz / 50 keV)
UCN spin-flipper was then changed and the cycle repeated.
The UCNs were confined to a 12.5 cm diameter, 3 m long Cu decay trap, coaxial with a 1 T
uniform field at the center of the spectrometer.
The ends of the decay trap were sealed with Becoated, 2.5 µm Mylar windows. β-particles were
recorded in detector assemblies situated at either
end of the solenoidal spectrometer, where the
field has fallen to 0.6 T. Each detector assembly is
composed of a position-sensitive multi-wire proportional counter (MWPC) with 25 µm entrance
and exit windows, backed by a plastic scintillator to measure the full energy of the β-particles.
Backgrounds were reduced by requiring a coincidence between the scintillator and MWPC, and
a cosmic ray veto. Over 3 × 105 decays were
observed in our analysis window between 200
keV and 600 keV, yielding a β-spectrum in good
agreement with expectations. The ratio of signal
to background was 21:1 in the region of interest.
A critical advantage of UCNs is that, because
of the very small number of UCNs in the system
compared to the number in neutron beams (low
UCN velocity means relatively few UCNs are required for a given decay rate), and because of
the low capture probability per collision (UCNs
are bottled in our system), neutron-generated
backgrounds are negligible. This is one of the
most troublesome systematic errors for cold- and
thermal-neutron-beam experiments. Analysis of
these results is ongoing, but a total uncertainty of
under 5% is expected for the β-asymmetry from
this preliminary run. The results of the 2007 run
period are summarized in Fig. 1.17.
In the coming run period, we plan to substantially improve our statistics, to reduce the thickness of the foils on the ends of the decay trap and
the detector, and to improve on the current limits for the depolarized UCN fraction. This should
0.3
0.2
0.1
0.0
0
200
400
600
Energy (keV)
800
1000
Figure 1.17: Depicted in (a) is the measured
asymmetry parameter A(E) (with arbitrary units on the y-axis), demonstrating the expected very small energy dependence.
In (b) is the
measured β-decay spectrum (filled
points) together with the simulated
expected spectrum using the PENELOPE code. The open circles are the
background measured when UCNs
are not present in the decay volume.
A
view of the TUNL-ITEP double-beta decay setup from the door of the TUNL
connex trailer at the Kimballton Underground Research Facilty (KURF) near
Blacksburg, Virginia.
Neutrino Physics
Chapter 2
•
ββ Decay
22
Neutrino Physics
2.1
2.1.1
ββ Decay
Double–Electron Capture on 112 Sn to the Excited 1871 keV State
in 112 Cd – A Possible Alternative to Double–Beta Decay
M.F. Kidd, J.H. Esterline, W. Tornow, TUNL
We report the first use of a coincidence technique to study neutrinoless double–electron
capture (0νECEC) to an excited state in the daughter nucleus. We investigated 0νECEC by
112 Sn leading to the possibly degenerate 1871 keV excited state in 112 Cd by searching for its
de-excitation γ rays of 1253 keV and 618 keV in coincidence. After an exposure of 1.59 kg ×
days of 112 Sn, no decays were observed. From this null result we determine a lower limit for
the half-life time of T1/2 >2.7(1.3) × 1019 yr (68%(90%) CL). We hope to achieve a sensitivity
in the 1023 to 1024 yr range with a sample of a few kg of
in an underground facility.
112 Sn
and improved γ-ray detectors
In 1955, Winter pointed out [Win55] the transitions to excited final states in daughter nuunique role of neutrinoless double-electron cap- clei.
ture (0νECEC) to very special daughter nuclei
which have excited states degenerate with the
parent nucleus. Bernabeu, De Rujula, and Jarlskog [Ber83] later identified 0νECEC as a tool for
measuring the electron neutrino mass and cited
the 112 Sn →112 Cd decay as a candidate. They
found that the sensitivity of this decay to the
neutrino mass is comparable to that of neutrinoless double-beta (0νββ) decay. In 2007, Dawson
et al. [Daw08] reported a lower limit of T1/2 >1.6
× 1018 yr (90% CL) for the 0νECEC decay of
112
Sn in their search for the decay of the 1871 Figure 2.1: Level scheme of 112 Cd, the daughter
nucleus of the ECEC process of 112 Sn.
keV 3rd excited 0+ state in 112 Cd, which is de112
generate with the
Sn ground state within the
uncertainty in the atomic mass difference of 5
keV (assuming ECEC of K–shell electrons).
Figure 2.1 summarizes the parts of the level
Assuming that the decay of the 1871 keV scheme of 112 Cd which are of interest to the
state in 112 Cd is observed, and that it can be present work. We searched for the decay of the
shown that this state is not produced by cosmic- 0+
3 state located at 1871 keV by detecting 1253
ray induced reactions, a clear signature for the keV and 618 keV γ rays in coincidence. We re0νECEC reaction would be obtained. As a report our result obtained after 102 days of measult, the Majorana nature of the electron neu- surement. We placed 3.91 g of tin enriched to
trino would be established, and a value for the (99.5±0.2)% in 112 Sn between two high-purity
effective neutrino mass could be obtained, pro- germanium (HPGe) detectors. Considering the
vided the resonance enhancement can be reliably small natural abundance of 112 Sn (0.97%), this
calculated. These are the compelling arguments corresponds to 0.40 kg of natural tin. Our 112 Sn
for why the study of 0νECEC reactions is con- target consisted of two strips 12.2 cm and 7.5 cm
ceivably a viable alternative to 0νββ studies.
long, both with 0.05 cm thickness and 1.6 cm
Here we describe our search for the 0νECEC width. These strips were folded once and taped
reaction on 112 Sn using the TUNL-ITEP double- crosswise to the front face at the center of one of
beta decay apparatus [Hor06], which was orig- our HPGe detectors.
inally designed to measure 2νββ half-lives for
We measured the coincidence efficiency for
Neutrino Physics
23
Figure 2.2: (Color online) (a) Expanded view of energy region centered around 618 keV in Det. 1 and
1253 keV in Det. 2. (b) same as (a) but centered around 1253 keV in Det. 1 and 618 keV
in Det. 2.
this geometry following the procedure described
in [Kid08]. The total coincidence efficiency for
the γ-ray pair of interest using just our 112 Sn
target was found to be (0.85±0.04)%. In order to increase the amount of target material,
we surrounded the cylindrical surface of our two
HPGe detectors by 55 rods of natural tin (purity 99.999%), each 10 cm long and 0.6 cm in
diameter, resulting in a total mass of 15.7 g of
112
Sn. For this specific geometry, a total coincidence efficiency of γγ = (0.22±0.01)% was measured [Kid08]. Although this additional amount
of 112 Sn compares favorably to the 3.91 g of enriched material, this smaller efficiency results in
only a 5% increase in effective 112 Sn target mass
compared to just the enriched sample.
We first plot the energy of detector 2 versus the energy in detector 1. By then zooming
in onto the regions of interest, i.e., 618 keV in
Det. 1 and 1253 keV in Det. 2 and 1253 keV
in Det. 1 with 618 keV in Det. 2, we obtained
the two energy distributions shown in Fig. 2.2.
There are no events at the energy combinations
of interest. These spectra were used to determine a lower limit for the half-life of 112 Sn for
the 0νECEC to the excited 1871 keV state in
112
Cd using our measured coincidence efficiency.
From the 618 keV/1253 keV data we obtained the
value of T1/2 > 2.7(1.3) × 1019 yr (68(90)% CL).
This result is based on 1.59 kg × days of exposure and represents an improvement by a factor
of eight over the value reported in [Daw08]. In
contrast to [Daw08], our result obtained with the
coincidence technique is almost background free.
Background will become significant only once we
increase our measuring time or sample material
by at least two orders of magnitude. Unfortunately, a larger isotopically enriched 112 Sn sample
was not available for our studies, as the sample
used in the present work is to our knowledge the
largest isotopically enriched 112 Sn sample that
currently exists.
We are currently in the process of recommissioning our apparatus after it was moved to the
Kimballton mine in Virginia. There, with 1450
m.w.e. overburden, we are continuing our present
work on 112 Sn. To increase the total mass of
112
Sn we have sandwiched a disk of natural tin
with 10.7 cm diameter and 1.0 cm thickness in
addition to our 3.91 g of highly enriched 112 Sn.
Although this disk will attenuate any γ-ray pairs
emitted by the enriched sample and, in addition,
will reduce the coincidence efficiency due to the
increased separation of the two HPGe detectors,
the net effect will still provide an increase in effective 112 Sn mass by 15% over that of the enriched
sample alone.
[Ber83] J. Bernabeu, A. D. Rujula, and C. Jarlskog, Nucl. Phys., B223, 15 (1983).
[Daw08] J. Dawson et al., Nucl. Phys., A799,
167 (2008).
[Hor06] M. J. Hornish et al., Phys. Rev. C74,
044314 (2006).
[Kid08] M. F. Kidd, J. H. Esterline, and
W.Tornow, (2008), submitted to Nucl.
Instrum. Methods in Physics Research.
[Win55] R. G. Winter, Phys. Rev., 100, 142
(1955).
24
Neutrino Physics
2.1.2
Progress at Kimballton Underground Research Facility
M.F. Kidd, J.H. Esterline, W. Tornow, P. Finnerty, R. Henning, H.O. Back, TUNL; R.B.
Vogelaar, Virginia Polytechnic Institute and State University, Blacksburg, VA
We report the establishment of a remote laboratory at Kimballton Underground Research
Facility (KURF) in collaboration with Virginia Polytechnic Institute and State University.
Two types of low-background measurements will be made at KURF: materials assay, and
searches for two neutrino double beta (2νββ) decay and neutrinoless double electron capture
(0νECEC).
beta decay apparatus was previously operated
above ground in the TUNL Low Background
Counting Facility (LBCF), a shielded room in
the basement of the Duke Physics Department.
It is designed to measure 2νββ decay to excited
final states via the detection of coincident de–
excitation gamma rays [Hor06]. In particular,
150
Nd 2νββ decays to the first 0+ excited state
150
in
Sm with the subsequent emission of a 334
keV and a 407 keV γ ray in coincidence. Due
to contamination of our enriched 150 Nd source
with 232 Th decay products, the background was
too high in the regions of interest to extract a
competitive half-life limit for this particular decay while operating at ground level [Kid07]. The
source was sent back to Oak Ridge National Laboratory for purification. Once it is returned, we
will take data with it at KURF to see if this background reduction will allow for a half-life limit
measurement.
Counts per Second
The Kimballton mine is located about 25
miles northwest of Blacksburg, Virginia. It is an
operating limestone mine with over 50 miles of
roads and a current maximum depth of 2300 feet.
Our present facility is located at the 14th level at
a depth of 1700 feet. This depth corresponds to
about 1450 m water equivalent. The mine has
drive-in access, which allows for the installation
of self-contained laboratory modules.
Prior to fall 2007, practically no laboratory
infrastructure existed at the facility aside from a
concrete pad and a connex trailer belonging to
the Naval Research Laboratory (NRL). By the
end of October 2007, the concrete pad had been
sealed, and a laboratory structure had been constructed, with the NRL trailer installed inside
(see Fig. 2.3). In early January 2008, the TUNL
connex was transported from Duke and installed
at KURF (see Fig. 2.5). Finally, a 600 gallon
(2270 liter) liquid nitrogen tank was delivered to
the facility.
10-1
10-2
-3
10
10-4
Figure 2.3: (Color online) On the left can be seen
the laboratory area with only the concrete pad present. On the right is the
completed laboratory structure containing the TUNL (left side) and NRL
(right side) trailers.
For more details about the materials assay
work in the NRL trailer, see Sect. 9.5.1. Here we
report on the 2νββ and ECEC related activities.
The TUNL-ITEP (Alikhanov Institute of
Theoretical and Experimental Physics) double-
-5
10
-6
10
200
400
600
800
1000
1200
1400
1600
Energy (keV)
Figure 2.4: (Color online) Normalized data taken
on 112 Sn at ground level (upper,
black) and at KURF (lower, red).
The apparatus can also be applied to investigate 0νECEC in 112 Sn (see Sect. 2.1). In this
case, the regions of interest are 618 keV and 1253
Neutrino Physics
25
keV. Figure 2.4 shows normalized spectra taken
at the TUNL LBCF and at KURF with an enriched 112 Sn sample in place. The strong line
seen at 392 keV is due to the decay of 113 Sn
(T1/2 =115 days) which is a contaminant in our
sample, resulting from prior exposure to neutron
beams. The Compton scattering from this line
masks the reduction we would see at 334 keV,
but the background reduction factors for each of
our regions of interest above 392 keV can be seen
in Table 2.1. While many of the γ-ray lines in
Fig. 2.4 are intrinsic to our setup and thus not
reduced by moving underground, the reduction
of cosmic-ray background is apparent. The 511
keV peak is reduced by a factor of ten, and the
neutron capture by 73 Ge at 596 keV is no longer
distinct as can be seen in Fig. 2.6.
We are currently continuing the measurements reported in Sect. 2.1 with our enriched
112
Sn sample.
Table 2.1: Background Reduction Factors
Energy
(keV)
407
618
1253
Reduction
Factor
2.17
2.66
2.42
Isotope
of Interest
150
Nd
112
Sn
112
Sn
Counts per Second
Figure 2.5: (Color online) A view of the TUNL-ITEP double-beta decay setup from the door of the
TUNL connex trailer.
511 keV
609 keV
596 keV
10-4
450
500
550
600
650
Energy (keV)
Figure 2.6: (Color online) A closer look at
Fig. 2.4. The reduction of the 511 keV
γ–ray line and the asymmetric peak
from neutron capture on 73 Ge is evident.
[Hor06] M. J. Hornish et al., Phys. Rev. C74,
044314 (2006).
[Kid07] M. F. Kidd et al., TUNL Progress Report, XLVI, 22 (2007).
26
Neutrino Physics
2.1.3
Partial Cross Section for Neutron-Induced Reactions on Cu, Ge
and Pb at En = 8 and 12 MeV for 0νββ Background Studies
E. Kwan, J.H. Esterline, B. Fallin, C.R. Howell, A. Hutcheson, H.J. Karwowski, J.H.
Kelley, M.F. Kidd, A.P. Tonchev, W. Tornow, TUNL; D.M. Mei, University of South
Dakota, Vermillion, SD;
The verification of the existence of the 0νββ decay process is of interest to particle physics.
The next generation of the ββ decay experiment will search for the
76 Ge
0νββ decay. The
search for such decays in long-lived isotopes requires extensive understanding of possible
background sources. The partial cross sections for neutron induced γ-ray transitions in Pb,
Cu, and enriched
76 Ge
have been measured at TUNL using pulsed mono-energetic beams at
En = 8 and 12 MeV. The intensities of γ rays in the energy range of 400 to 4000 keV emitted
from the residual nuclei were measured using an array of HPGe detectors. No significant
counts above the detection limit at the 0νββ Q-value of 2039 keV were observed in the
76 Ge
results. The current measurements will serve as benchmarks for future statistical model
calculations.
Results from experiments on atmospheric
and solar neutrinos showed that neutrinos are
not massless as the Standard Model predicts
[MWP06]. The search for the existence of neutrinoless double-beta decay (0νββ) plays an important role in the understanding of physics beyond the Standard Model. A possible observation of such decay would prove that the neutrinos
are their own antiparticles and would violate the
conservation of the total lepton number [Ell04].
Thus 0νββ decay serves as a sensitive probe of
the mass and mixing of the neutrino [Moh99].
Only a handfull of nuclei are thought to decay
by this process. A nucleus that is predicted to
undergo such decay is 76 Ge. If this decay process
were to exist in 76 Ge, two electrons with the sum
energy equal to the Q-value of 2039 keV would be
emitted. The best half-life limits and constraints
on the Majorana mass have been established
using enriched germanium-diode detectors and
have provided the most restrictive constraints on
the effective neutrino mass [Aal99, Kla04]. Currently, an upper limit on the neutrino mass of
0.46 eV can be extracted from the half life of
76
Ge obtained by the Heidelberg-Moscow group,
τ1/2 ≥ 1.1 x 1025 y [Aal02, Moh99]. The next
generation of ββ decay experiments will focus on
the 0νββ decay of 76 Ge [Aal02]. Future experiments will aim to increase detection sensitivity
to 1 event/ton-year or 1027 y in order to explore the mass region indicated by atmospheric
neutrino oscillations [Mei08].
The rarity of an event due to double beta decay requires the ability to minimize the intrinsic
background induced from the interaction of ionization radiation with materials. Trace levels of
background radiation due to natural radioactive
isotopes in the shielding and detectors can deposit energy into detectors producing a peak or
a continuum in the γ-ray energy spectrum that
can obscure the double beta decay line of interest.
Neutrons interacting with the shield and detector
materials will also contribute to the background.
These background components need to be understood and reduced or eliminated as sources
of contamination.
Future experiments, such as the one proposed
by the Majorana collaboration, plan to use hundreds of kilograms of 76 Ge to detect the 0νββ
events and several centimeters thick Cu and Pb
for passive shielding [MWP06]. Neutrons interacting with these materials may result in γ-ray
peaks that can interfere with the identification
of the 76 Ge 0νββ decay line. These interfering
peaks include the 2041 keV line from a 1− to
1+ transition in 206 Pb and the 2040 keV doubleescape peak from the 3062 keV 5/2+ to 5/2−
transition in 207 Pb. Due to the lack of excitation functions for the (n, xnγ) reaction from
these materials in the nuclear database, the cross
sections for neutron induced γ-ray transitions in
enriched 76 Ge, nat Pb and Nat Cu have been measured at TUNL using a pulsed mono-energetic
neutron beam at En = 8 and 12 MeV. An array
Neutrino Physics
27
Figure 2.7: (Color on line) The γ-ray spectra around 2039 keV from nat Cu (top left), nat Pb (top right),
and 76 Ge (bottom). The lower spectra in the upper panels and the upper spectrum in the
lower panel (shown in green) correspond to En = 8 MeV. The remaining spectra (black)
are for En = 12 MeV. The straight lines represent the background in the measured spectra.
of three high purity germanium (HPGe) clover
detectors were used to measure the yields of γ
rays in the energy range of 400-4000 keV.
Figure 2.8: (Color on line) Comparison of the partial cross section of Eγ (208 Pb) = 2614
keV from the current work (black)
with those found in literature (blue).
The iron foils that enveloped the Cu, Ge,
and Pb were used to measure the neutron fluence. The neutron-induced γ-ray spectra around
2049 keV from nat Cu, nat Pb and 76 Ge are shown
in Fig. 2.7 for En = 8 and 12 MeV. Peaks attributed to transitions in Cu, Pb and Ge are labeled with their energies. No significant peaks at
2039 keV above the detection limit (144 counts
for Cu at 12 MeV) were observed. In the current work, the contributions from (n, n0 γ), (n, γ),
and/or (n, 2nγ) on 206−208 Pb could not be distinguished and are expected to contribute to the
γ-ray yields. The partial cross sections were obtained for the most significant peaks. The results
from the present work for the partial cross section for the first excited state of 208 Pb at Eγ =
2614.6 keV is compared with references in EXFOR in Fig 2.8. Currently, simulations using
the code mcnpx are being carried out to determine the γ-ray absorption in the Ge target and
the contribution to the Ge yields due to neutrons
scattering from the target into the detectors and
interacting with the crystals.
[Aal99]
C. E. Aalseth et al., Phys. Rev., C59,
2108 (1999).
[Aal02]
C. E. Aalseth et al., Phys. Rev., D65,
092007 (2002).
[Ell04]
S. R. Elliott and J. Engel, J. Phys.
G:Nucl. Part. Phys., 30, R183 (2004).
[Kla04]
H. V. Klapdor-Kleingrothaus et al.,
Phys. Lett., B586, 198 (2004).
[Mei08]
D. M. Mei et al., Phys. Rev., C77,
054614 (2008).
[Moh99]
R. Mohapatra, Nuc. Phys., A77, 376
(1999).
[MWP06] The
Majorana
Collaboration,
Technical report, (2006),
http:/majorana.pnl.gov/documents/
Majorana White%20Paper 2006Nov
22.pdf.
28
Neutrino Physics
2.1.4
Development of a 1.4 kg, Segmented Ge Ionization Detector Enriched to 85% 76 Ge for Underground Measurements of ββ Decay,
Axions and Other Forms of Dark Matter
M.W. Ahmed, H.O. Back, M. Boswell, R. Henning, J.D. Kephart, M.F. Kidd, L. Leviner,
W. Tornow, A.R. Young, TUNL
We present progress in the implementation of a segmented, enriched germanium detector
(the SEGA detector), analysis of segmentation and pulse-shape data to reconstruct singlesite events, and development of low background contacts.
The SEGA detector (Segmented Enriched
Germanium Assembly) is a 1.4 kg, N-type Ge
detector, with 6-fold azimuthal and 2-fold axial
segmentation. This detector is unique, in that it
is also enriched to 85% 76 Ge, making it the first
enriched, segmented Ge detector. The SEGA detector will serve as a possible prototype detector
element for the one-ton Majorana experiment,
designed to probe Majorana neutrino mass scales
down to roughly the 20 meV level. For more details on this experiment, please see the website:
http://majorana.npl.washington.edu.
The strategy to eliminate background in the
Majorana ββ decay experiment rests on four
primary techniques: the high resolution achievable with Ge diode technology, finely segmenting the detector, implementing pulse-shape analysis (PSA), and utilizing ultra-low radioactivebackground materials. The first of these relies
on the fact that neutrinoless ββ decay should
normally result in the deposition of a monoenergetic “line” at 2.04 MeV in our detector. Thus,
the intrinsically high resolution (nominally 0.2%)
of Ge detectors allows an excellent rejection of
most backgrounds present in a typical underground counting experiment. Segmentation and
PSA rely on the fact that ββ decay is a “singlesite” event, in that all of the detected energy released in the decay is carried by two β particles
with very small (less than 2 mm) range in the
Ge. Many background events, on the other hand,
are “multi-site” events, and can deposit energy
at several points in the crystal separated by centimeters or more. Perhaps the most important
example of these backgrounds comes from high
energy γ rays, which typically Compton scatter
one or more times before depositing their full energy in the crystal. In order to evaluate our ability to identify and reject γ-ray backgrounds with
the SEGA detector, we performed a series of measurements with 2 MeV and 3 MeV γ-ray beams
at the HIγS facility in September 2005.
Analysis of the HIγS data is essentially complete, with detailed simulations of the segmentation response in reasonable agreement with our
measurements (see, for example, Fig. 2.9). When
combined with our previous measurements of the
leakage currents, resolution for various different
incident γ-ray energies, peak energy vs. applied
bias, dead layers, and cross-talk, we have now
performed a thorough evaluation of the basic performance of SEGA and demonstrated that an enriched 76 Ge, segmented detector should serve as
an effective tool for next generation double betadecay measurements.
In the past year, the SEGA project was integrated into the responsibilities of the detector
working group of the Majorana collaboration. As
delineated in our 2006 report, SEGA will serve as
a testing ground for low-background contacts and
small parts for the segmented N-type Ge detector
option for Majorana. We plan to achieve this goal
in a two-step process, in which we first implement
our proposed low-background contact geometry
for SEGA in an intermediate, transfer cryostat,
to ensure adequate performance and reliability
for the contacts. We will then move SEGA into a
low background cryostat at WIPP. H. Back is coordinating component design and the integration
of SEGA into these two new hardware cryostat
configurations.
New engineering designs for the lowbackground contact assembly, developed by T.
Burritt at the University of Washington, incorporate the string configuration and cabling
planned for the Majorana demonstrator modules. Ultralow-background, electroformed copper and Kel-F plastic contacts are the structural
Neutrino Physics
Counts
Counts
700
600
500
400
300
200
100
0
700
600
500
400
300
200
100
0
29
S1 Simulated
500
1000
1500
2000
2500
3000
Energy (keV)
2000
2500
3000
Energy (keV)
S1 Measured
500
1000
1500
Figure 2.9: (Color online) Simulated and measured traces for segment S1. The HIγS beam is initially
incident on S1. The black trace is experimental data, and the blue trace is a GEANT4
(MaGe) simulation.
materials utilized, with the necessary mandrells
for electroforming of the larger Cu contact and
inner cryostat parts already fabricated and in the
queue to begin plating. In addition, a prototype
assembly is now complete, and will be reviewed
by ORTEC and TUNL personnel in the near
future.
Cryostat designs are being developed by H.
Back and J. Amsbaugh (University of Washington). The transfer cryostat and electronics are already mostly fabricated, with some design mod-
ifications to accomodate the new contact assembly and minimize noise underway. The low background cryostat is at an earlier stage of design, although the basic strategy is now established, utilizing the cold finger and Dewar of the MEGA detector and a new cryostat fabricated from OFHC
copper. Contact and cryostat development is
roughly on schedule, with the actual transfer to
the new contact assembly planned for later in
2008.
T
he Electron Cyclotron Resonance Source (ECR) at the Laboratory for Experimental Nuclear Astrophysics (LENA).
Nuclear Astrophysics
Chapter 3
•
Nucleosynthesis in AGB Stars
•
Explosive Nucleosynthesis
•
Cosmochronology
•
Reaction Rates
32
3.1
3.1.1
Nucleosynthesis in AGB Stars
Folding Potential and R-matrix Calculations for
16
O(p,γ)17 F
C. Iliadis, TUNL; C. Angulo, Université Catholique de Louvain, Louvain-la-Neuve, Belgium; M.
Lugaro, University of Utrecht, Netherlands; P. Descouvemont, Université Libre de Bruxelles,
Belgium; P. Mohr, Diakoniekrankenhaus Schwäbisch Hall
We evaluate existing nuclear cross section data for 16 O(p,γ)17 F that were obtained since 1958
and, if appropriate, correct published data for systematic errors that were not noticed previously. The data are interpreted by using different models of nuclear reactions, that is, a single
particle potential model and R-matrix theory. A new astrophysical S-factor is obtained that
is far more precise and accurate than previous results. Based on our work, the
16 O(p,γ)17 F
reaction now has the most precisely determined thermonuclear rates of any charged-particle
reactions in the A≥ 12 mass range. Our results have implications for the massive AGB star
origin of certain presolar grains.
The 16 O(p,γ)17 F reaction is characterized by
a number of exceptional attributes. At lower
bombarding energies it provides a textbook example of a nonresonant reaction cross section
since the lowest-lying resonance is located at the
relatively high laboratory energy of 2.66 MeV.
The absence of low-energy resonances and the
high binding energy of 16 O are among the main
reasons that the cross section can be described
in terms of simple nuclear reaction models. This
reaction has been studied many times at low energies and it is generally assumed that the different measurements are in agreement. For these
reasons it is surprising that the thermonuclear
reaction rates evaluated by the NACRE collaboration have relatively large errors (for example,
to ±35% at T=0.06-0.1 GK) [Ang99]. This issue is particularly important since recent studies
show that variations of the 16 O(p,γ)17 F reaction
rate sensitively influence the 17 O/16 O isotopic
ratio predicted by models of intermediate-mass
asymptotic giant branch (AGB) stars. Specifically, it has been demonstrated that fine-tuning
the 16 O(p,γ)17 F reaction rate may account for
the measured anomalous 17 O/16 O abundance ratio in the extraordinary presolar spinel grain OC2
[Lug07].
in the original papers to permit a straightforward correction. Fitting of the reaction model
to the data is performed only for the results of
Refs. [Cho75, Mor97] since these papers present
the cross sections for the individual transitions
to the ground and first excited state in 17 F. Our
S–factor extrapolation can then be tested using
the low–energy total cross section data of Ref.
[Hes58].
For our analysis we considered only the data
of Refs. [Cho75, Mor97, Hes58]. Other data
sets [Tan59, Rol73] had to be disregarded, mainly
because the yields had been converted to cross
sections using 40–year–old stopping power values, and not enough information was provided
The data are analyzed using two different
models, a direct capture model using folding potentials and an R–matrix approach. Preliminary
results indicate that these models give results
that are consistent with one another and with
the data. The statistical errors arising from the
It was necessary to correct all of the data sets
considered in the present work. First, we discovered that the original data of Ref. [Mor97] had
not been corrected for coincidence summing effects. These are particularly important for this
reaction since the main primary transition to the
first excited state gives rise to a secondary transition to the ground state. We performed these
corrections by using geant4 simulations of the
original setup. Second, considering all the error
sources discussed in Ref. [Cho75] we find that
the authors have underestimated their published
cross sections. We estimated more reliable errors by making reasonable assumptions based on
the details given in their paper. Third, we corrected the low-energy data of Ref. [Hes58] by
using modern stopping powers derived from the
code srim.
least–squares fitting amount to only a few percent. We added quadratically systematic errors
that arise from reasonable variations of model parameters (that is, the strength parameter for the
folding potential and the channel radius for the
R–matrix approach). Our results indicate that
the extrapolated S–factor has an error of about
9% at a center-of-mass energy of 90 keV, which is
located at the center of the energy region important for the hot bottom burning in AGB stars.
This factor-of-four reduction in the error compared to previous results will significantly improve certain predictions of AGB star models.
Integration of the new S-factor directly yields
the thermonuclear reaction rate for 16 O(p,γ)17 F.
In the astrophysically important temperature
range of T=0.01-2.5 GK, the rate uncertainties
now amount to about 7%, representing a fourfold improvement over previously published results from the NACRE collaboration. We used
this new rate in a model of a 6.5 M AGB star
of solar metallicity with the aim of predicting improved 17 O/16 O ratios during hot bottom burning. While 17 O/16 O ratios measured in certain
presolar grains could be reproduced within the
large uncertainties of previous rate predictions,
we now find that, with our much improved precision, the AGB model does not reproduce the
33
measurements. Consequently, there is no clear
evidence to date for any stellar grain origin from
massive AGB stars. Stellar model uncertainties,
such as different mixing prescriptions and mass
loss rates, still need to be carefully evaluated
in this context. For more information, see Ref.
[Ili08].
[Ang99] C. Angulo et al., Nucl. Phys., A656, 3
(1999).
[Cho75] H. C. Chow et al., Can. J. Phys., 53,
1672 (1975).
[Hes58] R. E. Hester et al., Phys. Rev., 111,
1604 (1958).
[Ili08]
C. Iliadis,
(2008).
Phys. Rev., C77, 045802
[Lug07] M. Lugaro et al., Astron. Astrophys.,
461, 657 (2007).
[Mor97] R. Morlock et al., Phys. Rev. Lett., 79,
3837 (1997).
[Rol73] C. Rolfs, Nucl. Phys., A217, 29 (1973).
[Tan59] N. Tanner,
(1959).
Phys. Rev., 114, 1060
34
3.1.2
22
Ne+α Reaction Rates
R. Longland, C. Iliadis, C. Ugalde, TUNL
Reaction rates for the s-process neutron-producing
reaction,
22 Ne(α,γ)26 Mg
22 Ne(α,n)25 Mg
reaction and its competing
have been recalculated using a Monte Carlo uncertainty propagation
code. A reanalysis of existing resonance data has been performed to obtain the most realistic
rates for these reactions. These rates can now be used in stellar nucleosynthesis studies.
22
25
The Ne(α,n) Mg neutron source is an important reaction for the weak and strong sprocess components in massive stars and asymptotic giant branch (AGB) stars, respectively. In
massive stars, the flux of neutrons available during the He-shell burning stage helps us understand the yield from the weak s-process. In
AGB stars, the main source of neutrons is the
13
C(α,n)16 O reaction. However, during the thermal pulse in these stars, the 22 Ne source produces a brief burst of neutrons, which affects the
branchings in the s-process path. Knowing the
reaction rates of the 22 Ne α-particle capture reactions with good precision helps us in our understanding of the internal structure of these stars.
The 22 Ne(α,n)25 Mg and 22 Ne(α,γ)26 Mg reactions proceed through the same 26 Mg compound nucleus at an excitation energy of approximately Ex ∼ 11 MeV. At this excitation energy,
the level density of the nucleus is relatively high,
leading to about 57 observed states in the energy region important to s-process nucleosynthesis. Of these states, only 5 have been observed directly through α-capture reactions, while the others have been observed through other reactions,
particularly neutron capture on 25 Mg [Koe02].
In the present work, a complete reevaluation of
the reaction rates and their uncertainties using
a Monte Carlo error propagation code [Lon07] is
summarized.
This helped to reduce reaction rate uncertainties at low temperatures considerably. There is
an unexplained energy ambiguity between the direct measurements and neutron capture measurements. In this work, we have attempted to match
the states as well as possible, but it is clear that
more experimental data are needed to correct this
problem. The uncertainties introduced in this
level-matching are not reflected in the plots below, as there is no clear method for obtaining
them.
1
Uncertainty Ratio
Introduction
0.1
0.01
0.01
0.1
1
Temperature (GK)
10
Figure 3.1: (Color online) Normalized reaction
rate uncertainties (68%) for the
22 Ne(α,γ)26 Mg reaction.
Whenever possible, the resonances have been
integrated numerically. This method accounts for
Resonance Parameters
the contributions of the resonance tails to the reThis work includes analysis of upper-limit action rates at low temperatures. For many inα-particle partial widths from direct measure- termediate states in the 26 Mg nucleus, the γ-ray
ments [Jae01, Wol89]. Upper limits are cal- partial width is not well known. We have folculated assuming that reduced widths are dis- lowed the suggestion of Ref. [Koe02] and used
tributed according to a Porter-Thomas distribu- the average width measured in this region, which
tion. New spectroscopic factors from α-particle is Γγ = 3 eV. Using this value gives us a good
transfer measurements performed using a 6 Li starting point for comparison with previous work.
beam [Uga07] have been used in this analysis. The lowest measured resonances in both of these
reactions are at Er ≈ 830 keV, and they have
widely been considered to be the same state. As
pointed out in Ref. [Koe02], in order for these to
be the same state, it would require an extremely
large value of Γγ . For this analysis, the two states
have been treated as separate. Additionally, the
state in the 22 Ne(α,n)25 Mg reaction has been assigned J π = 2+ in Ref. [Jae01], although it is not
clear where this assignment comes from. There
are no 2+ states in this excitation energy region
in the compound 26 Mg nucleus, so we can only
assume that this assignment is erroneous. Uncertainties due to this quantum number assignment
have not been calculated, but could contribute
significantly to the total reaction rate uncertainties.
22
Ne(α,γ)26 Mg Reaction Results
The reaction rate uncertainties for the
Ne(α,γ)26 Mg reaction obtained through this
reanalysis are shown in Fig. 3.1. We can see that
there are large uncertainties between T = 0.03
and 0.1 GK. These are partly due to the uncertainties in the measured resonances in this region.
The main reason for these, however, is the large
uncertainty that comes from fitting the spectroscopic factors of states below the neutron threshold, which have been observed in transfer reactions [Uga07]. More experimental information on
these states is clearly needed.
22
22
35
Ne(α,n)25 Mg Reaction Results
As can be seen from Fig. 3.2, the largest uncertainties in the 22 Ne(α,n)25 Mg reaction exist
between T = 0.03 and 0.3 GK. These uncertainties come mainly from uncertainties in the low
energy resonances that have significant resonance
strength and width uncertainties. Uncertainties
due to the systematic energy shift mentioned earlier are not present in this plot and could contribute significantly to the total uncertainties.
Summary
The reaction rate uncertainties for the
Ne(α,n)25 Mg s-process neutron source and the
competing 22 Ne(α,γ)26 Mg reaction have been
reevaluated using a Monte Carlo error propagation code. Unlike previous evaluations, this
study has included upper limits on unobserved
resonances in a physical way, by sampling from a
Porter-Thomas distribution of possible values for
the reduced width. Significant uncertainties that
could not be calculated using this method exist
in resonance parameters in the currently available data. These include an energy ambiguity
between directly measured resonances and neutron capture data that could produce significant
uncertainties. In order to improve the uncertainties in these reaction rates, direct measurements
of both reactions should be performed in the full
energy range of Er = 600 − 1500 keV.
22
[Jae01] M. Jaeger et al., Phys. Rev. Lett., 87,
202501 (2001).
2
Uncertainty Ratio
1.5
[Koe02] P. E. Koehler, Phys. Rev., C66, 055805
(2002).
1
0.8
0.6
[Lon07] R. Longland and C. Iliaids, TUNL
Progress Report, XLVI, 36 (2007).
0.4
0.01
0.1
1
Temperature (GK)
10
Figure 3.2: (Color online) Normalized reaction
rate uncertainties (68%) for the
22 Ne(α,n)25 Mg reaction.
[Uga07] C. Ugalde et al.,
025802 (2007).
Phys. Rev., C76,
[Wol89] K. Wolke et al., Z. Phys. A, 334, 491
(1989).
36
3.1.3
Reaction Rate Uncertainties and
dust
26
Al in AGB Silicon Carbide Star-
M.A. van Raai, M. Lugaro, University of Utrecht, Netherlands; A.I. Karakas, Mt. Stromlo
Observatory, Austrailia; C. Iliadis, TUNL
We show that large uncertainties in the 26 Al+p reaction rate have important implications
when comparing AGB star models to the 26 Al/27 Al ratios observed in stardust mainstream
SiC grains. We present two scenarios to explain the observed distribution: one involves using
the lower limit of the rate and invoking a relatively long time scale of grain formation, possibly
connected to processes occurring in binary systems. The other involves using the upper limit
of the rate and invoking extra-mixing processes.
Stardust is a class of presolar grains each of
which presents an ideally uncontaminated stellar sample. Mainstream silicon carbide stardust
is formed in the extended envelopes of carbonrich asymptotic giant branch (AGB) stars and
contains the radioactive nucleus 26 Al as a trace
element.
measured in SiC grains. For the upper limit
of the 26 Al(p,γ)27 Si reaction rate, the predicted
26
Al/27 Al ratios are instead about 100 times
lower and lie below the range observed in SiC
grains. When considering models of different
masses and metallicities, the spread of more than
an order of magnitude in the 26 Al/27 Al ratios
measured in stellar SiC grains is not reproduced.
The aim of our work is to analyze in detail the
We propose two scenarios to explain the
effect of nuclear uncertainties, in particular the
spread of the 26 Al/27 Al ratios observed in mainlarge uncertainties of up to 4 orders of magnitude
stream SiC grains, depending on the choice of
related to the 26 Al(p,γ)27 Si reaction rate, on the
the 26 Al+p reaction rate. One involves different
26
production of Al in AGB stars and to compare
times of stardust formation, the other involves
model predictions with data obtained from labextra-mixing processes. Stronger conclusions on
oratory analysis of SiC stardust grains. To this
the interpretation of the Al composition of AGB
end, we use a detailed nucleosynthesis postprostardust will be possible after more information is
26
27
cessing code to calculate the Al/ Al ratios at
available from future experiments on the 26 Al+p
the surface of AGB stars of different masses (bereaction. For more information on this work, see
tween 1.75 and 5 M ) and metallicities (Z=0.02,
Ref. [vR07].
0.012 and 0.008). For the lower limit and rec26
27
ommended value of the Al(p,γ) Si reaction
rate, the predicted 26 Al/27 Al ratios replicate the [vR07] M. A. van Raai et al., Astron. Astrophys.,
478, 521 (2007).
upper values of the range of 26 Al/27 Al ratios
38
3.2
3.2.1
Explosive Nucleosynthesis
New Simulations of the Classical Nova Outburst
S. Starrfield, F. Timmes, Arizona State University, Tempe, AZ ; C. Iliadis, TUNL; W.R. Hix,
Oak Ridge National Laboratory, Oak Ridge, TN ; W.M. Sparks, Los Alamos National Laboratory,
Los Alamos, NM
We investigate the effects that improving the nuclear reaction library have on our simulations
of classical nova explosions. We find that the changes in the rates have affected the nucleosynthesis predictions of our calculations and, to a lesser extent, the gross features of the
evolution. For example, with the new rate library the ejected abundances of both 26 Al and
27 Al increase, while the abundance of 22 Na declines from the values predicted in our earlier
work. These results influence the prospects for observing γ-ray emission from classical novae
with present and future space-based detectors.
Nova explosions occur on the white dwarf
(WD) component of a cataclysmic variable binary stellar system in which the WD is accreting matter lost by a companion. When sufficient
material has been accreted by the WD, a thermonuclear runaway (TNR) occurs and ejects material in what is observed as a classical nova explosion. We have continued our studies of TNRs
on WDs under conditions which produce both
mass ejection and a rapid increase in the emitted
light. The studies involve examining the effects
of changes in the nuclear reaction rates on the
gross features and on the nucleosynthesis during
the outburst for two different WD masses (1.25
and 1.35 M ).
In order to improve our calculations over previous work, we have incorporated a modern nuclear reaction network into our one-dimensional,
fully implicit, hydrodynamic computer code. We
find that the updates in the nuclear reaction rate
libraries change the amount of ejected mass, the
peak luminosity, and the resulting nucleosynthesis. Because the evolutionary sequences on the
1.35 M WD reach higher temperatures, the effects of library changes are more important for
this mass. As a byproduct of this change, we discovered that the pep reaction (p+e− +p → d+ν)
was not included in our previous studies of CN
explosions (or, to the best of our knowledge, in
those of other investigators). Although the energy production from this reaction is not important in the sun, the densities in WD envelopes
can exceed 104 g/cm3 , and the presence of this reaction increases the energy generation during the
time that the pp chain is operating. The effect
of this increase is to reduce the evolution time to
the peak of the TNR and, thereby, to reduce the
accreted mass as compared to the evolutionary
sequences calculated without including this reaction. This work has been submitted recently for
publication.
3.2.2
39
X-Ray Burst Nucleosynthesis Sensitivity Studies
C. Iliadis, TUNL; A. Parikh, J. Josè, F. Moreno, EUETIB, Barcelona, Spain
We study, for the first time, the sensitivity of x-ray burst nucleosynthesis to current uncertainties in reaction rates. To this end, about 50,000 post-processing calculations are performed
with a nuclear reaction network containing about 600 nuclides and more than 3500 nuclear
interactions.
Type I x-ray bursts are violent stellar events
that take place in the H/He-rich envelopes of accreting neutron stars. We investigate the role
played by uncertainties in nuclear processes on
the nucleosynthesis accompanying these explosive phenomena. Two different approaches are
adopted in the framework of post-processing calculations. In the first, nuclear rates are varied individually within uncertainties. Ten different models, covering the characteristic parameter
space for these stellar events, are considered. The
second and somewhat complementary approach
involves a Monte Carlo code in which all nuclear
rates are randomly (and simultaneously) varied
within uncertainty limits. In all, we performed
about 50,000 post-processing calculations, with
a network containing 606 nuclides (1 H to 113 Xe)
and more than 3500 nuclear processes.
Several results are worth noting. First, the
total number of reactions affecting the energy
output for any model is small. Second, some
of those reactions are known with better precision than a factor of 10. Third, there is no way
to overcome this problem in the context of postprocessing calculations. Indeed, a self-consistent
analysis with a hydrodynamic code capable of
self-adjusting both the temperature and the density of the stellar envelope seems mandatory to
address this issue for the few reactions of con-
cern. And fourth, the presence of reactions affecting the energy output is particularly dangerous within a Monte Carlo context, as the results
rely on the simultaneous variation of all rates,
whereas most of the results obtained from an individual reaction-rate variation approach remain
unaffected. It is also worth mentioning that varying the rates of the triple-α reaction or any βdecay rate within realistic uncertainty limits has
no effect on either the yields or on the nuclear
energy output.
In summary, we identify a limited number
of reactions that play a significant role in x-ray
burst nucleosynthesis studies. Indeed, our results
can help to guide and motivate future measurements by experimental nuclear physicists. Finally, we compare the two general approaches
to the sensitivity study: individual reaction-rate
variations, and simultaneous variation of all rates
through Monte Carlo techniques. The highly
coupled environment characteristic of an x-ray
burst provides the opportunity to test for the
effects of correlations in the uncertainties of reaction rates. We find similar results from both
approaches, with minor differences attributed to
such correlation effects. Our work (including detailed results in tabular format) has been accepted recently for publication.
40
3.3
3.3.1
Cosmochronology
The Astrophysical 187 Re/187 Os Ratio: Measurement of the
2nγ)186m Re Destruction Cross section
187
Re(n,
E.J. Pooser, North Georgia College and State University, Dahlonega, GA; A. Hutcheson, H.J.
Karwowski, J.H. Kelley, E. Kwan, S.L. Hammond, C. Huibregtse, A.P. Tonchev, W.
Tornow, TUNL; F.G. Kondev S. Zhu Argonne National Laboratory, Argonne, IL
We are continuing a program to measure (n,2n) reaction cross sections on 187 Re with an
emphasis on population of the 186m Re isomeric state (T1/2 =2 × 105 y). Using quasi-monoenergetic neutron beams from the TUNL tandem van de Graaff accelerator, we will measure
the 187 Re(n,2n)186,186m Re cross sections in the range of En = Ethresh. − 16 MeV. We plan
to carry out measurements using traditional activation techniques in which 186 Re β-decay is
observed, as well as (n,2nγ) measurements where γ-ray transitions that populate
observed. These data will reduce uncertainties in the
The half-life of 187 Re (4.35 × 1010 years) is
comparable to the age of the universe, making the Re/Os chronometer well suited for dating cosmological events such as the formation of
galaxies and r-process nucleosynthesis [Luc80].
The Re/Os pair is particularly well suited for
use as a cosmochronometer because 187 Re and
187
Os are produced by independent nucleosynthesis processes. 187 Re is formed in the r-process,
which can be attributed to supernovae explosions
in early galactic formation. On the other hand
187
Os is mainly formed by the s-process, and it
is shielded from population via the r-process by
the long-lived 187 Re isotope. Therefore, our understanding of the r- and s-processes raises the
credibility for using the 187 Re/187 Os system as
an r-process cosmochronometer.
187 Re/187 Os
186m Re
are
cosmochronometer.
bution to the 187 Re abundance is negligible.
The reliability of this cosmochronometer rests
on two assumptions. First, the s-process is understood and the 187 Os abundance can be deduced from the 186 Os abundance. Second, the
increase in the 187 Os abundance when compared
with the expectation is the result of 187 Re decay.
As a result of these assumptions, the ratio of the
excess 187 Os to the 187 Re abundance in cosmological samples such as meteorites can be used to
date the period of 187 Re decay and, therefore, the
time since r-process nucleosynthesis occurred.
More subtle effects that must be accounted
for in using the cosmochronometer have also
been evaluated. Measurements have shown a
strong modification of the 187 Re lifetime depending on atomic charge state. As the atomic charge
There has been significant activity in under- state increases to full ionization, the lifetime
10
standing potential systematic problems associ- decreases from 4.35 × 10 years to 33 years
ated with the Re/Os chronometer pair. Accu- [Bos96]. This effect is relevant in stellar inrate measurements of the (n,γ) capture cross teriors; however it is unlikely that the metesections on the Os isotopes have been carried orites being evaluated by this cosmochronomeout [Mos07]. These cross-section measurements ter would have been exposed to the extreme conare critical for determining the s-process popu- ditions of a stellar interior after their formation
187
Os
lations of 186 Os, which is shielded from the r- [Hay05]. Another possible correction to the
process by 186 W (stable); of 187 Os, which is fed s-process abundance is due to neutron capture reby 187 Re, thus forming the cosmochronometer; actions on its thermally excitable nuclear state at
and of 188 Os, which is populated in the s-process 9.8 keV. Therefore, neutron capture reactions for
187
Osg.s. (n, γ)188 Os and 187 Os9.8keV (n, γ)188 Os
by neutron capture on 187 Os (see Fig. 3.3). Uncertainty still remains in the s-process population [Seg05] must be included in the s-process reacof 187 Re from neutron capture on 186 Re. How- tion network code to estimate the s-process abunever, since the half-life of 186 Re is only 3.4 days, dances.
it is commonly assumed that its s-process contriThe primary aspect of the Re/Os cos-
41
Os 184
Os 185
Os 186
Os 187
Os 188
Os 189
Os 190
Os 191
Os 192
stable
94 d
stable
stable
stable
stable
stable
15 d
stable
Re 183
Re 184
Re 185
Re 186
Re 187
Re 188
Re 189
Re 190
70 d
38 d
stable
3.7 d
4e10 y
17 h
24 h
3 min
W 182
W 183
W 184
W 185
W 186
W 187
W 188
stable
stable
stable
75 d
stable
24 h
69 d
S-process
Figure 3.3: Nuclear landscape in the vicinity of
mochronometer that we want to focus on is
reactions that convert 187 Re into nuclei other
than 187 Os. In environments with energetic
neutrons the (n,2n) reaction must be properly taken into account. The cross section for
187
Re(n,2n)186 Reg.s. reaction can be measured
using straightforward activation techniques, since
the half-life of 186 Reg.s. is 3.7 days. However,
the long and poorly known half-life of 186m Re,
T1/2 = 2 × 105 years, complicates measurements
of this part of the 187 Re(n,2n) destruction cross
section. Surprisingly, only two measurements
of these cross sections appear in the literature.
Measurements at En =14.8 MeV [Yam94] indicate σ=445(156) mb for the (n,2n) reaction to
186m
Re, and ENDF reports a measurement of
σ ≈ 2.2 b for the (n,2n) reaction to 186 Reg.s. .
Both of these measurements were carried out using activation techniques which depend directly
on the accuracy of the half-lives of 186,186m Re
(3.7186(5) days and ≈ 2×105 years, respectively)
In our previous experiment we carried out
a measurement of the 187 Re(n,2nγ)186m Re reaction using a 12 MeV pulsed and quasimono-energetic neutron beam (flux ≈ 104
neutrons/(cm2 s)). Analysis yielded evidence for
the population of the isomeric state, via observation of a 144 keV transition that is tentatively
thought to feed the isomer. While positive results were observed, uncertainties arising from
the target composition and geometry have prevented meaningful results from being deduced.
We have already obtained new Re target foils
and have plans for a next cycle of measurements
that will provide more meaningful results for the
187
Re(n,2nγ)186m Re cross section. We also plan
to use a pair of high-purity planar germanium
187 Re
and
187 Os.
detectors that will provide γγ coincidences, possibly revealing the level structure above the isomeric state.
In an experiment planned for July 2008 we
will neutron activate a set of nine natural Re
targets with a 12 MeV neutron beam. Because
this measurement will emphasize results from activation, we will reconfigure the target so that a
flux near 108 neutrons/(cm2 s) will irradiate the
target. Past experience has revealed issues with
thermal neutron capture, so the target package
will be wrapped in a Cadmium screen for thermal neutron suppression. The targets will be irradiated for five days, and will then be arranged
into a three by three matrix, for counting by a
high-purity germanium detector. Over the first
several months we expect to be most sensitive to
decays related to the 187 Re(n,2n)186 Reg.s. reaction; later we hope to observe decays related to
187
Re(n,2n)186m Re activation.
[Bos96] F. Bosch et al., Phys. Rev. Lett., 77,
5190 (1996).
[Hay05] T. Hayakawa et al., Astrophys. J., 628,
533 (2005).
[Luc80] J. M. Luck et al., Nature, 283, 256
(1980).
[Mos07] M. Mosconi et al., Prog. in Part. and
Nucl. Phys., 59, 165 (2007).
[Seg05] M. Segawa et al., Nucl. Phys., A758,
553c (2005).
[Yam94] N. Yamamuro, Nucl. Sci. Eng., 118,
249 (1994).
42
3.4
3.4.1
Reaction Rates
Matching of Experimental Thermonuclear Reaction Rates to Statistical Model Results at High Temperatures
J.R. Newton, C. Iliadis, R. Longland, TUNL
The Gamow-peak concept, used for the extrapolation of reaction rates toward higher temperatures, breaks down as an approximation of narrow resonant reaction rates at high temperatures. A new method of matching statistical models to experimental reaction rates is
presented. The definition of a new effective thermonculear energy region (ETER) is used to
define a temperature at which the statistical model calculations will be normalized to the
experimental reaction rate contributions. This is generally much higher than that currently
calculated using the Gamow peak.
Fraction of total rate
0.4
(a)
ETER
0.3
Median + ETER
0.2
exp
E max
0.1
0
0
500
1000
1500
2000
1.2
Cumulative fractions
Knowledge of thermonuclear reaction rates at
high temperatures is necessary for the modeling
of advanced stellar burning, supernovae, and xray bursts. These high temperatures often extend into ranges where the experimentally measured energy regions have been exhausted. The
discussion here revolves around particle-capture
reactions where the major contribution to the total reaction rate will come from a sum of the individual narrow resonances. In any experiment
designed to measure these resonances there will
always be a maximum energy resonance which
can be experimentally measured. This may be
due to accelerator limitations or to the fact that
the energy levels in the compound nucleus are
starting to overlap and individual resonances can
no longer be resolved. The existence of this cutoff will not affect the reaction rates calculated for
low temperatures, but at high enough temperatures, the unmeasurable resonances will begin to
contribute to the reaction rate, so that the experimental data are not sufficient. It is necessary
to determine the cutoff temperature above which
the measured resonances are no longer sufficient
for rate calculations. The current prescription
for this is defined using the Gamow peak. It is
important to remember that the Gamow peak is
strictly defined for nonresonant reactions but is
often used as an approximation for the thermonuclear energy-burning region of a reaction which
is dominated by narrow resonances. This is discussed in detail in [New07].
92%
(b)
ETER
0.8
50%
0.4
Median + ETER
8%
0
0
500
1000
1500
2000
E (keV)
Figure 3.4: (Color online) (a) Fractional contributions of narrow resonances to the total
reaction rate at a given temperautre,
T . The highest-lying resonance corresponds to the experimental cutoff enexp
ergy, Emax
; (b) Cumulative distribution of fractional resonant rates; the
8th , 50th and 92nd percentiles define
the ETER (see text).
Given a maximum resonance energy, a match-
ing temperature can now be defined using the
Gamow peak. This will be the temperature beyond which the experimental range no longer covers a significant enough portion of the experimental region and must therefore be described by a
statistical model. The matching temperature can
be found analytically by:
exp
GP
GP
Emax
= E0 (Tmatch
) + n∆E0 (Tmatch
),
(3.1)
where E0 is the position of the Gamow peak and
∆E0 is the 1/e width of the Gaussian approximation of the Gamow peak [Ili07]. The quantity
n is an empirical parameter which is usually chosen to be 1. At T > Tmatch , a significant portion of the Gamow peak lies above the maximum
experimentally measured resonance energy and
statistical models are now necessary. These statistical model rates should then be normalized to
the experimental rates at T = Tmatch .
It has been shown in detail that the Gamow
peak is no longer a proper description of the effective thermonuclear energy region at high temperatures [New07]. These are exactly the temperatures that result when calculating Tmatch
using the Gamow peak. This requires a new
definition of the effective thermonuclear energy
range (ETER) which describes the actual statistical rate distribution. The ETER is obtained
by computing the cumulative distribution of the
fractional resonant rate contributions. The 50th
percentile, or median, of this distribution is defined as the position of the ETER. The width
of the ETER is the energy range spanned by the
8th through 92nd percentiles. This is displayed in
Fig. 3.4. The width then covers 84% of the total
rate, in analogy with the width of the Gamow
peak. The matching temperature can be redefined as:
exp
ET ER
ET ER
Emax
= E 0 (Tmatch
) + n∆E 0 (Tmatch
)
(3.2)
A large discrepancy in matching temperature
between the two methods was seen in a vast
majority of the 21 reactions studied, including
(p, γ), (α, γ) and (p, α) reactions. A significant
change in matching temperature may then lead
to a large variation in the final calculation of the
reaction rate, since the statistical model rates are
normalized to the experimental rates at a significantly different temperature. The temperature
dependence of the two reaction-rate curves will
determine the extent of the variations in the final reaction rate.
The reaction rates calculated using the ETER
are more reliable for two important reasons.
43
First, the ETER is defined by the actual distribution of reaction-rate contributions, as opposed
to the Gamow peak which is an approximation
that has been shown to break down at high temperatures. Also, the statistical model is known
to be more accurate as the temperature and density of states increases, making a higher matching
temperature more reliable.
25
8000
26
Al(p,γ ) Si
Reaction Rate
6000
4000
2000
0
2
Figure 3.5:
4
6
Temperature (GK)
8
10
(Color online) Reaction rates of
calculated using the
Gamow peak method (upper, black)
and the ETER method (lower, red)
of determining the matching temperature. The dashed curves give the corresponding error bands. Resonance
strengths are taken from [End90,
End98] and Hauser-Feshbach calculations are obtained using the code
NONSMOKER [Rau00].
25 Al(p,γ)26 Si
Figure 3.5 illustrates the difference in final rate calculations using the ETER and the
Gamow peak methods of calculating the matching temperature for normalizing to the HauserFeshbach model. It is clear that there is a large
discrepancy between the two calculations. A paper describing this work has been recently submitted for publication.
[End90] P. M. Endt,
(1990).
Nucl. Phys., A521, 1
[End98] P. M. Endt,
(1998).
[Ili07]
C. Iliadis, Nuclear Physics of Stars,
Wiley-VCH, 2007.
[New07] J. R. Newton et al., Phys. Rev., C75,
045801 (2007).
[Rau00] T. Rauscher and F. Thielmann, At.
Data Nucl. Data Tables, 75, 1 (2000).
44
3.4.2
A New Reaction-Rate Evaluation for Proton-Induced Reactions on
A=16–40 Nuclei
C. Iliadis, A.E. Champagne, C. Ugalde, R. Longland, J.R. Newton, TUNL; A. Coc, Université de Paris-Sud, Orsay, France; R. Fitzgerald, National Institute of Standards and Technology, Gaithersburg, MD
We briefly describe a new effort at TUNL to evaluate thermonuclear reaction rates for A=16–
40 target nuclei. Special emphasis is placed on reliability and the presentation of meaningful
uncertainties. This database will be crucial for improving theoretical models of stellar objects
(and events), such as red giants, AGB stars, classical novae, supernovae and x-ray bursts.
Charged-particle reaction rate evaluations
represent the backbone of large-scale stellar network calculations, which are indispensable for
building realistic stellar models. Such evaluations were periodically provided by Fowler
and collaborators and culminated in the results
(widely adopted at the time) of Ref. [Cau88],
covering the mass range of A=1–30. A decade
later, a European effort (the NACRE collaboration [Ang99]) published an updated chargedparticle reaction rate evaluation for the A=1–28
mass range. The NACRE effort included a number of novel improvements, among them the first
systematic presentation of reaction rate errors.
Two years later we published a new rate evaluation with a slightly different focus. Our main
interest at the time was proton-induced reaction
rates on A=20–40 target nuclei [Ili01]. We also
derived reaction rate errors, as in the NACRE
work, and presented for the first time evaluated
rates for unstable target nuclei.
have been frequently obtained by questionable
methods. A major step forward in this direction
occurred with the publication of Ref. [Tho99],
which described, for the first time, a statistically
sound technique for calculating uncertainties if
the total rate is mainly determined by narrow resonances and non-resonant processes. However,
the method presented there was analytical in nature and, because of a number of approximations,
frequently does not provide reliable results. This
is especially the case if the error in the resonance
energy is large (say, ≥10 keV). Thus we began
work on a code that utilizes Monte Carlo techniques for calculating meaningful rate uncertainties. So far, we have evaluated the rates of about
25 reactions with the new Monte Carlo method.
We plan to submit our evaluation for publication by the end of 2008 and, shortly afterward,
to make the reaction rates available to the community for use in stellar model calculations.
In the six years since the publication of our
reaction rate evaluation, there have been many
new measurements of astrophysically important
quantities. Thus we began work recently on a
new evaluation of reaction rates on stable and unstable target nuclei, expanding the target range
to A=16–40.
In our new evaluation effort we have significantly improved the determination of reaction
rate uncertainties. In the past, such uncertainties
[Ang99] C. Angulo et al., Nucl. Phys., A656, 3
(1999).
[Cau88] G. R. Caughlan and W. A. Fowler, At.
Data Nucl. Data Tables, 40, 283 (1988).
[Ili01]
C. Iliadis et al., Astrophys. J. Suppl.
Ser., 134, 151 (2001).
[Tho99] W. J. Thompson and C. Iliadis, Nucl.
Phys., A647, 259 (1999).
The polarized
3
He experimental setup at DFELL inside the HIγS target room
used in measurements of the Gerasimov-Drell-Hearn (GDH) sum rule.
Sub-Nucleonic Degrees of Freedom
Chapter 4
•
Compton Scattering
•
Nucleon Spin Structure
48
4.1
Compton Scattering
4.1.1
Compton@MaxLab Collaboration
S. Stave, M.W. Ahmed, S.S. Henshaw, H.R. Weller, TUNL; G. Feldman, George Washington University, Washington, DC ; M. Kovash, University of Kentucky, Lexington, KY ; A. Nathan,
L. Myers, University of Illinois, Champagne, IL; Lund Collaboration, Lund University, Lund,
Sweden;
The Compton@MaxLab collaboration is committed to making measurements of the Compton
scattering cross sections from the deuteron as a function of energy and angle below 120 MeV.
These data will be used to determine the isoscalar electric (α) and magnetic (β) polarizabilities
of the nucleon. During the running period in late 2007, Compton-scattering data at 60 ◦ , 120◦ ,
and 150◦ were collected at Max-lab for about 250 hours of beam on a liquid deuterium target
with tagged photons in the energy range from 66 to 97 MeV. Significant improvements in
the intensity of the photon beam at Max-lab, made prior to the late 2007 run, allowed the
desired statistics to be acquired. Data taking began for the next photon energy range (95
to 115 MeV) in June 2008, but due to a change in the photon tagger, the accuracy of these
data will be statistics-limited until the next run, which is tentatively scheduled for December
2008.
The Compton@MaxLab collaboration is committed to making measurements of the Compton scattering cross sections from the deuteron as
a function of energy and angle below 120 MeV.
These data will be used to determine the isoscalar
electric (α) and magnetic (β) polarizabilities of
the nucleon.
The collaboration continued collecting data
on deuterium at Max-lab in November 2007.
Scattered photons were detected with the three
NaI spectrometers, Cats, Buni and Diana using FERA and CAMAC electronics and a root
based data acquisition system. The detectors
were placed at scattering angles of 60◦ , 120◦,
and 150◦ in the lab—a change from our previous measurements that was made based on theoretical physics considerations mentioned in Ref.
[Hen07]. The upgraded Lund photon tagging facility was used. The energy of the electron beam
incident on the radiator was 144 MeV, and tagged
photons covered the range from 66 to 97 MeV.
Scattering data were collected from a 70-mm
thick liquid deuterium (LD2) target, which was
a significant improvement over the 48-mm thick
target used in our previous measurements and
was one of the important changes from the earlier runs. In addition, the counting rate was increased by a factor of two over previous runs by
moving the NaI detectors closer to the LD2 tar-
get. The photon tagger rates were approximately
500 kHz per channel. Given the relatively low
true event rate, it was necessary to sum the data
for each angle over the full range of the tagged
beam energies to improve the statistical accuracy
of the data. This was made possible by sorting
the energy spectra from each NaI detector according to the missing energy, which is defined as
the difference between the tagged and detected
photon energies. During the running period in
November 2007, data were acquired for approximately 250 hours of beam time. The number of
counts accumultaed during this run allowed the
data to be split into seven energy bins with about
±5% statistical accuracy in each bin. One significant problem that ocurred was that the LD2
target was only 85% full, due to a heat bridge
formed through some bad insulation. This problem was corrected for the June 2008 run, and the
target was able to be filled to 100% of its capacity.
For the June 2008 run, the energy of the electron beam was kept the same at 144 MeV, but
the tagger was changed to the end-point tagger
in order to reach higher tagged photon energies
while keeping the end-point energy below the
pion production threshold. The tagged photon
energy range is smaller for the end-point tagger,
extending from 95 to 115 MeV. Also, the radia-
tor for the end-point tagger is farther away from
the collimator. This reduced the angular size of
the collimator by a factor of three. This reduction, coupled with the 20 MeV range versus the
31 MeV range of the main tagger, lowered the
counting rate by a factor of five. Approximately
the same number of hours of data were taken,
but the statistical accuracy will limit the result
to one large energy bin or possibly two energy
bins.
The tentative plan for the rest of 2008 is to
run for more time at the same energy and detector angles in December, in order to improve
the statistics. A new collimator that is better
matched with the end tagger radiator’s distance
will be made. This should improve the counting
rate by about 70%.
The analysis of the data is progressing well,
with refinements to the analysis software being
made with each pass through the data. It has
been determined that cosmic rays are sufficient to
find and correct for gain shifts in the photomultiplier tubes for the Buni detector. The Cats and
Diana detectors have LED pulsers which are used
to correct for gain shifts. An additional check for
49
gain shifts was performed with Buni during the
June 2008 run. A 228 Th source was used to monitor and adjust the gain in the Buni NaI shield
detectors. The source was placed in the detector
collimator daily, and any gain shifts were immediately visible in the spectra. The day-to-day gain
shifts were typically less than a few percent, with
5% being the maximum.
TUNL’s contribution to this work has been to
help with data collection and data analysis during the running periods. We have also helped in
target development, both in assisting in the development of another LD2 target and in loaning
Lund a deuterated scintillating target for the experiment. The use of a scintillating target was to
help reduce the background contribution by having a coincidence with recoiling deuteron signals
from the target. Future plans are to continue to
be available to take shifts during data runs as
well as to contribute to the planning process of
future experiments.
[Hen07] S. Henshaw et al., TUNL Progress Report, XLVI, 54 (2007).
50
4.2
4.2.1
Nucleon Spin Structure
Measurement of Single Target-Spin Asymmetry in Semi-Inclusive
Deep Inelastic Pion Electroproduction on a Transversely Polarized
3
He Target
W. Chen, H. Gao, X. Qian, Y. Qiang, W.Z. Zheng, X.F. Zhu, X. Zong, TUNL; Others,
Istituto Nazionale di Fisica Nucleaire, Frascati, Italy; Temple University; Rutgers University; University of Illinois at Urbana-Champaign; University of Virginia; College of William & Mary
A new experiment on neutron transversity employing a high-pressure polarized 3 He target
is currently underway at Jefferson Lab. This A-rated experiment, together with data on the
proton, will provide powerful constraints on the transversity distribution on both u-quark
and d-quark in the valence quark region in the nucleon.
Measuring parton distribution functions
(PDFs), which represent the flavor and spin
structure of the nucleon, is important to reveal
information on quantum chromodynamics in the
confinement region. In particular, within the
parton model, the structure functions describing the nucleon structure in electron scattering
can be written as PDFs. Furthermore, under
the factorization assumption, the cross section
in semi-inclusive deep inelastic scattering (DIS)
can be written as the product of PDFs and a
fragmentation function which describes the parton hadronization process due to the color force.
Detailed studies of PDFs have been carried out
in various processes.
At leading twist, after integrating over the
transverse momenta of the quarks, three quark
distribution functions describe the internal dynamics of the hadrons completely: the unpolarized parton distribution f1 , the longitudinal
polarized parton distribution g1 , and the quark
transversity distribution, h1 . The f1 function
has been extracted with excellent precision over
a large range of x and Q2 after several decades
of experimental and theoretical efforts. The g1
function has been determined with reasonable
precision over a smaller region of x and Q2 from
polarized DIS experiments at CERN, SLAC, and
DESY in the last two decades, and recently at
JLab and at RHIC from polarized proton-proton
scattering. What remains elusive is the transversity function, h1 , a chirally odd quark distribution function. It probes the relativistic nature
of the quarks inside the nucleon, and the lowest
moment of hq1 measures a simple local operator,
known as the “tensor charge”, which is analo-
gous to the axial charge and can be calculated
from lattice QCD.
Because of its chiral-odd nature, the experimental determination of the transversity function requires an additional chiral-odd function.
Examples are double-polarized Drell-Yan processes, target single-spin azimuthal asymmetries from semi-inclusive DIS pion electroproduction, double-spin asymmetries in Λ production from e-p and p-p reactions and other processes, and single-spin asymmetries from double
pion production from e-p scattering. The target single-spin azimuthal asymmetry can arise
from the following mechanisms: the so-called
Collins mechanism and the Sivers mechanism
for semi-inclusive DIS electroproduction of pions.
The quark transversity function, in combination
with the chiral-odd Collins fragmentation function [Col93], gives rise to an azimuthal (Collins)
asymmetry in sin(φh + φS ), where φh and φS
are azimuthal angles of the hadron (pion) and the
target spin axis with respect to the virtual photon
axis and relative to the lepton scattering plane.
The Sivers asymmetry [Men89, Siv90, Ans99]
refers to the azimuthal asymmetry in sin(φh −
φS ) due to the correlation between the transverse
target polarization of the nucleon and the transverse momentum of the quarks [Bro02, Bur04].
The HERMES SSA results [Air05] were obtained from a transversely polarized proton target from semi-inclusive electroproduction of pions in DIS kinematics. Signals due to the unknown Collins fragmentation function in conjunction with the previously unmeasured quark
transversity distribution have been seen in the extracted moment of hsin(φh + φS )i from the data
for both the positive pions and the negative pions. The Sivers asymmetry due to the correlation between the quark transverse polarization
and quark transverse momentum was also extracted for the first time from the moment of the
azimuthal hsin(φh −φS )i distribution. The HERMES data show rather larger negative π − Collins
moments. This surprising feature might be explained by the possibility that the corresponding disfavored fragmentation could be of unexpected importance and may enter with a sign
opposite to that of the favored case. A very interesting observation from the HERMES data is
that the Sivers moment extracted from the positively charged pion is positive over the entire x
and z range of the experiment, while the Sivers
moment from the negatively charged pion seems
to be consistent with zero.
The COMPASS collaboration reported measurements [Com05, Com07] of the Collins and
Sivers asymmetries of charged hadrons on the
deuteron. Both asymmetries are consistent with
zero within experimental uncertainties. Some
cancellation between the neutron and proton in
single-spin asymmetries may exist in measurements using a transversely polarized deuteron
target, and this may explain the smallness of the
COMPASS Collins and Sivers asymmetries.
Two experiments have been approved at JLab
with A rating. They are experiments E06-010
(Spokespersons: J.P. Chen, X.D. Jiang and J.C.
Peng) and E06-011 (E. Cisbani, H. Gao, and X.D.
Jiang). These two neutron-transversity experiments will focus on measuring the target singlespin asymmetry in the semi-inclusive deep inelas−−→
tic 3 He(e, e0 π ± )X reaction with a transversely polarized 3 He target at JLab in Hall A with a 5.7
GeV electron beam. The main objective of these
experiments is to measure the neutron “transversity”, which is essential to extract the transversity distribution of the u and d quarks. The neutron transversity experiments will take advantage
of the high 3 He polarization of the rubidiumpotassium hybrid target cell, which was successfully used in JLab experiment E02-013, reaching
a polarization of 52% during two months of running in 2006.
Currently, the neutron transversity collaboration is making good progress to ensure that the
experiment will start in 2008. Our group is playing a very important role in this experiment.
Yi Qiang, a postdoc from our group, is leading the effort to build a whole new polarized
3
He target system. The new system will provide optical pumping in 3 directions (longitudinal, horizontal and vertical), as required for the
“transversity” measurement. In addition to the
two sets of existing Helmholtz coils, a new set
of Helmholtz coils providing a vertical holding
51
field was assembled and tested. There are also
two new ovens built with vertical pumping capability. The second oven is from the final design,
which has lighter weight, less internal light reflection and better insulation than the first design.
The EPR and NMR polarimetries have been set
up and tested with the new oven system. Due to
the requirement of frequent spin flips, a spin-state
flagging system was designed and built with the
signal from the 3 He NMR frequency sweep measurement. The system was proved to be working
quite well and will mark individual events with
their proper spin state. All of the planned 20
new cells have been filled by target laboratories
at the University of Virginia and at William &
Mary. The characterization of these cells is on
track.
Xin Qian, a graduate student at Duke University, will take this neutron transversity experiment as his thesis experiment. He is actively
working on the preparation of the large acceptance BigBite spectrometer, which will serve as
the electron arm of the experiment. His work includes the background study, wire chamber test,
optics studies, and tracking software development and test. So far, all detectors in the BigBite
spectrometer have been assembled and are currently under test and calibration.
Data from this experiment, when combined
with proton data from HERMES [Air05] and
COMPASS [Com05, Com07], will provide powerful constraints on the transversity distribution
of both the u-quark and d-quark in the valence
quark region of the nucleon.
[Air05]
A. Airapetian et al., Phys. Rev. Lett.,
94, 012002 (2005).
[Ans99] M. Anselmino et al., Phys. Rev., D60,
054027 (1999).
[Bro02] S. Brodsky et al., Phys. Lett., B530,
99 (2002).
[Bur04] N. Burkardt, Phys. Rev., D69, 057501
(2004).
[Col93]
J. C. Collins, Nucl. Phys., B396, 61
(1993).
[Com05] Compass Collaboration,
Lett., 94, 202002 (2005).
Phys. Rev.
[Com07] Compass Collaboration, Nucl. Phys.,
B765, 31 (2007).
[Men89] T. Meng et al., Phys. Rev., D40, 769
(1989).
[Siv90]
D. W. Sivers,
(1990).
Phys. Rev., D41, 83
P
hotograph of the original configuration of the reactor YAGUAR in the reactor hall at the All-Russian Research Insitute of Technical Physics, Snezhinsk,
Russia. As a part of the n − n scattering experiment we have added a well below
the reactor; the neutron detector is located at the bottom of the highly collimated well approximately 12 m from the reactor center. To reduce backscattering
effects, we have also added a several meter high tower above the reactor which
extends through the ceiling.
Few-Nucleon Interaction Dynamics
Chapter 5
•
•
Nucleon-Nucleon Interactions
The A=3 System
•
The A=4 System
•
Reaction Dynamics of Light Nuclei
54
5.1
5.1.1
Nucleon-Nucleon Interactions
Direct Measurement of the nn-Scattering Length (The DIANNA
Collaboration)
G.E. Mitchell, C.R. Howell, W. Tornow; Others, All Russian Institute of Technical Physics,
Snezhinsk, Russia; Duke University, Durham, NC ; Gettysburg College, Gettysburg, PA; Joint Institute for Nuclear Research, Dubna, Russia; North Carolina State University, Raleigh, NC.
The relative values of the neutron-neutron and proton-proton 1 S0 scattering lengths (ann and
app ) are crucial for resolving the issue of charge symmetry in the nuclear force. However,
after some 50 years there are still discrepancies in the value of Ann determined via indirect
methods. Essentially all of the construction, calculations, and test measurements have been
completed for a direct measurement of ann using the Russian pulsed reactor YAGUAR.
A direct measurement of the nn-scattering
length can be performed by observing the scattered neutrons after collisions in a thermal neutron gas. If only the colliding neutrons contribute
to the detector counting rate, and if the parameters of the neutron field are determined and the
geometry is known, then the detector counting
rate measures the nn-scattering cross section.
by the same neutron source. Measurements at
different reactor power levels are used to distinguish true signals (which depend on the neutron
flux density squared) from background events
(which depend linearly on the flux density). An
evacuated tube contains a collimation system,
and the neutron detector is placed at the bottom
of a hole beneath the central through-channel.
The neutron detector is located approximately 12
We have formed the DIANNA collaboration
m from the central plane of the through-channel
(Direct Investigation of Ann Association) in orand reactor core.
der to perform the first direct measurement of
Because our initial analysis was rather idealthe nn scattering length. The collaboration involves neutron physicists from the Joint Institute ized (background estimates were based on a simof Nuclear Physics, Dubna, Russia, and reac- plified geometry, the epithermal tail of the neutor physicists and transport code theorists from tron spectrum was neglected, and the stationary
the All Russian Institute of Technical Physics at approximation was applied), we addressed each
Snezhinsk, Chelyabinsk region, Russia. This ex- of these issues with more realistic simulations.
periment is supported by the International SciThe effect of the epithermal neutrons was conence and Technology Center – Project 2286. The sidered by Crawford et al. [Cra04]. Detailed
experiment has become feasible due to the avail- analysis indicates that although the shape of the
ability of the unique aperiodic pulsed reactor neutron time-of-flight spectrum after scattering
YAGUAR at Snezhinsk. The key points are that is appreciably different from the initial spectrum,
the instantaneous flux density is extremely high the integral over the Maxwellian part of the re(about 1018 /cm2 s) and that the reactor has a alistic scattering spectrum differs by only about
through-channel. Details of the reactor char- 6% from that of a pure Maxwellian nn-scattering
acteristics and comparison with possible experi- spectrum.
ments at other locations are provided by Mitchell
The stationary approximation assumes neuet al. [Mit03]. A general description of the protron sources and thermal neutron densities that
posed experiment was given by Furman et al.
have reached their asymptotic values. However,
[Fur02]. The experimental set up is shown in
there is a significant transition time. We thereFig. 5.1.
fore have calculated the neutron density for difAfter the neutrons collide in a neutron gas ferent time slices (of 20 µs) and for different vertiin the through-channel, the scattered neutrons cal spatial slices of the through-channel (the neuare detected externally. In this arrangement the tron cavity). The hardness of the neutron spec“target” and the “beam” are neutrons produced trum changes with time. A detailed description
55
Figure 5.1: Experimental set up.
of the time dependence is given by Sharapov et
al. [Sha06].
Extensive modeling of the background with
a realistic geometry was performed in parallel by the transport code experts at Snezhinsk
and by the Dubna group [Muz06]. The former
calculations were performed with the multipurpose Monte Carlo code prism-d developed at
Snezhinsk and the latter calculations with the
code mcnp. The two sets of calculations agree
with each other and agree with preliminary experimental measurements. These results indicate
that the background level will be about 25-30%
of the anticipated count rate.
Initial measurements at full reactor power
suggested a modification of the collimation system to reduce the effects of the γ flash. After
these modifications, measurements were made in
the pulsed mode. The detector responded well,
and displayed a linear response to the detector
gas pressure.
Two experimental issues have developed. The
borated collimators contain a great deal of sulfur. Since sulfur has been contaminating the vacuum pumps, it is essential either to develop new
borated collimators without sulfur or to cover
the present collimators with a thin, low-neutronscattering material. Another issue is an unexpectedly large background that is hypothesized
to be an outgassing phenomenon. At present
we are actively involved in studying and modeling proposed mechanisms for the observed background.
[Cra04] B. E. Crawford et al., J. Phys., G30,
1269 (2004).
[Fur02] W. I. Furman et al., J. Phys., G28,
2627 (2002).
[Mit03] G. E. Mitchell et al., In Proc. of the Intl.
Conf. on Advanced Neutron Sources, p.
79, Forschungzentrum Jülich, Germany,
2003.
[Muz06] A. Y. Muzichka et al.,
A789, 30 (2006).
Nucl. Phys.,
[Sha06] E. I. Sharapov et al., In Proc. XII
Intl. Seminar on Interactions of Neutrons with Nuclei, p. 130, JINR, Dubna,
Russia, 2006.
56
5.1.2
Neutron Capture Experiments
G.E. Mitchell, B. Bayarbadrakh, R. Chankova, A. Chyzh, S. Sheets, C. Walker, TUNL;
OTHERS, Charles University, Prague and Rez Institute of Nuclear Physics, Rez, Czech Republic;
North Carolina State University, Raleigh, NC ; LANSCE Division and C-INC Division, Los Alamos
National Laboratory, Los Alamos, NM ; N Division and Nuclear Chemistry Division, Lawrence Livermore National Laboratory, Livermore, CA.
The neutron capture reaction is very important for a wide variety of pure and applied
physics. Our measurements utilize the DANCE array (at LANSCE/LUHAN)—a highly efficient calorimeter consisting of 160 BaF2 detectors. In favorable cases the high degree of
segmentation can be used to perform neutron resonance spectroscopy. The study of the statistical γ-ray cascade from different resonances and for different multiplicities provides unique
opportunities to test models of the photon strength function.
The neutron capture reaction is very important for a variety of pure and applied physics,
ranging from stewardship science (radiochemical
applications) to astrophysics to advanced fuel cycle issues. We are pursuing these efforts via two
primary approaches.
The first uses the Detector for Advanced
Neutron Capture Experiments (DANCE) at the
Manuel Lujan Center at Los Alamos National
Laboratory. DANCE is an array of barium fluoride crystals (162 segments with 160 crystals).
Many of the details of this system as well as proposed applications are given on line in the proceedings of a workshop “New opportunities and
challenges with DANCE” [Dan04]. The high efficiency of DANCE (this is a calorimeter that identifies capture by the γ-ray total energy) makes
possible the study of very small samples, including radioactive samples. The high degree of segmentation makes possible the separation of the
statistical gamma cascade for different multiplicities. The basic limitation of this calorimeter system is setting the energy gate on the total γ-ray
energy in order to accumulate sufficient statistics.
The range of this gate is typically one MeV.
Our other major effort involves the two-step
cascade (TSC) method. In our version of this
approach the thermal neutron beam from the
Rez research reactor is captured, and the resulting γ rays are detected with high-resolution germanium detectors. By requiring a coincidence
between two detectors and a total energy corresponding to the second transition ending at a
specific low-lying final state, one obtains a very
clean spectrum. For the target 95 Mo, we have
studied the capture reaction at Los Alamos with
DANCE and at Rez via the TSC method.
At DANCE we have measured neutron capture on 94,95 Mo. For s-wave resonances on 95 Mo
(ground state J π = 5/2+ ) the compound state
s-wave resonances can have J = 2 or 3. The
ground states of 96 Mo has J = 0. If the decay is
statistical, one expects on average a higher multiplicity for the J = 3 states. In addition, the
energy spectra for resonances with different spin
are different, especially for the multiplicities M
= 2 and 3. The combination of the average multiplicity M and the M = 2 and 3 spectra leads to
a very clear signature. There is excellent agreement with previous information on 96 Mo. There
are also clear indications of differences for the pwave states (J = 1, 2, 3, 4 and negative parity).
These results are presented in a recent Physical
Review article [She07].
We also have focused on the photon strength
function (PSF) and the level density (LD). We
tested various models by simulating the statistical cascade with the program dicebox. In addition, our predictions with dicebox for the average multiplicities and for the spread in these
values, agreed very well with the experimental
data. This suggests that the extreme statistical
model works well in this mass region. To examine this issue further, we have performed a TSC
measurement at Rez on 95 Mo.
This measurement complements and extends
the DANCE results; we were able to study the
TSC to 11 different final states. We determined
a rather standard set of values for the level density and the PSFs that fit all of the data very well.
We also see no evidence for the large low energy
enhancement of the PSF observed in (3 He,3 He0 )
and (3 He,α) reactions, although the data may be
consistent with a smaller enhancement. A paper
on this work at Rez has been published [Krt08].
We then adopted the same parameter set that
provided excellent agreement with the TSC data
and applied that to our DANCE data. The result was very good agreement, as demonstrated
in Fig. 5.2. Thus, there is strong evidence that
the extreme statistical model works very well in
this mass region. These DANCE results are being prepared for publication.
M=2
Intensity (arb. units)
30
M=3
200
20
100
10
0
0
M=4
400
400
J π = 2+
200
0
0
600
M>4
200
2
4
6
2
4
8 0
γ-Ray Energy (MeV)
6
8
0
Figure 5.2: Comparison between experimental
spectra (solid histogram) and spectra
simulated with the Monte-Carlo code
DICEBOX (shaded histogram). See
text for discussion.
57
tion.
Measurement of the neutron capture reaction on 152,154,157,160 Gd has been performed at
DANCE. The 152 Gd target had an enrichment
which is high compared with natural enrichments, but low on an absolute scale. Although
the cross sections will be difficult to determine,
much new information about the resonance decay can still be obtained. Analysis of the data on
157
Gd is most advanced. This isotope has a very
high cross section and is extremely interesting for
applications.
Our intent is to study all of the stable Gd
isotopes, both for practical reasons and to enable
a systematic study of the variation of (for example) scissors mode resonances with mass and
deformation. The LANSCE PAC has approved
experiments to study 155,156,158 Gd. These are
scheduled in the late 2008 run cycle. Our first
emphasis will be on 155 Gd, which, like 157 Gd,
has a high Q value.
Much of this research is supported by an academic alliances grant from the National Nuclear
Security Agency.
[Agv07] U. Agvaanluvsan et al., 2007, Lawrence
Livermore National Laboratory technical report UCRL-TR-234006.
[Dan04] 2004, DANCE workshop proceedings,
http://wnr.lanl.gov/dance/workshop 2004/.
We measured the capture cross sections for
Eu, which are important both for radio- [Krt08] M. Krticka et al., Phys. Rev., C77,
chemical applications and for astrophysics. A
054319 (2008).
technical report that emphasizes the cross section results has been prepared [Agv07]. A paper [She07] S. A. Sheets et al., Phys. Rev., C76,
on the gamma cascade information is in prepara064317 (2007).
151,153
58
5.2
5.2.1
The A=3 System
Neutron-Deuteron Analyzing Power Ay (θ) at En = 21.0 MeV
G.J. Weisel, Penn State Altoona, Altoona, PA; M.W. Ahmed, A.S. Crowell, J.H. Esterline,
C.R. Howell, M.R. Kiser, W. Tornow, TUNL; B.J. Crowe III, North Carolina Central University, Durham, NC ; R.S. Pedroni, North Carolina A&T State University, Greensboro, NC ; H.
Witala, Jagiellonian University, Cracow, Poland.
In a 10-day run in August 2007, we completed measurements of n-d Ay (θ) at En = 21.0 MeV
at twelve detector angles. This experiment is part of TUNL’s effort to obtain n-d A y (θ) data at
~ n)3 He. Our
the highest energies available using our traditional neutron source reaction, 2 H(d,~
preliminary data at 21.0 and 22.5 MeV conflict with the predictions of three-body calculations
of the n-d analyzing power and confirm the three-nucleon analyzing-power puzzle at these
energies.
Our interest in n-d analyzing-power data
originated in the three-nucleon analyzing-power
puzzle, in which precision n-d Ay (θ) data are
found to be significantly higher than the predictions of rigorous three-nucleon calculations based
on nucleon-nucleon and three-nucleon potential
models. As of the year 2002, the discrepancy
was well established at incident neutron energies
from 1.2 to 16.0 MeV. TUNL’s measurements of
n-d Ay (θ) at En = 19.0 MeV during late 2002
and 2003 confirmed the analyzing power puzzle
at that energy [Wei04].
In the present work, we seek data at still
higher neutron energies. Our two objectives are,
first, to determine how high in energy the Ay (θ)
puzzle persists and, second, to provide highprecision data which can aid the theoretical development of potential models and three-nucleon
calculations. During the summers of 2005, 2006,
and 2007, we conducted measurements of n-d
Ay (θ) at En = 21.0 and 22.5 MeV. We used the
TUNL Shielded Source Target Area and a polarimeter consisting of a 4 He gas cell and two neutron side detectors. The n-d Ay (θ) measurement
itself used six pairs of liquid-scintillator neutron
side detectors and a deuterated scintillator center
detector. The work required four runs of about
ten days each, and involved two students from
the Research Experience for Undergraduates program.
Because we could not secure an appropriate tritiated target in order to make use of the
3
~ n)4 He source reaction, we relied on the traH(d,~
~ n)3 He source reaction and on raisditional 2 H(d,~
ing the terminal voltage of our FN tandem accelerator. This arrangement made it more difficult
to move to higher energies than we had hoped.
The FN tandem could not always be brought reliably to the terminal voltage (10 MV) necessary
for the En = 22.5 MeV measurement and we
therefore used En = 21.0 MeV as a fall-back option. For two of our experimental runs (July 2005
and August 2006), we were able to get to 22.5
MeV, while for the two others (June 2006 and
August 2007), we had to be content with 21.0
MeV. At 22.5 MeV, we obtained a good angular distribution with data for six detector angles,
while at 21.0 MeV we achieved a more detailed
angular distribution, containing data for twelve
angles.
The data analysis of both the n-d and nHe scattering used pulse-shape descrimination
(PSD) to reduce the γ-ray background. After
PSD filtering, gates were set in the time-of-flight
spectra of each side detector to sample the elastic and accidental events. These gates were used
to sort spectra for the center detector and the
4
He scintillator pulse height. Accidental backgrounds were subtracted from all pulse-height
spectra. The well-known Ay (θ) for n-4 He elastic scattering was used to determine the neutron
beam polarization from the asymmetry measurement of the polarimeter. For n-d scattering, a
linear fit was used to determine the background
after accidental subtraction in the pulse-height
spectra. This additional background ranged from
5% to 15% of the total counts within our yield
gates. Preliminary Monte-Carlo calculations of
4
n-d scattering show that multiple-scattering and
edge-effect events in the deuterated scintillator
account for 50% to 100% of this background.
After removal of the multiple-scattering contributions, a preliminary treatment of the residual
background was accomplished by taking an average of two Ay (θ) results: one using a polarized
residual background and one using an unpolarized background.
Ay(θ)
0.2
21.0 MeV preliminary data
0.1
0
-0.1
-0.2
0
Ay(θ)
0.2
20
40
60
80
100 120 140 160 180
θcm(deg)
22.5 MeV preliminary data
0.1
0
-0.1
-0.2
0
20
40
60
80
59
TUNL’s preliminary data for En = 21.0 and
22.5 MeV are displayed in Fig. 5.3. The data
at 22.5 MeV are a combination of the July 2005
and August 2006 experiments, while the data at
21.0 MeV combine June 2006 and August 2007
results. The error bars reflect only statistical uncertainties and are about twice as large as those
for our 19.0 MeV n-d Ay (θ) data because we
counted for only about one quarter of the time
at the higher energies. The data are compared
to a Legendre polynomial fit (dotted curve) and
to three-nucleon calculations based on the CDBonn NN potential (solid curve). The discrepancies between the theoretical predictions and the
data are especially notable at the minimum of
Ay (θ) at center-of-mass angles around 110◦ and,
to a lesser degree, at the peak at angles around
140◦. Thus, the present data confirm the presence of the three-nucleon analyzing power puzzle
for n-d scattering at En = 21.0 and 22.5 MeV.
In addition, the discrepancies found in this n-d
study are consistent with those seen in our recent review comparing p-d Ay (θ) data above En
= 16 MeV with rigorous three-nucleon calculations [Tor08].
100 120 140 160 180
θcm(deg)
Figure 5.3: (Color online) Preliminary TUNL
data for the n-d analyzing power at
En = 21.0 and 22.5 MeV compared to
three-body predictions using the CDBonn NN potential (solid curve). The
dotted curve is a Legendre polynomial
fit. The error bars include only statistical uncertainties.
[Tor08] W. Tornow, J. Esterline, and G. Weisel,
2008, submitted to J. Phys. G: Nucl.
Phys.
[Wei04] G. Weisel et al., TUNL Progress Report,
XLIII, 20 (2004).
60
5.2.2
Measurements of the Neutron-Deuteron Breakup Cross Section for
the Space- and Coplanar-Star Configurations at 19 MeV
B.J. Crowe III, L.C. Cumberbatch, D.M. Markoff, L.D. Threat, North Carolina Central
University; A.S. Crowell, J.H. Esterline, B. Fallin, C.R. Howell, T.B. Clegg, A.C.
Couture, TUNL; R.S. Pedroni, North Carolina A & T University;
We constructed a new scattering chamber and used it to simultaneously measure the cross
sections in nd breakup for the space- and coplanar-star configurations at 19 MeV. Our goal
is to measure these cross sections to a statistical accuracy of about ± 5% in 1-MeV neutron
energy bins and with a systematic uncertainty less than ± 5%. The experimental setup was
optimized and about 2/3 of the data production runs were completed this year. A signal-tonoise ratio better than 7:1 was achieved in the neutron-proton coincidence time spectrum for
both configurations. With these measurements, we obtained a cross section in nd breakup for
the space star configuration of 0.44 ± .03 mb/sr2 /MeV about the space star point.
To better understand the cause of the “space
star anomaly” — the discrepancy between theory and experiment for the nd breakup cross section in the space-star configuration — we are
developing experimental techniques and instrumentation to measure these cross sections at new
and previously measured energies. A discussion
of the experimental technique and the scattering
chamber built for these nd-breakup cross-section
measurements at 19.0 MeV has been presented
in Ref. [Cro06].
During this year, we built and tested the scattering chamber and used it at TUNL to collect
452 hours of data for the space and coplanar star
configurations. A picture of the experimental
setup used for these measurements is shown in
Fig. 5.4. The scattering chamber consists of two
proton detector arms that are positioned symmetrically about the incident neutron beam axis
at a scattering angle (θ) of 52.1◦ . This arrangement enables two simultaneous measurements to
be made of the same configuration for increased
statistical accuracy. For the space-star configuration measurements, the neutron detectors (black
cylinders shown in Fig. 5.4) were placed at θ =
51.2◦ and azimuthal angles (φ) of 60◦ and 120◦.
The neutron detectors used for the cross-section
measurements of the coplanar-star configuration
were positioned at θ = ±16.9◦ and φ = 0◦ and
180◦ . The space and coplanar star neutron detectors have flight paths from the CD2 target foil
of 50 cm and 70 cm, respectively. The data event
trigger was generated by a charged-particle trajectory in either proton arm, i.e., the coincidence
of the signals from the ∆E detector (a thin scintillating foil detector) and the E detector (a plastic scintillator) in one of the proton arms. Star
configuration events were defined in software as
the coincidence between signals from a neutron
detector and the proton detector on the opposite
side of the incident-beam axis.
Figure 5.4: (Color online) Experimental setup for
the nd-breakup measurements made
with the new scattering chamber. The
dome-topped target chamber and proton detector arms are shown in the
photograph. The recoil proton telescope (located center foreground) is
used to measure the neutron beam
flux via np scattering.
Particle-identification (ID) histograms were
generated for events detected in each chargedparticle arm by plotting the time-of-flight (TOF)
of the particle from the ∆E to the E detector
versus the pulse height of the signal from the E
detector (see Fig. 5.5). The separation between
the proton, recoil electron, and deuteron bands
is clear. Pulse-shape-discrimination (PSD) techniques were applied to the light from the liquid
scintillator in each neutron detector to reject the
detected gamma rays.
Figure 5.5: Histogram of the pulse height of the
plastic scintillator versus the time of
flight of charged particles detected in
one of the charged-particle arms attached to the center scattering chamber.
A time-of-flight spectrum for neutrons in coincidence with protons is shown in Fig. 5.6 for
the space-star configuration. The peak is the
neutron-proton coincidence. The application
of particle ID techniques to select the protons
and eliminate the deuterons and electrons detected in the charged-particle arm plus the use of
PSD techniques to reject γ rays detected in the
neutron detector significantly reduced the background in the neutron time-of-flight spectrum,
giving a signal to noise ratio of about 7 to 1.
The counts for the true and accidental coincidence events are plotted onto the neutron energy
axis in 0.5-MeV bins as shown in Fig. 5.7. These
data were analyzed with a neutron detector energy threshold of about 1 MeV, or one half the
Compton recoil edge from the γ-ray calibration
source, 137 Cs. These histograms represent 67%
of the accumulated data for the space-star configuration.
The main focus in the coming year will be
on data analysis and completing the data taking at 19.0 MeV. The primary challenge in data
analysis is to complete the development of the
Monte-Carlo simulation code needed to fold the
finite geometry, beam energy spread and detector responses of our experiment into the cross
sections from three-nucleon calculations, so that
direct comparisons can be made with our mea-
61
sured cross sections.
Figure 5.6: Time of flight spectrum of neutrons
in coincidence with protons from the
breakup reaction. The following conditions were applied to this spectrum:
(1) PSD cut to reject events with γ
rays detected in the neutron detector,
(2) neutron detector energy threshold of around 1 MeV, (3) selection of
events with protons detected in the
charged-particle arm. This spectrum
was generated from half of the total accumulated data. The time calibration for this spectrum is 0.1465
ns/channel.
Figure 5.7: Projection of counts accumulated for
the space-star configuration onto the
neutron energy axis into 0.5-MeV
wide bins.
The counts from the
“true+accidental” and “accidental”
events are represented by the open
and shaded plots, respectively.
[Cro06] B. Crowe III et al, TUNL Progress Report, XLVI, 138 (2006).
62
5.2.3
SCRE Experiments in the Breakup Reaction 2 H(p,pp)n
A. Imig, T.B. Clegg, A.H. Couture, H.J. Karwowski, C.R. Howell, E.P. Carter, T.V.
Daniels, TUNL;
The three-nucleon system serves as a fertile testing ground for models of the nucleon-nucleon
(NN) interaction. Traditionally, NN models have been tested against the static properties
of the deuteron. Now, because the dynamics can be described using the Faddeev formalism,
more rigorous tests are possible using comparisons against observables in nucleon-deuteron
(N d) elastic scattering or breakup reactions. An interesting choice among distinct exit-channel
configurations for N d breakup is the symmetric constant relative energy configuration. There,
a cross section discrepancy of about 35% has been observed between theoretical calculations
and recent experimental results. New measurements have been performed at TUNL in an
attempt to resolve this discrepancy.
Considerable progress has been made in theoretical models of the three-nucleon (3N) interaction.
Still, there are glaring open questions: e. g., the
Ay -puzzle [Wit91], and a discrepancy between recent experimental results and theoretical calculations for the breakup reaction. In particular,
for four different theoretical approaches for predicting the 2 H(p,pp)n reaction for the symmetric
constant relative energy (SCRE) configuration,
theoretical and experimental cross sections disagree by about 35% [Ley06].
In this configuration the three outgoing particles lie in one plane in the center-of-mass system
and are emitted with equal kinetic energies at
relative angles of θc.m. = 120◦. The SCRE final
state can be characterized by α, the angle between the outgoing neutron momentum and the
reversed beam axis.
We measured cross sections for different angles α in and out of plane. The height of the
original chamber shell limited measurements to
∼ 12◦ out of plane. Therefore, we designed and
built a new aluminum ring with a height of 10 cm
(Century Metal Spinning Co., Bensenville, IL),
serving as a vertical extension for the scattering chamber. For these measurements, arcs were
developed for mounting detectors out of plane.
These are suitable for a wide range of configurations and it is possible to set detectors up to 30◦
out of plane (see Fig. 5.8).
We decided to use a gaseous D2 target to introduce different, and hopefully smaller experimental uncertanties than those present in measurements reported by others. Filling the whole
chamber with gas has many advantages [Imi07].
To enable this, a generator of deuterium from
electrolysis of heavy water is located near the
scattering chamber. We tried both sealing the
chamber entrance with a thin Havar foil, and using a newly developed differential pumping system (see Sect. 9.5.9). The former method causes
substantial, inconvenient radioactive activation
of the Havar, but this technique was used after
differential pumping was finally rejected. Among
the reasons for rejecting it was the relatively high
loss rate of expensive deuterium, even when using Ar gas to buffer its flow through the differential pumping system. The scattering chamber is filled with deuterium gas at a pressure of
∼76 Torr. Measurement of the chamber pressure, needed to determine the target density, is
performed with a precision of 0.1 Torr using a
capacitance manometer (MKS Instruments, Inc.
Baratron Model 122A).
The active target volume for coincidence experiments with a gas target is determined by the
combination of the double slit systems of both
coincidence detectors. Rectangular detector collimator openings, which are relatively insensitive
to vertical beam position variations, are preferable. Asymmetric front detector apertures are
required for the defined volume to be similarly insensitive to horizontal beam positions. Choosing
different widths of the front apertures results in
a defining (small) and non-defining (large) detector for the coincidence region along the incident
beam path.
For measuring coincident cross sections, one
must determine the non-defining detector’s solid
angle. This can be measured by proton elastic
scattering from a target of known cross section
(e.g., H, Au or deuterium in CD2 ). A monitor
detector with a well-defined solid angle located
in the plane is used to determine the product
of target thickness and integrated beam current.
Recently, however, we developed a more accurate
way to determine this solid angle by actual physical measurements (see Sect. 9.5.10). To obtain
the product of incident beam current, target density, and G-factor [Sil59] of the defining detector,
we use p-d elastic scattering taken simultaneously
with the p-d breakup data.
63
(α = 0◦ ) and space star (α = 90◦ ) kinematics.
Figure 5.9: (Color online) Plot of E p1 versus
E p2 with kinematical curve obtained
for the SCRE configuration at angle
α=0◦ , after cuts on the coincidence
time difference spectrum and protons
given by the particle identification in
the telescope.
Figure 5.8: (Color online) Detector setup in the
scattering chamber. To accommodate
the arcs for detectors out of plane, the
vertical extension ring on the chamber is needed. Coincidence detectors
on one side were E-∆E telescopes to
enable particle identification.
The electronic coincidence setup is first
checked using p-p scattering coincidences for all
detectors in the horizontal plane. Then detectors are moved to appropriate positions to define
the α-angle desired. Kinematical curves of pd
breakup events are then measured using an incident proton beam with Ep =9.5 MeV on a D2
target. Figures 5.9 and 5.10 display the 2D coincident energy distributions of two outgoing protons after cuts on the coincidence-time-difference
spectrum as well as on protons, given by the particle identification in the ∆E-E-telescope, were
applied.
Drawn in the pictures is the calculated kinematical locus of the pd breakup for coplanar
Figure 5.10: (Color online) Same as Fig. 5.9 except for the case α=90◦ .
[Imi07] A. Imig et al., TUNL Progress Report,
XLVI, 137 (2007).
[Ley06] J. Ley et al., Phys. Rev., C73, 064001
(2006).
[Sil59]
E. A. Silverstein, Nucl. Instrum. Methods, 4, 53 (1959).
[Wit91] H. Witala, Nucl. Phys., 528, 48 (1991).
64
5.2.4
~ Elastic ScatStudies of the Spin-Correlation Coefficients for p~+3 He
tering
T.V. Daniels, C.W. Arnold, J.M. Cesaratto, T.B. Clegg, A. Couture, A. Imig, H.J.
Karwowski, TUNL
The UNC Few Body Group has constructed a polarized 3 He target [Kat05] with the aim of
measuring spin-correlation coefficients for p+3 He elastic scattering below 4 MeV. We hope
to extract a unique set of experimental phase shifts and mixing parameters for the system
by including these new data in a global phase-shift analysis, which can then be compared to
recent theoretical predictions based on “realistic” nuclear potential models. Over the past
year we have completed data-analysis except for the correction for systematic asymmetries
caused by the target’s magnetic field.
Our group uses scattering experiments with
light nuclear systems to better understand the
interaction between individual nucleons. “Realistic” potential models, which accurately fit the
database of two-nucleon data, underpredict Ay ,
the spin-dependent asymmetry between scattering to the left and right of the beam, in 3- and 4nucleon systems [Glö96, Fis06]. To better understand this discrepancy, we seek a consistent set
of phase shifts and mixing paramters for p+3 He
elastic scattering below 4 MeV. The most recent
global analysis of the system failed to produce a
unique result, and the ambiguity is largest for the
spin-correlation coefficients [Geo03].
Data analysis over the last year has produced
angular distributions of Ayo , Aoy , and Ayy at Ep
= 2.25, 3.13, 4.00, and 5.54 MeV between 30 and
150◦ in the lab, as well as Axx over a similar
energy and angle range, but not at 4 MeV. We
have also analyzed the measurements of systematic asymmetries associated with the polarized
target’s magnetic field. The magnetic field steers
the scattered protons slightly, so that those which
are detected at a given angle were originally scattered at a slightly different one. This gives rise to
a systematic left/right asymmetry in the following way. With the magnetic field pointed, for example, “up” with respect to the laboratory floor,
protons detected at a given angle to the right
of the beam were actually scattered at a slightly
more forward angle, while those detected on the
left were scattered at a slightly more backward
angle. Given the strong angle-dependence of the
scattering cross-section and our estimates of the
amount of steering, we believe this effect is responsible for asymmetries of the size observed.
A geant4 simulation of the target is being used
to better understand this effect, whose contribution we must subtract from our measured distributions.
A preliminary attempt was made to include
the cross-section and beam-analyzing-power data
of Ref. [Fis06] in a global phase-shift analysis, following Ref. [Geo03]. While the analysis was not able to fit the additional data accurately, we found one of the two previouslyreported minima in chi-squared to be less pronounced. While the analysis is now somewhat
closer to a unique solution, its continuing ambiguity indicates the need for second-order polarization observables. We hope to see greater
improvement when we include our corrected distributions of spin-correlation coefficients, since
those observables are far more sensitive to the
existing ambiguity in the phase-shifts than are
the cross-section or analyzing powers.
[Fis06] B. M. Fisher et al., Phys. Rev., C74,
034001 (2006).
[Geo03] E. A. George and L. D. Knutson, Phys.
Rev., C67, 027001 (2003).
[Glö96] W. Glöckle et al., Phys. Rep., 274, 107
(1996).
[Kat05] T. Katabuchi et al., Rev. Sci. Instrum.,
76, 033503 (2005).
66
5.3
5.3.1
The A=4 System
Neutron-3 He Analyzing Power between En = 1.60 and 5.54 MeV
J.H. Esterline, A.S. Crowell, B. Fallin, C.R. Howell, A. Hutcheson, M.F. Kidd, M.R.
Kiser, W. Tornow, TUNL; B.J. Crowe III, North Carolina Central University, Durham, NC ;
R.S. Pedroni, North Carolina A&T State University, Greensboro, NC ; G.J. Weisel, Penn State
Altoona, Altoona, PA
We report on the analysis of data obtained over the past four years on the analyzing power
of neutron-3 He scattering at five neutron energies between 1.60 and 5.54 MeV, for which ab
initio calculations have only recently become available.
As motivated and outlined in [Est07], we measured the n-3 He analyzing power Ay (θ) using
neutrons obtained from polarized ion beams and
the 3 H(p,n)3 He and 2 H(d,n)3 He source reactions.
The setup consisted of an active 3 He gas target surrounded by a symmetric array of NE213 liquid scintillator neutron detectors, in addition to a 4 He-based polarimeter, as is shown
in Fig. 5.11. Data were obtained at a total of
approximately thirty angles for each of the five
neutron energies. Acquired using the CEBAF online data acquisition system (CODA), these data
were first analyzed using paw++. Conversion
to analysis in root is in progress and is the fo-
cus of present work. Our preliminary results, not
yet corrected for finite-geometry and multiplescattering effects (computations of which are also
in progress), are shown in Fig. 5.12 alongside rigorous theoretical predictions [Del07] and the Rmatrix computations of Ref. [Hal04].
[Del07] A. Deltuva and A. Fonseca, 2007, private
communication.
[Est07] J. Esterline et al., TUNL Progress Report, XLVI, 66 (2007).
[Hal04] G. Hale, 2004, private communication.
NE−213
neutron detectors
3
polyethylene
shield/collimator
He
scintillator
target
67
Neutron
polarimeter
helium gas cell
Θ1
Θ pol
deuterium gas
target / tritiated
titanium foil
Θ2
Θ3
Θ4
Figure 5.11: The layout of the experiment, specifically for forward scattering (the detector ring would
be rotated 180◦ for backward angles). Changing Θi enabled all the angles for a given
energy to be measured.
0.8
En = 1.60 MeV
En = 2.26 MeV
En = 3.14 MeV
En = 4.05 MeV
0.4
0
Ay(θ)
0.6
0.2
−0.2
60
En = 5.54 MeV
180
Legend:
0.6
R−matrix (Hale)
AV18 (Deltuva)
CD−Bonn (Deltuva)
TUNL preliminary
0.2
−0.2
120
0
60
120
180
θc.m.
Figure 5.12: (Color online.) A comparison of our data to theoretical predictions.
agreement with rigorous calculations at low En .
Note the good
68
5.4
5.4.1
Reaction Dynamics of Light Nuclei
Measurements of
11
B(~
p,α)8 Be
R.H. France III, J.K.P. Metzker, Georgia College & State University, Milledgeville, GA; S.S.
Henshaw, M.W. Ahmed, B.A. Perdue, S. Stave, H.R. Weller, TUNL; R.M. Prior, M.C.
Spraker, North Georgia College & State University, Dahlonega, GA;
Measurements of the differential cross section and vector analyzing power (Ay ) of the 11 B(~
p,α)8 Be
reaction were measured at energies from approximately Ep = 500 keV to Ep = 2.74 MeV to
study the resonances at 675 keV, 1.4 MeV, and 2.64 MeV. The latest experiment was run
in June 2007 as a continuation of earlier runs in June 2006 and December 2005. Extensive
analysis of the Ep = 675 keV resonance is ongoing, with results at variance with previously
published analyses.
One of the more serious problems in developing practical nuclear-fusion power involves reactor activation by the high flux of neutrons from
standard fusion fuels (e.g., t(d, n)α). Thus there
is interest in developing advanced aneutronic fusion fuels such as 11 B, which undergoes fusion
via the 11 B(p,α)2α reaction. This may be possible with advanced non-equilibrium collidingbeam reactors [Ros04]. At fusion energies this
reaction is dominated by broad resonances at Ep
= 675 and 1388 keV.
Fusion energy researchers have requested a
detailed investigation of the 11 B(~
p,α)8 Be reaction, specifically requesting detailed information
on α-particle energy spectra as well as accurate
cross sections and analyzing powers at the lowest
energy resonance. The first experimental runs
were completed in June 2006, with early results
reported at the APS Division of Nuclear Physics
meeting in Nashville [Lew06, Ric06].
During June 2007, energies around the Ep =
675 keV resonance, from approximately 500 keV
to 1 MeV, were studied in 50 keV steps using
proton beams of energies from 1.20 MeV through
1.60 MeV and a standard commercial aluminum
foil as a beam degrader. Over this range of energies, the beam current on target was adjusted
from 0.5 nA to 10 nA, and the beam energy resolution (on target) varied from approximately 60
to 70 keV.
Becker et al. [Bec87] extracted cross sections
from their data using a calculated line shape
based upon a single level assumption (i.e. the
assumption that higher energy levels in 8 Be had
minimal effects). However, measured α-α phase
shift data show significant deviations from the
single-level model at higher energies (correspond-
ing to higher excitation energy in the 8 Be and
hence lower energy α-particles in our spectrum).
Using the published phase shift data
[Tom63], we were able to extract a line shape
more consistent with data, taking into account
effects from higher energy states in 8 Be (see
Fig. 5.13). We are presently working on calculations of the secondary α background (which
comes from the decay of 8 Be). However, this is a
complicated three-body calculation, which needs
to be done correctly using the correct angular
distribution for the primary α-particles.
hmult
1000
800
600
400
200
0
0
1
2
3
4
5
6
Figure 5.13: (Color online) Comparison of our
lineshape (calculated histogram)
based upon α-α phase shift data and
the analytical single level lineshape
used by Becker (thin line) et al.
[Bec87].
Until we calculate the actual secondary αparticle line shape, we estimate this background
using the shape of the measured α particle energy spectrum. The cross section was determined
using two assumptions for the background. First
we assumed no or minimal background under the
primary peak and fitted just that peak directly
with the calculated line shape, see Fig. 5.14. This
fit provides an upper limit for the cross section
value at each angle. The value of χ2 for these fits
was around 1.7.
E p = 653 keV
Data
Total Fit Func
Entries
1536937
Original a1 Lineshape
Fitted a1 Lineshape
Integral
2.559e+04
θlab= 110°
Fitted a1 Lineshape Extrapolated
100
Counts / Channel
80
69
rent best estimate of the peak cross section on
resonance is 1684 ± 143 mb. This places the
cross section between 23% and 45.5% larger than
the earlier value of Becker et al. [Bec87]. Our
method of fitting the data uses a more complete
model for estimating the line shape based upon
measured data, rather than the purely theoretical
single level approximation used by Becker et al.
[Bec87]. We expect to get a more precise measure of the cross section when a fully calculated
secondary α-particle spectrum is employed.
60
E p = 653 keV
Data
h29
Entries
Total
Fit Func
40
θlab= 110°
Original Background Lineshape
Fitted a1 Lineshape
100
1500
2000
2500
3000
3500
4000
4500
5000
Energy (keV)
Figure 5.14: (Color online) Data on resonance
fitted with the correct line shape
assuming no background under the
peak. This shows the full calculated
line shape based upon measured α-α
phase shifts.
The second fit forced an exaggerated linear
background, which provides a lower bound for
the cross section value, see fig. 5.15. The value of
χ2 for these fits was somewhat higher at around
2.5.
Assuming no background, the extracted value
of the total cross section on resonance is 1822 ± 5
mb, with the uncertainty listed being purely statistical. With a forced linear background, the
extracted value of the total cross section on resonance is 1544 ± 4 mb, with the uncertainty
listed again being purely statistical. In both cases
any background would be from the secondary αparticles. The linear background result is 23%
higher than the previous results of Becker et al.
[Bec87]. The higher result is 45.5% higher than
the results of Becker et al. [Bec87].
In conclusion, based on the average value of
the two extreme results described above, our cur-
Counts / Channel
20
0
1536937
Integral 2.583e+04
Original a1 Lineshape
Fitted Background Lineshape
80
60
40
20
0
2000
3000
4000
5000
6000
Energy (keV)
Figure 5.15: (Color online) Data on resonance fitted with the correct line shape forcing a linear background.
[Bec87] H. Becker, C. Rolfs, and H. Trautvetter,
Z. Phys., A327, 341 (1987).
[Lew06] T. Lewis et al., Bull. Amer. Phys. Soc.,
51, 56 (2006).
[Ric06] A. J. Richards et al., Bull. Amer. Phys.
Soc., 51, 60 (2006).
[Ros04] N. Rostoker, A. Qerushi, and M.
Binderbauer, J. Fusion Energy, 22, 83
(2004).
[Tom63] T. A. Tombrello and L. S. Senhouse,
Phys. Rev., 129, 2252 (1963).
70
5.4.2
Measurements of
11
B(α,α)11 B
R.H. France III, E.J. Sand, A.M. Smith, Georgia College & State University, Milledgeville, GA;
C.R. Driessen, University of Wisconsin, Stevens Point, WI ; S.S. Henshaw, M.W. Ahmed, B.A.
Perdue, S. Stave, H.R. Weller, TUNL; R.M. Prior, M.C. Spraker, North Georgia College
& State University, Dahlonega, GA
Measurements of the differential elastic scattering cross section of the 11 B(α,α)11 B reaction
were made at energies from Eα = 4.2 MeV to Eα = 4.5 MeV and from Eα = 6.0 MeV to Eα
= 7.0 MeV in 100 keV steps.
α-particles from a 5.8 µg/cm2 carbon target (also
placed at 45◦ with respect to the beam) at energies from 4.20 MeV to 4.30 MeV in 10 keV steps.
These α-particle energies cover a well known 4+
resonance located at Eα = 4.256(11) MeV with
a width of Γ = 27(4) keV [AS82].
Our primary source of background is α particles elastically scattered from carbon deposited
onto our target during the course of the experiment. At forward angles, this background peak
is not separable from the α particles scattered
from the 11 B that are of interest, although they
remain separable at backward angles throughout
our energy range (see Fig 5.16). To account for
this background, we took 12 C(α,α) elastic scattering data at every energy where 11 B target data
were taken. The target for the elastic scattering
measurements from 12 C was a 5.8 µg/cm2 carbon foil. These data, combined with the carbon
peaks seen in the backward-angle 11 B data allow
us to deduce the carbon background underlaying
the 11 B data at forward angles.
40
35
30
Counts
In order to extract energy efficiently from a
fusion reactor using 11 B fuel, it is necessary to
understand the elastic scattering of the α particles produced via the 11 B(p,α)2α reaction. These
α particles are produced with energies up to
Eα ≈ 7 MeV. In an experiment complementary
to the 11 B(p,α)2α measurements discussed previously, we started a systematic measurement of
the elastic scattering cross section of 11 B(α,α)11 B
in June 2008.
During June 2008, α-particle beams with energies from Eα = 6.0 MeV to 7.0 MeV and
from 4.2 MeV to 4.5 MeV in 100 keV steps were
used. The beam intensities were 1 to 25 pnA.
The target was composed of 3 µg/cm2 of isotopically pure 11 B sandwiched between two layers of
gold and placed at 45◦ with respect to the incident beam axis. The forward gold layer was 15
µg/cm2 and the backing layer was 80 µg/cm2 .
Eight surface barrier detectors were placed 10 cm
from the target at laboratory angles of 45◦ , 60◦ ,
75◦ , two at 90◦ , 110◦ , 130◦, and 150◦ .
At all energies used, scattering of the α particles from the gold layers was purely Rutherford in
nature. The elastically scattered α particles from
gold were detected at all angles and energies and
will be used to calibrate the relative solid angles
of each detector and for beam current calibration.
The thickness of the 11 B layer was measured
using α-particle beams at Eα = 1.5 MeV and 2.4
MeV. At these energies the scattering of the αparticles from the boron layer is expected to be
purely Rutherford in nature at the forward angles. At the time this report was prepared, this
target thickness data had not yet been analyzed.
Online comparisons of our data with the previously published results of Weller et al. [Ott72],
showed comparable results, except for an apparent beam energy discrepancy. To precisely calibrate our beam energy, we elastically scattered
25
20
15
10
5
0
1000
1100
1200
1300
E [keV]
1400
1500
Figure 5.16: Elastically scattered α-particles from
the 11 B target and 12 C contamination at θlab = 150◦ . During this run
the beam energy was Eα = 7.0 MeV.
Further measurements to cover the energy
ranges of Eα = 2.0MeV to 4.1 MeV and 4.6 MeV
to 5.9 MeV are planned. Analysis of the present
data is ongoing at TUNL and will continue at
Georgia College & State University in the fall.
71
[AS82] F. Ajzenberg-Selove, Nucl. Phys., A375,
1 (1982).
[Ott72] W. R. Ott and H. R. Weller, Nucl. Phys.,
A198, 505 (1972).
72
5.4.3
Cross Section Measurements of the
160 keV
10
B(d,n0 )11 C Reaction Below
S. Stave, M.W. Ahmed, M.A. Blackston, A.S. Crowell, S.S. Henshaw, C.R. Howell, P.
Kingsberry, B.A. Perdue, H.R. Weller, TUNL; A.J. Antolak, B.L. Doyle, P. Rossi, Sandia National Laboratory, Albuquerque, NM ; R.M. Prior, M.C. Spraker, North Georgia College
and State University, Dahlonega, GA
Data were taken to investigate the plausibility of using 140 to 160 keV deuterons and the
11
0 ) C reaction as a source of 6.3 MeV neutrons. An analysis of the data indicates
10 B(d,n
an n0 neutron cross section that is lower than previous estimates by at least three orders of
magnitude, thus making the use of this reaction at these energies impractical. A comparison of
the data with distorted-wave Born-approximation and Hauser-Feshbach calculations suggests
a statistical compound nucleus rather than a direct reaction.
action dynamics at these low energies, the cross
section results have been compared with results
from calculations using the distorted-wave Bornapproximation (DWBA) and a detailed HauserFeshbach calculation (see Fig. 5.17). The angular distribution is consistent with the HauserFeshbach calculation, suggesting a statistical
compound nucleus reaction rather than a direct
reaction.
103
TUNL Data
TUNL fit S=11420 keV b
102
10
Cross section [mb]
There is an interest in using the 10 B(d,n)11 C
reaction as a source of 6.3 MeV neutrons for
active interrogation of special nuclear materials
[Hal07] and explosives [Goz94]. A previous experiment indicated a large (> 10 mb) total neutron cross section at low energies [Fir98]. However, there are no corroborating data in the low
energy region. The next sets of available data
start near 400 keV [May77, Mic90]. Data at
higher energies [VW69, Guz84, Fir98, And81]
show a general trend but have rather large discrepancies. New data were taken at the Triangle
Universities Nuclear Laboratory to investigate
the plausibility of using low energy deuterons and
the 10 B(d,n0 )11 C reaction as a portable source of
6.3 MeV neutrons. Analysis of the data at and
below incident deuteron energies of 160 keV indicates an n0 neutron cross section that is lower
than previous estimates by at least three orders
of magnitude. In separate runs, deuterons with
two different energies (160 and 140 keV) were
stopped in a 10 B target. The resulting n0 neutrons of approximately 6.3 MeV were detected
at angles between 0◦ and 150◦ . The angle integrated yields were used to determine the astrophysical S-factor for this reaction, assuming
a constant value for the S-factor below 160 keV.
The cross sections reported between 130 and 160
keV were calculated using the extracted value
of the S-factor. The measured n0 cross section is several orders of magnitude smaller than
previous results, thus eliminating 10 B(d,n0 )11 C
as a portable source of neutrons with low energy deuteron beams on the order of tens of microamps. In order to gain insight into the re-
Firouzbakht
HF
1
DWBA
10-1
10-2
10-3
10-4
10-5
20
40
60
80
100 120
Ed [keV]
140
160
180
200
Figure 5.17: (Color online) The new TUNL data
(open circles) along with the constant S-factor fit (dashed line) and
the statistical and systematic errors added in quadrature (gray
band). Also included are the HauserFeshbach (HF) and DWBA calculated results as a function of energy.
The three lower energy data points
(solid circles) are total neutron cross
sections from Ref. [Fir98].
The cross section values and the results of the
DWBA and Hauser-Feshbach calculations were
all published this year in Physical Review C
73
[Sta08].
[Hal07] J. M. Hall et al., Nucl. Instrum. Methods, B261, 337 (2007).
[And81] B. Anders, P. Herges, and W. Scobel, Z.
Phys., A301, 353 (1981).
[May77] J. W. Mayer and E. Rimini, Ion Beam
Handbook for Material Analysis, Academic Press, New York, 1977.
[Fir98]
M. L. Firouzbakht et al.,
Nucl.
Medicine and Biology, 25, 161 (1998).
[Goz94] T. Gozani, Nucl. Instrum. Methods,
A353, 635 (1994).
[Guz84] B. J. Guzhovskij, S. N. Abramovich,
and V. A. Pereshivkin, Vop. At. Nauki.
i Tekhn., 2, 55 (1984).
[Mic90] R. W. Michelmann, J. Krauskopf, and
K. Bethge, Nucl. Instrum. Methods,
B51, 1 (1990).
[Sta08] S. Stave et al., Phys. Rev., C77, 054607
(2008).
[VW69] K. Von Wohlleben and E. Schuster, Radiochimica Acta, 12, 75 (1969).
E
xperimental setup for neutron induced cross section measurements at the
shielded neutron source area at TUNL
The Many-Nucleon Problem
Chapter 6
•
Random Matrix Theory
•
Preequilibrium Nuclear Reactions
•
Nuclear Data Evaluation
Neutron-Induced Reactions
γ-Ray-Induced Reactions
•
•
•
Radioactive Decays
76
6.1
6.1.1
Random Matrix Theory
Random Matrices and Chaos in Nuclear Physics
G.E. Mitchell, TUNL; J.F. Shriner, Jr., Tennessee Technological University, Cookeville, TN ;
A. Richter, Technische Universität Darmstadt, Darmstadt, Germany; H.A. Weidenm üller,
Max-Planck-Institut für Kernphysik, Heidelberg, Germany.
The last comprehensive review of random matrix theory in nuclear physics was given by
Brody et al. over 25 years ago. Weidenmüller and Mitchell have prepared a review on this
topic for the Reviews of Modern Physics.
Following the Bohigas-Giannoni-Schmit conjecture (that quantum analogs of classically
chaotic systems obey the Gaussian orthogonal
ensemble or GOE) [Boh84] and the application
of supersymmetric techniques to random matrix
theory (RMT) formalism [Efe83], interest in and
applications of random matrix theory have increased at a very rapid pace. The last major general summary of RMT by Guhr, Müller-Groeling
and Weidenmüller [Guh98] has over 800 references!
ans (in matrix form). The ensemble is defined in
terms of the probability distribution for the matrix elements, thus the name random matrices.
The ensemble is chosen such that the Hamiltonians incorporate generic features. The spectral
distribution functions are calculated as averages
over the ensemble and are compared with the
actual fluctuation properties of nuclear spectra.
Since nuclei are invariant under time reversal, the
nuclear Hamiltonian can be chosen real and symmetric.
However, the last comprehensive review concerning the application of RMT to nuclear
physics was by Brody et al. and was published
in 1981 [Bro81]. At the request of Reviews of
Modern Physics we (Weidenmüller and Mitchell)
have prepared an article entitled Random Matrices and Chaos in Nuclear Physics [Wei08]. This is
intended to be the first part of a two-part series;
the first part focuses on spectra and the second
part is to focus on reactions.
Following a brief definition of the Gaussian
orthogonal ensemble (GOE) and an interpretation of the results for the ensemble (average level density, universality, information content, etc.), we consider the standard GOE fluctuation measures (Porter-Thomas distribution,
nearest neighbor spacing distribution, DysonMehta statistic, etc.). We then provide a comparison of the experimental results with the predictions of RMT. We focus on neutron and proton resonances, low-lying levels, high spin states,
low-lying modes of excitation, and eigenvector
distributions. We then consider both the models and the data for isospin symmetry violation
and for tests of time reversal invariance. Most of
the rest of the review is devoted to the issue of
chaos in nuclear models. In addition to consideration of the shell model and the collective model
in various forms, we also consider random matrix
models that were inspired by nuclear structure
concepts, such as the two-body random ensemble.
Random matrices were introduced into nuclear physics by Wigner around 1960. This
step was preceded by and probably motivated by
Bohr’s insight that nuclei are systems of great
complexity. Bohr’s idea that the nucleus is
a complex, strongly interacting system is most
clearly demonstrated in the compound nucleus
model. Wigner first applied random matrix ideas
to the spacings of neutron resonances. The random matrix approach characterizes spectra by
their fluctuation properties. The distribution of
spacings of nearest neighbors is the first and obvious measure for spectral fluctuations, but there
are numerous other relevant measures. To implement this approach one needs a statistical theory of spectra, and random matrices provide this
tool. One considers an ensemble of Hamiltoni-
Comparison of GOE predictions with experimental data indicate that there is good agreement on spectral fluctuations not only near neutron threshold, but also in the ground state domain. Extensions of the GOE are used to suc-
cessfully describe isospin mixing and to test time
reversal invariance. Studies of the spherical shell
model, of the Nilsson model, and of the interacting boson model show a joint tendency towards
strong mixing of the unperturbed configurations
and towards GOE fluctuation properties. This
resolves the apparent dichotomy between Bohr’s
compound nucleus and the independent particle
model. Typically, the GOE limit is not fully attained.
The evidence strongly supports the view that
chaos is a generic property of nuclei. However, we
are far from having a complete theoretical understanding of chaos in nuclei. Also, in contrast to
few-degrees-of-freedom systems, there is no theoretical framework such as the semiclassical approximation that would establish the connection
between classical chaos and spectral fluctuation
properties of the RMT type.
In last year’s progress report [Mit07] we described the need for a measure that was less sensitive to experimental mistakes such as missing
levels. We suggested the use of the thermodynamic energy that was first proposed by Dyson
and Mehta over 40 years ago but has never been
applied in practice to experimetal data. We
demonstrated that this measure is less sensitive
to missing or spurious levels than is the standard
Dyson-Mehta statistic. This work has now been
published [Shr07] and we plan to apply this measure to experimental data in the near future.
The fragmentation of doorway states into a
complicated background has always been a phe-
77
nomenon of interest (e.g., the fine structure of
analog states), but the detailed study of theoretical predictions has always been hampered
by statistical limitations. In an effort to overcome this difficulty, we have simulated this doorway state mixing into a complicated background
with coupled microwave billiards. This work was
performed in collaboration with Achim Richter’s
group in Darmstadt. The fragmentation of a
doorway was simulated by coupling a very small
billiard with a large billiard. The data are now
being analyzed.
[Boh84] O. Bohigas, M. J. Giannoni, and C.
Schmit, Phys. Rev. Lett., 52, 1 (1984).
[Bro81] T. A. Brody et al., Rev. Mod. Phys., 53,
385 (1981).
[Efe83] K. B. Efetov, Adv. Phys., 32, 53 (1983).
[Guh98] T. Guhr, A. Müller-Groeling, and H. A.
Weidenmüller, Phys. Rep., 299, 189
(1998).
[Mit07] G. Mitchell, M. P. Pato, and J. F.
Shriner, Jr., TUNL Progress Report,
XLIV, 76 (2007).
[Shr07] J. F. Shriner Jr. et al., Nucl. Instrum.
Methods Phys. Res. A581, 831 (2007).
[Wei08] H. A. Weidenmüller and G. E. Mitchell,
Rev. Mod. Phys., (2008), to be published.
78
6.2
6.2.1
Preequilibrium Nuclear Reactions
Study of Preequilibrium Reactions at GEANIE/WNR
G.E. Mitchell, D. Dashdorj, TUNL; Others, LANSCE and T-16, Los Alamos National Laboratory, Los Alamos, NM ; North Carolina State University, Raleigh, NC ; N Division, Lawrence
Livermore National Laboratory, Livermore, CA.
As a part of a broader program to study reaction dynamics of neutron-induced reactions for
radiochemical applications, partial γ-ray cross sections are measured at the LANSCE/WNR
facility. The prompt γ rays are detected with the GEANIE germanium array. The focus
is on improved understanding of preequilibrium emission through measurement of the spin
population of the residual nuclei.
Partial γ-ray cross sections for neutroninduced reactions are being measured as a part of
a broader program to study reaction dynamics for
radiochemical applications. Energetic neutrons
are provided by the Los Alamos National Laboratory spallation neutron source located at the
LANSCE/WNR facility. The prompt-reaction γ
rays are detected with the large-scale Comptonsuppressed germanium array for neutron-induced
excitations (GEANIE). Neutron energies are determined by the time-of-flight technique. Our initial study was for a highly enriched 48 Ti sample
[Das05]. The primary radiochemical interest is in
the (n, n0 ) and (n, 2n) reactions at relatively low
energies (below approximately 20 MeV). However, data were obtained for a wide variety of
reactions, including charged particle production,
and for neutron energies between 1 and 250 MeV.
Partial γ-ray cross sections were determined for
transitions in 43−48 Ti, 44−48 Sc, and 42−45 Ca.
ative population of the final states in the residual nucleus. This relative population is usually
called the spin distribution. The spin distribution for the residual nuclide can be measured by
observing the γ decay.
The first step was to determine the impact
of changes in the spin distribution on the γ-ray
deexcitation cascade and thus on the γ-ray partial cross section. This was investigated using
the gnash reaction code. The preequilibrium
reaction spin distribution was calculated using
the Feshbach, Kerman and Koonin (FKK) formulation. The FKK preequilibrium spin distribution was incorporated into the gnash calculations and the γ-ray production cross sections
were calculated and compared with our experimental data. The difference in the partial γ-ray
cross section using spin distributions with and
without preequilibrium effects is large. The probability of a γ transition from a high spin state is
strongly suppressed because of the prequilibrium
spin distribution. This is illustated in Fig. 6.1
(left panel). Overall the model calculations of
the partial γ-ray cross sections are in good agreement with the measured values. Thus the measurement of the γ-ray cascade should provide a
reliable determination of the relative contribution
of the preequilibrium mechanism. Two papers on
these measurements have been published, one in
Nuclear Science and Engineering (on the partial
cross sections in general) [Das07a] and another
in Physical Review C (on the effects of the spin
distribution) [Das07b].
These results were compared with model calculations that include both compound nuclear
and preequilibrium emission. The model calculations were performed using the stapre code for
En up to 20 MeV and the gnash code for En up
to 120 MeV. The conventional method for determining the relative contribution of the preequilibrium reaction mechanism is to measure neutron angular distributions and to use the amount
of forward peaking to infer the preequilibrium
contribution. However, in practice these experiments are extremely difficult and therefore the
data are rather sparse, with large uncertainties.
An alternate approach is based on the fact that
A longer term goal is to establish the reliabilthe mechanism determines not only the charac- ity of this approach (understanding the preequiteristics of the emitted neutrons, but also the rel- lbrium mechanism through measurement of the
79
0.500
Sm, Eγ = 505.5 keV
GEANIE
CN+GNASH
FKK+GNASH
6+1 → 4+1
0.400
0.5
FKK
Compound
150
0.4
Sm, Spin distribution
0.300
σ γ (b)
Eout = 11 MeV
Probability
150
0.3
0.200
0.2
0.100
0.1
0
0.000
0
1
2
3
4 5 6
J (h/2π)
7
8
9
10
0
5
10
15
20
25
30
35
En (MeV)
Figure 6.1: Left panel: Comparison of the spin distribution for incident neutron energy E n = 20 MeV
and outgoing neutron energy Eout = 11 MeV, calculated with the FKK model (solid histogram) and the compound nuclear reaction (dotted histogram). The smooth curve is a
Gaussian fit to the FKK result. Right panel: Partial γ-ray transition cross section for the
Eγ = 505.5-keV line in the 150 Sm(n, n0 γ)150 Sm reaction, compared with calculations with
the code GNASH. Dotted line: GNASH with the preequilibrium reaction assuming the
compound nuclear spin distribution. Solid line: GNASH with the preequilibrium reaction
assuming the FKK spin distribution.
γ decay of the final states) throughout the periodic table. We performed analysis of previously
measured data on 150 Sm. Cross section measurements were performed on prompt γ-ray production as a function of incident neutron energy (En
= 1 to 35 MeV on an enriched (95.6%) 150 Sm
sample). Above 8 MeV the preequilibrium reaction process dominates the (n, n0 ) reaction. The
spin distributions for the residual nuclei were calculated with the FKK theory and incorporated
into gnash. The difference in the partial cross
section with and without preequilibrium effects is
considerable. This is illustrated in Fig. 6.1 (right
panel). These results have been presented at key
international conferences such as Ref. [Das08]
Future efforts will continue to focus on a comparative study of preequilibrium effects throughout the nuclear periodic table. As our next
project we plan to analyze data previously obtained on tungsten.
Most of this research is supported by an academic alliances grant from the National Nuclear
Security Agency.
[Das05]
D. Dashdorj et al., 2005, LLNL Technical Report UCRL-TR-209474.
[Das07a] D. Dashdorj et al., Nucl. Sci. and Engineering, 157, 65 (2007).
[Das07b] D. Dashdorj et al., Phys. Rev., C75,
054612 (2007).
[Das08]
D. Dashdorj et al., In Compound Nuclear Reactions and Related Topics, p.
185, 2008, AIP Conference Proceedings
1005, Melville, NY.
80
6.2.2
Preequilbrium Reaction Phenomenology
C. Kalbach Walker, TUNL
Work this year has significantly advanced the development of a model for light-projectile
breakup reactions to be included in the preequilibrium reaction model code PRECO. Global
descriptions of the peak energies, widths and shapes have been derived. Work is continuing
on the angular distribution systematics and on the absolute value of the breakup cross section
as a function of target nucleus, breakup channel and incident energy.
For light-particle (A ≤ 4) induced reactions
at incident energies of 14 to 200 MeV, the semiclassical exciton model of preequilibrium nuclear
reactions provides a simple way to describe the
continuum energy and angular distributions of
light particles emitted during energy equilibration. Its advantages are simplicity, physical
transparency, utility, and adaptability.
The TUNL code system, preco, is based on
the exciton model and has been used (either alone
or in Hauser-Feshbach codes) in applied projects,
in support of rare isotope accelerator studies, and
in other basic research. Model development uses
simple physical concepts and relies on available
data to direct choices between alternative formulations and to provide values for key model parameters that cannot be obtained from independent sources.
The exciton model, its associated phenomenological direct-reaction models, the code, and the
global input set are being continually refined and
benchmarked against an ever-broader range of
data from the literature, thus facilitating the reliable calculation of unmeasured or unmeasureable
reactions. Regrettably, work has slowed in recent
years due to funding cutbacks. The current version of the code, preco-2006, was released in
April 2007. Since then, work has centered on
projectile-breakup reactions.
One of the unusual strengths of preco is its
ability to treat reactions with complex particles
in the entrance and/or the exit channel. These
reactions are well-described for incident nucleons at energies up to around 100 MeV and for
α particles at energies up to around 50 MeV. For
loosely bound projectiles—d, t and 3 He—and for
α particles at higher energies, the lack of a model
for projectile breakup makes the description incomplete. Projectile breakup is expected to significantly reduce the amount of the total reac-
tion cross section going into the main excitonmodel equilibration calculations, and without its
inclusion, a definitive assignment of the initial
particle-hole configuration in the exciton model
cannot be made for complex-particle projectiles.
Meanwhile, interest in deuteron-induced reactions is increasing. The International Atomic
Energy Agency is beginning a Coordinated Research Project (CRP) to upgrade the Fusion Energy Nuclear Data Library. One of the important additions will be “data” (mostly generated
by computer models) for deuteron-induced reactions. This is needed to support development
of the International Fusion Materials Irradiation
Facility.
Given the importance of the projectilebreakup mechanism from a basic physics perspective and for energy applications, a phenomenological model is being developed for inclusion in
the next release of preco. This work will be part
of the CRP. Projectile breakup is here defined as
the emission of a projectile fragment with a fairly
narrow energy distribution peaked at an energy
corresponding to the projectile velocity and with
an angular distribution that is sharply peaked
at forward angles. When the undetected fragment interacts with the target nucleus, it forms
a composite system which then undergoes energy
equilibration. Particle emission occuring during
and after that equilibration will also need to be
considered.
The inclusion in the database of data for d,
He and α-particle projectiles allows the role
of the incident particle’s dissociation energy to
be investigated and leads to a more robust and
global model than one developed for a single projectile type. Work this year has improved the description of the peak energies and widths begun
last year. Now the angular distribution systematics are being studied.
3
The simplest estimate of the breakup peak
energy is E0 = Einc Ab /Aa , where Aa and Ab
are the mass numbers of the projectile and the
detected fragment, respectively, and where Einc
is the projectile energy in the laboratory system. The actual peak energy can be shifted
from this value by Coulomb deceleration in the
entrance channel and by Coulomb acceleration
in the exit channel. In the case of dissociative
breakup, in which both projectile fragments continue forward, the requirement of supplying the
projectile’s dissociation energy lowers the peak
energy, but experimental results for both 3 He and
α-particle breakup exclude this as a dominant
mechanism. Instead, they point to absorptive
breakup, in which the non-observed fragment interacts strongly with the target, and the observed
fragment is largely a spectator. For incident α
particles, this observation is supported by coincidence measurements [Koo79].
Using the experimental peak positions for the
heaviest targets, an estimate of the Coulomb
shifts in the peak positions has been used to
extract an effective target-projectile separation,
D0 , at the point of interaction.
81
the experimentally-derived and fitted values for
r0 as a function of the incident energy.
The full width at half maximum (FWHM) of
the breakup peaks is largely independent of emission angle. It can be described by the formula
1
A
F = 62 1 −
1−
exp(Einc /173)
155(Sa,b)2
−3 Θ(Aa − Ab − 1.5),
(6.2)
where F , Sa,b , and Einc are all given in MeV
Here, Sa,b is the projectile dissociation energy
in the observed breakup channel and Θ is the
Heaviside function. The last term in the equation
lowers the FWHM for (3 He,p), (α,p) and (α,d)
breakup relative to the channels where only a single nucleon is absorbed by the target. A comparison of this formula with the data for deuteroninduced reactions is shown in Fig. 6.3.
Figure 6.3: FWHM for deuteron breakup peaks
at the incident energies shown in the
figure. The points show the values
extracted from data in the literature,
while the lines are obtained from Eq.
(6.2).
Figure 6.2: Effective radius parameter for projectile breakup.
The points show
the values inferred from the Coulomb
shifts in the energies of experimental
breakup peaks for the indicated projectiles. The solid curve shows the fitted dependence.
The widths, the Gaussian shapes, and, in extreme cases, even the positions of the breakup
peaks can be modified by either the Coulomb barrier or the maximum-energy cutoff in the spectrum. These effects have been described.
Work is continuing on the absolute value of
the breakup intensity and on the angular distriThese results have been fit with the formula bution systematics. Preliminary results indicate
D0 = r0 A1/3 + 1.2, where r0 is an effective ra- that the cross section for a given breakup chandius parameter that depends only on the incident nel and incident energy is proportional to (D0 )2
energy. It is found to have the form
and that the angular distributions generally fall
off exponentially with the emission angle.
5
r0 = 1.2 +
.
(6.1)
1 + exp(Einc /30 MeV)
Here A is the target mass number, and both D0 [Koo79] R. W. Koontz et al., Phys. Rev. Lett.,
43, 1862 (1979).
and r0 are given in femtometers. Figure 6.2 shows
82
6.2.3
Preequilbrium Model Comparisons for 96 MeV (n,xn) Reactions
C. Kalbach Walker, TUNL; Others, Uppsala University, Sweden; Swedish Defense Research
Agency; George Washington University, Washington DC ; University of Caen, France; Centre de
Recherche, Caen, France; CEN Bruyères-le-Châtel, France; Kailua-Kona, Hawaii; Japan Atomic
Energy Agency; Nuclear Research Consultancy Group, Petten, the Netherlands; Kyushu University,
Japan
Double differential cross sections for inclusive inelastic neutron scattering at 96 MeV on
four targets have been measured in Uppsala, Sweden. The data are compared with calculated results from five reaction codes using different preequilibrium models and no adjustable
parameters. There is general agreement between the results from different codes, and all
reproduce the general features of the data, but there are also important differences.
An international group has undertaken to reanalyze existing data that were initially measured at the Svedborg Laboratory in Uppsala,
Sweden to study neutron elastic scattering at
96 MeV. The reanalysis uses a novel technique
to extract inclusive inelastic scattering doubledifferential cross sections. The motivation for
this work was the data needs for accelerator
driven technologies for burning nuclear waste.
The four target nuclides studied are 12 C, 56 Fe,
89
Y, and 208 Pb, and the data cover the upper 45
MeV of the emission energy range at laboratory
angles between 26 and 65 degrees. The observed
energies thus exclude the evaporation peaks and
emphasize the preequilibrium part of the spectrum. As part of this work, various groups provided blind calculations of the observed quantities, each using their standard input parameter
values. The codes employed are:
• hms-alice, using the hybrid Monte-Carlo
simulation model (calculations by M.
Blann);
• brieff, an intranuclear cascade (INC)
model code (calculations by H. Duarte);
and
• qmd, a quantum molecular dynamics code
similar to the INC but replacing point particles with Gaussian wave packets (calculations by S. Chiba).
While details of the comparisons are still being worked out, this work provides a fortuitous
intercomparison of the predicitive ability of various models. Bearing in mind that not all the
codes include all of the same reaction mechanisms
(for instance hms-alice and brieff obviously do
not include collective state excitations), the over• preco, the TUNL preequilibrium code all agreement is encouraging, but there are also
based on the two-component exciton model significant differences that need to be better understood, and all of the models currently have
(calculations by C. Kalbach Walker);
problems in reproducing the 12 C data. Overall,
• talys, a more complete Hauser-Feshbach preco’s results are the most successful, though
model reaction code using a slightly differ- this may partially result from the use of these calent version of the two-component exciton culations in one step of the data analysis. This
model (calculations by A. Koning);
possibility still needs to be investigated.
84
6.3
Nuclear Data Evaluation
6.3.1
Nuclear Data Evaluation Activities
J.H. Kelley, E. Kwan, J.E. Purcell, C.G. Sheu, D.R. Tilley, H.R. Weller, TUNL
The Nuclear Data Evaluation Group at TUNL is part of the United States Nuclear Data Program (USNDP) and the International Nuclear Structure and Decay Data network (NSDD).
After the retirement of Fay Ajzenberg-Selove in 1990, TUNL assumed responsibility for evaluation of nuclides in the mass range A = 3 − 20. The status of the published evaluations and
preliminary reviews is presented in Table 6.1.
Along with producing evaluations of the A =
3-20 nuclei in the “Energy Levels of Light Nuclei” series that is published in Nuclear Physics
A, the Nuclear Data Evaluation Group has
been charged with providing the corresponding updates to the Evaluated Nuclear Structure Data Files (ENSDF) database that is maintained at the National Nuclear Data Center
(NNDC) at Brookhaven National Laboratory.
We also provide a web-based service for the nuclear science and applications communities at
http://www.tunl.duke.edu/nucldata/.
6.3.1.2
Publications
Table 6.1 displays the status of our most recent
published evaluations and preliminary reviews.
A first draft of a new evaluation for A = 3 is
anticipated for late 2008.
Table 6.1: Current publication status
Nuclear Mass
Publication
Institution
Published:
A
A
A
A
A
A
A
A
A
=
=
=
=
=
=
=
=
=
3
4
5−7
8 − 10
11 − 12
13 − 15
16 − 17
18 − 19
20
Nucl.
Nucl.
Nucl.
Nucl.
Nucl.
Nucl.
Nucl.
Nucl.
Nucl.
Phys.
Phys.
Phys.
Phys.
Phys.
Phys.
Phys.
Phys.
Phys.
A474
A541
A708
A745
A506
A523
A564
A595
A636
(1987)
(1992)
(2002)
(2004)
(1990)
(1991)
(1993)
(1995)
(1998)
1
1
3
155
1
1
1
1
247
TUNL
TUNL a
TUNL a,b
TUNL c
Penn d
Penn d
TUNL
TUNL
TUNL e
Nuclear
Structure
The ENSDF files contain concise nuclear structure information such as tables of adopted level
energies and tables of properties for levels that
have been observed in various nuclear reactions
and decays. The ENSDF files are updated concurrently with the last published reviews in the
“Energy Levels of Light Nuclei” series.
Work on the A = 3 and A = 11-13 ENSDF
files is presently underway.
6.3.1.3
6.3.1.1
Evaluated
Data Files
World Wide Web Services
Our group continues to develop web-based services for the nuclear science and applications
communities. The website layout and contents
are constantly revised and kept up to date to ensure high-quality service and accurate information. Figure 6.4 displays the usage of our website
from the nuclear science communities since April
2002.
The following items are currently available:
• A new area of our website is being developed to provide evaluated data from recent work on ground-state β and chargedparticle decays, as well as compiled data
from earlier measurements. All the β-decay
and more than half of the charged-particle
decay data for nuclides A = 3-20 are currently online.
Reviews in Progress:
A = 3
A = 11 − 13
TUNL
TUNL
a Co-authored with G.M. Hale, LANL.
b Co-authored with H.M. Hofmann, Universität ErlangenNürnberg, Germany.
c Co-authored with D.J. Millener, BNL.
d F. Ajzenberg-Selove, University of Pennsylvania.
e Co-authored with S. Raman, ORNL.
• Thermal Neutron Capture Data provides
evaluated data from recent work on thermal neutron capture, as well as compiled
data from earlier measurements.
• PDF and HTML documents are online
for TUNL’s and Fay Ajzenberg-Selove’s re-
views from 1959 to the present. The PDF
versions include hyperlinks for references,
Tables of Recommended Level Energies,
Electromagnetic Transitions Tables, General Tables, Energy Level Diagrams, and
Erratum to the Publications. The HTML
documents are more comprehensive than
the PDF documents, as they include hyperlinks to: tables in the PDF and PS formats;
reactions and reaction discussions; TUNL
and NNDC references; Energy Level Diagrams; and General Tables.
Re-creation of PDF files for our publications is presently in progress, and the files
are available from 1970 to the present.
The goal is to provide the most current
NNDC reference keys and to correct all errors found since the articles went to press.
We will continue to work on A = 5-20 for
1959–1968 PDF documents.
• Energy Level Diagrams for the publication
years 1959-present are provided in GIF,
PDF and EPS/PS formats.
• Tables of Energy Levels provides a brief
listing of the tables of recommended energy levels [in PDF and PS formats] from
85
the most recent publication for nuclides in
A = 4-20.
• General Tables that reference theoretical
work related to TUNL’s most recent reviews are available on our website for the
masses A = 5-10. The tables include dynamic links to the NSR database.
• ENSDF information for A = 3-20 nuclides
is available through the NNDC site.
• A link to NuDat, which allows users to
search and plot nuclear structure and nuclear decay data interactively, is available
through the NNDC site.
• Palm Pilot Physics Page provides links to
Palm applications and databases that are
of interest to the Nuclear Physics community.
• Links to the National Nuclear Data Center and other useful sites, as well as to the
online electronic journals that the nuclear
science communities use most often are provided, along with a sitemap that contains a
complete listing and links to everything on
the website.
Figure 6.4: (Color online) Overview of web usage deduced from Analog Web Analysis Package
86
6.4
6.4.1
Neutron-Induced Reactions
Neutron-Induced Partial γ-Ray Cross-Section Measurements on 235,238 U
A. Hutcheson, C.T. Angell, M. Boswell, A.S. Crowell, J.H. Esterline, B. Fallin, C.R.
Howell, J.H. Kelley, H.J. Karwowski, M.R. Kiser, A.P. Tonchev, W. Tornow, TUNL;
J.A. Becker, D. Dashdorj, R.A. Macri, Lawrence Livermore National Laboratory, Livermore,
CA; R.O. Nelson, Los Alamos National Laboratory, Los Alamos, NM ; R.S. Pedroni, NC A&T
State University, Greensboro, NC ; G.J. Weisel, Penn State Altoona, Altoona, PA
Precision measurements have been performed on
235,238 U
targets at Triangle Universities
Nuclear Laboratory using a pulsed, monoenergetic neutron beam. The excitation function
of the (n, 2n) reaction has been studied with incident energies between 5 and 14 MeV and
a beam flux of 104 n s−1 cm−2 at the target position. Multiple (n, n0 γ) and (n, 2nγ) partial
cross sections have been measured using clover and planar high purity germanium (HPGe)
detectors, and results are compared with the Hauser-Feshbach model.
A pulsed and collimated neutron beam of 5
cm (FWHM) diameter was produced via the
2
H(d,n)3 He reaction. Using a deuteron beam
current of 1 µA the neutron flux was on the order
of 104 n/(cm2 s) at a distance of 215 cm from the
deuterium gas cell (filled to 7.8 atm). The total
neutron energy spread ranged from ∆En = 1.1
MeV at En = 6 MeV to ∆En = 0.36 MeV at En
= 14 MeV. The repetition rate was 2.5 MHz with
a typical pulse width of 2 ns. A sample energy
spectrum at En = 10 MeV taken with a clover
detector is shown in Fig. 6.5.
1000
800
Counts
In support of the Stewardship Sciences Academic Alliances initiative, precision measurements have been performed on actinide targets
at TUNL. This activity represents a joint effort between TUNL, LANL, and LLNL to measure the (n,xnγ) cross sections using in-beam
γ-spectroscopy. The excitation function of the
(n,2nγ) reaction on 235,238 U has been studied
with incident neutron energies between 5 and 14
MeV. Our goal is to improve the partial crosssection data for the National Nuclear Security
Administration’s (NNSA) Stockpile Stewardship
Program with special emphasis on the cross sections for the 239 Pu(n,2nγ)238 Pu reaction. The
relatively close spacing of the levels in these nuclei and the background from fission neutrons
make direct measurements of neutron inelastic
scattering quite difficult. However, measurements of the emitted γ rays can provide useful
information on inelastic scattering and (n,xnγ)
reaction cross sections. Reaction channels leading to even-mass nuclei are particularly suitable
for this technique. In this case, the γ-ray energies
are sufficiently large and the decay yields are high
enough to observe most of the decays with highresolution γ-ray detection systems. The method
used is both direct and indirect: the cross section is inferred from partial γ-ray cross sections
measured as a function of En . Advanced HauserFeshbach theory, implemented in the gnash and
talys codes, will link the partial and the total
cross sections.
The measurements were carried out in the
shielded neutron source target area at TUNL.
238
238
150
160
Energy (keV)
170
U(n,n’) U
158.8 keV
237
238
U(n,2n) U
148.6 keV
600
400
200
140
Figure 6.5: (Color online) Prompt γ-ray spectrum
taken with a 238 U target.
In addition, “empty target” measurements
have been taken at each of these incident neutron
energies in order to investigate beam-correlated
The TUNL pulsed and monoenergetic neutron beam has proven to be an excellent tool
for high-precision γ-ray spectroscopy and thus
can contribute significantly to the National
Stockpile Stewardship program. Cross sections
have been measured for the 238 U(n, n0 γ) and
235,238
U(n, 2nγ) reactions for 5 MeV ≤ En ≤
14 MeV, and results are generally in good agreement with existing data [Fot04, McN99, Ols79]
and provide cross-section data for transitions in
energy regions where none previously existed.
Comparison with model calculations is underway. Sample data for measured cross sections
are shown in Figs. 6.6, 6.7, and 6.8
This work is supported in part by the NNSA
under the Stewardship Science Academic Alliances Program through DOE Research grant
DE-FG52-06NA26155.
TUNL
Olsen
GNASH
TALYS
60
50
40
30
20
10
0
2
Cross Section (mb)
6
En (MeV)
8
10
600
TUNL
McNabb
GNASH
500
400
300
200
100
700
TUNL
Fotiades
GNASH
TALYS
4
Figure 6.7: (Color online) 238 U(n, n0 γ) partial
cross sections for the 519.5 keV transition.
0
5
600
87
70
Cross Section (mb)
events not resulting from the target that may interfere with the γ-ray spectra. A number of lines
resulting from scattering from materials found in
the detectors and/or shielding (e.g., Pb, Ge, Al,
and Bi) have been observed. Furthermore, because the reaction partial cross sections are normalized to 56 Fe(n, n0 γ) partial cross sections, the
angular distribution of γ-rays emitted from 56 Fe
were also measured at incident neutron energies
of En = 8 and 12 MeV. In each of these measurements, a combination of clover and planar HPGe
detectors were utilized. A detailed description of
the NNSA setup can be found in Ref. [Hut07].
Cross Section (mb)
10
En (MeV)
15
20
Figure 6.8: (Color online) 235 U(n, 2nγ) partial
500
400
[Fot04]
300
[Hut07] A. Hutcheson et al., Nucl. Instrum.
Methods, B261, 369 (2007).
200
100
0
0
N. Fotiades et al., Phys. Rev., C69,
024601 (2004).
5
En (MeV)
10
238 U(n, n0 γ)
15
Figure 6.6: (Color online)
partial
[McN99] D. McNabb et al., In Symp. on Capture Gamma Ray Spectroscopy, p. 384,
Santa Fe, NM, 1999.
[Ols79]
D. Olsen, G. Morgan, and J. McConnell, In Conf. on Nucl. Cross Sections for Technology, p. 677, Knoxville,
1979.
88
6.4.2
Neutron-Induced Reaction Cross-Section Measurements on GaAs
G. Rusev, B. Fallin, C.R. Howell, A. Hutcheson, J.H. Kelley, A.P. Tonchev, W.
Tornow, TUNL; R.S. Rundberg, D.J. Vieira, J.B. Wilhelmy, Los Alamos National Laboratory, Los Alamos, NM ; J.A. Becker, D. Dashdorj, R.A. Macri, M.A. Stoyer, C.Y. Wu,
Lawrence Livermore National Laboratory, Livermore, CA
Cross sections for the reactions (n, p) and (n, 2n) on 69 Ga, 71 Ga, and 75 As have been measured at
En = 9.5, 10.8 and 14.5 MeV using the neutron-activation technique. Monoenergetic neutron
beams were produced via the 2 H(d, n)3 He reaction at the TUNL 10 MV tandem accelerator
facility. The neutron-induced reaction cross-section data are compared to literature values
and to predictions from the code EMPIRE, which is based on the Hauser-Feschbach model.
GaAs is an important semi-conducting material. Its electrical properties depend strongly
on the purity and the defects of the monocrystal. Exposure of a semiconductor to a flux of
fast neutrons leads to transmutations and thus
increases the impurity content of the material.
Defects in the crystal can be produced by all
types of neutron-induced reactions, due to the
high energy of fast neutrons. Therefore, the
neutron-induced reaction cross sections on GaAs
have been studied extensively over the past five
decades. In support of our application-driven research, we carried out neutron-reaction experiments on GaAs at TUNL to provide more accurate data than available in the literature. These
data are needed to interpret the results of past
test explosions of nuclear devices.
Figure 6.9: (Color online) Cross section for the
(n, p) reaction on 69 Ga. The results
from the present work, shown as filled
circles, are compared with data from
Refs. [Bor62], [Nes03], and [Shi04]
depicted with open symbols.
We performed neutron-activation measure-
ments on GaAs at energies of En = 9.5, 10.8 and
14.5 MeV. The neutron beams were produced via
the 2 H(d, n)3 He reaction. A 3 cm × 1 cm gas cell,
sealed to the accelerator vacuum by a thin Havar
foil, and filled with deuterium gas at a pressure of
3 atm, was bombarded by a 2 µA deuteron beam,
producing quasi-monoenergetic neutrons. A 1 cm
× 1 cm GaAs sample was placed 3.3 cm downstream of the gas cell. The average neutron flux
at the GaAs-sample position varied from 1 × 107
to 5 × 107 cm−2 s−1 depending on the deuteronbeam energy. Each sample was combined with
foils of nat Al, nat Ni, and nat Au for neutron-flux
determination purposes. After irradiation, the
activity of the GaAs sample and the monitor foils
was determined in a low-background area with
three measuring stations consisting of 60% highpurity Ge detectors and Pb shields. The experimental setup and the analysis of the data are
presented in more detail in Ref. [Ton08].
The results of the present work are shown in
Figs. 6.9, 6.10, 6.11, 6.12, and 6.13. They are
compared with data from the literature and predictions from the code empire using a default
set of parameters. Due to the conflicting data for
some of the reaction cross sections, we plan future
investigations of the neutron-induced reaction
cross sections on 69 Ga, 71 Ga, and 75 As at various neutron-beam energies, starting from the reaction threshold, and using isotopically enriched
samples. The results will be compared with predictions from the Hauser-Feschbach model.
This work was performed under the auspices of the U.S. Department of Energy
at Los Alamos National Laboratory by the
Los Alamos National Security, LLC under
Contract No.
DE-AC52-06NA25396 and at
Lawrence Livermore National Laboratory by
89
Lawrence Livermore National Security, LLC under Contract No.
DE-AC52-07NA27344.
(n, p) reaction on 71 Ga.
The results from the present work, shown
as filled circles, are compared with
data from Ref. [Nes03] depicted with
open triangles.
(n, 2n) reaction on 75 As.
The results from the present work, shown
as filled circles, are compared with
data taken from Refs. [Bor68] and
[Kon93] depicted with open symbols.
This work was also supported in part by the
National Nuclear Security Administration, Office of Nonproliferation Research and Engineering (NA-22), of the United State Department
of Energy and by the National Nuclear Security Administration under DOE grant DE-FG5206NA26155.
[Bir94] I. Birn et al., Nucl. Science and Eng.,
116, 125 (1994).
[Bor62] M. Bormann et al., Z. Phys., 166, 477
(1962).
(n, p) reaction on 75 As. The results
from the present work, shown as
filled circles, are compared to data
taken from Refs. [Bor67], [Oku67],
[Kon93], and [Bir94] depicted with
open symbols.
[Bor65] M. Bormann et al., Nucl. Phys., 63, 438
(1965).
[Bor67] M. Bormann et al., Nucl. Data for Reactors Conf., 1, 225 (1967).
[Bor68] M. Bormann et al., Nucl. Phys., A115,
309 (1968).
[Kon93] C. Konno et al., JAERI Reports, 1329
(1993).
[Nes03] C. D. Nesaraja et al., Phys. Rev., C68,
024603 (2003).
[Oku67] S. Okumura,
(1967).
[Shi04] T. Shimizu et al., Ann. Nucl. Energy,
31, 975 (2004).
(n, 2n) reaction on 69 Ga. The results from the present work, shown
as filled circles, are compared to data
taken from Ref. [Bor65] depicted
with open triangles.
[Ton08] A. P. Tonchev et al., Phys. Rev., C77,
054610 (2008).
90
6.4.3
Mixed-Symmetry States and Anomalous Decays in
94
Zr
E. Elhami, S.N. Choudry, M.T. McEllistrem, S. Mukhopadhyay, J.N. Orce, M. Scheck,
S.W. Yates, University of Kentucky, Lexington, KY ; A.P. Tonchev, C.T. Angell, M. Boswell,
B. Fallin, C.R. Howell, A. Hutcheson, H.J. Karwowski, J.H. Kelley, Y. Parpottas,
W.Tornow, TUNL
In a recent study of the even-even nucleus 94 Zr54 with the (n,n0 γ ) reaction, an interesting
new situation was encountered. The 2+ state at 1671.4 keV was identified as the lowest
mixed-symmetry state in this nucleus, because the 752.5-keV transition from this state to the
first excited state has a large B(M1) value of 0.31 ± 0.3 µ2N . To expand our knowledge of
this anomalous behavior in 94 Zr, we have performed γγ coincidence measurements following
inelastic neutron scattering at the TUNL shielded neutron source area.
Mixed-symmetry (MS) states have emerged
as a structural feature of many nuclei and have
been shown to act as building blocks upon which
multiphonon excitations can occur in weakly deformed nuclei. The expected signatures for MS
states are strong M1 transitions to selected symmetric states, with matrix elements of about 1
µN . MS states in the N = 52 nuclei, 92 Zr, 94 Mo,
and 96 Ru have been indentified, and two-phonon
excitations have been observed [Tso05, Fra03,
Pie01].
If MS states are a general feature of nuclei,
they should also be evident in odd-mass nuclei.
In 93 Nb, the only stable odd-mass N = 52 isotone, 5/2− and 3/2− states corresponding to
the π(2p1/2 −1 )⊗(2+ M S 94 Mo) coupling have been
found, and the M1 and E2 transition strengths of
their decays to the π(2p1/2 −1 )⊗(2+ 1 94 Mo) symmetric one-phonon states have been determined
[Orc06].
In an extension of these studies to N = 54
nuclei, 96 Mo was examined to see if it follows the
systematic behavior of MS excitations [Les07].
The 2+ MS state was identified; however, the
B(M1; 2M S + → 21 + ) is a factor of two smaller
than that in 94 Mo, even though the number
of active bosons in 96 Mo is greater. The 1+
two-phonon MS strength is fragmented into two
states, and candidates were proposed for the 2+
and 3+ two-phonon states [Les07, Elh07]. To
clarify the situation, it is desirable to study additional N = 54 nuclei.
The E2 transition from the 1671.4-keV state
to the ground state has an unusually large B(E2)
of 7.8 ± 0.7 W.u., which is larger than the
+
B(E2;2+
1 → 0gs ) of 4.9 ± 0.3 W.u. The M1
transition strength is in agreement with interacting boson model IBM-2 predictions in the U(5)
vibrational limit, whereas the large B(E2;2+
MS
0+
gs ) value significantly exceeds the E2 strength
predicted by the IBM-2. For the first time, the
E2 transition to the ground state from a higherlying excited 2+ state has been observed to have
a larger E2 transition strength than the 2+
1 →
0+
gs decay.
The 2 H(d,n)3 He reaction was used as a neutron source. Neutrons produced from the gas cell
were formed into a 4.5-cm beam using a double
truncated copper collimator, 78.7 cm long. The
20.45 g isotopically enriched (98.57% ) 94 Zr sample was contained in a cylindrical vial with a diameter of 2.6 cm and a height of 3.9 cm. The
sample was hung coaxially with the beam, and
three 100% efficient (relative to 3 in. × 3 in.
NaI) clover detectors were placed in a horizontal arrangement approximately 6 cm from the
center of the zirconium oxide sample. The γγ
coincidences were measured at incident neutron
energies of 5.0 MeV.
Data were stored in event mode, using a
scripted version of SpecTcl. A two-dimensional
matrix was also constructed offline by a .tcl script
in the SpecTcl environment by considering pairwise coincidences, and it was analyzed off-line
with Tv spectral analysis software. Since a continuous beam was used for these measurements,
a quadrant of one of the clover detectors was
used as a universal time reference for recording
any event in all the other quadrants of the three
clover detectors. The coincidence resolving time
between any 2 clovers was set electronically to
about 120 ns to record coincident events. More
stringent time constraints were applied when the
Eγ - Eγ coincidence matrix was constructed offline. An example of a coincidence spectrum,
+
gated on the 2+
1 → 0gs transition, is shown in
91
550.8
18
0
400
600
800
1000
E(keV)
1200
1400
1589.1
1411.1
4
1232.4
752.5
8
1138.8
12
381.5
Counts(x100)
Coincidence Spectrum gated on 918.8 keV
1600
Figure 6.14: (Color online) Coincidence spectra from gate set on the 918.8 keV γ ray from the first
94 Zr excited state.
Fig.6.14.
Over the past year, 38 new γ-ray transitions
were identified and placed in the level scheme up
to 3.7 MeV. Even though the 2+
5 state at 2908
keV exhibits the signatures of a member of the
two-phonon mixed-symmetry multiplet, B(M1;
+
+
2
2+
5 → 23 ) = 0.18(2) µN , and the 23 state appears
to be a two-phonon symmetric state, these characterizations cannot be ascribed with certainty
because of the anomalous behavior of the 2+
1,M S
state. An example of further anomalous behavior is the very interesting decay of the 4+
2 state,
+
+
with B(E2; 4+
→
2
)
>
B(E2;
4
→
2+
2
1
1
1 ).
94
Overall, the decay scheme of Zr indicates that
the levels belong to two different groups, and the
vibrational model seems to be inappropriate in
explaining their decay behavior [Elh08]. Shellmodel calculations are being performed to shed
light on the structure of this interesting nucleus.
This work is partially supported by the U.S.
National Science Foundation under Grant No.
PHY-0652415.
[Elh07] E. Elhami et al.,
011301(R) (2007).
Phys. Rev. C75,
[Elh08] E. Elhami et al., Phys. Rev. C (2008),
in press.
[Fra03] C. Fransen et al.,
024307 (2003).
Phys. Rev. C67,
[Les07] S. R. Lesher et al., Phys. Rev., C75,
034318 (2007).
[Orc06] J. N. Orce et al., Phys. Rev. Lett. 97,
062504 (2006).
[Pie01] N. Pietralla et al.,
031301 (2001).
Phys. Rev. C64,
[Tso05] C. Fransen et al.,
054304 (2005).
Phys. Rev. C71,
92
6.5
6.5.1
γ-Ray-Induced Reactions
Photodisintegration Cross Section Measurements for 142 Nd and 150 Nd
and Low-energy E1 γ-ray Strength Functions
C.T. Angell, TUNL; H. Utsunomiya, S. Goko, A. Makinaga, T. Kaihori, Konan University,
Kobe, Japan; H. Toyokawa, AIST, Tsukuba, Japan; Y.W. Lui, Texas A&M University, College
Station, TX
p-process nucleosynthesis has received much attention lately. New cross-section measurements
are needed to constrain key theoretical inputs in reaction models. The photo-neutron disintegration cross sections were measured near threshold for 142 Nd, and, for the first time, 150 Nd.
The measurements were made using the monoenergetic γ-ray beam at the AIST TERAS facility in Tsukuba, Japan. The technique and motivation were presented previously. The final
results of the analysis are presented here.
Direct measurements of the cross-sections determining the reaction rate is unfeasible for most
nuclei involved in the p-process because they are
radioactive. For nuclei whose cross-sections are
unknown, the statistical model is used to calculate the cross sections, using theoretical models
to calculate the input parameters. A key input
parameter is the γ-ray strength function (γSF)
[Arn03]. Microscopic models must be tested and
constrained to be reliable for calculating parameters of nuclei far from stability. The quasiparticle random phase approximation (QRPA)
predicts an order of magnitude increase in the
relative low-energy γSF from 142 Nd to 150 Nd. By
measuring the (γ, n) cross-section for these two
nuclei near the neutron separation energy, the
low-energy γSF can be constrained, and a key
test of the QRPA can be made.
Further details on the motivation and technique for this work are given in Ref. [Ang07].
The neutron photodisintegration crosssection was measured for 142 Nd and 150 Nd at the
AIST TERAS facility in Tsukuba, Japan [Uts06],
in January 2007. A quasi mono-energetic beam
of γ rays, generated via laser inverse-Compton
scattering, was used. The photodisintegration
neutrons were measured using a composite detector. It consists of 20 3 He proportional counters
embedded in three concentric rings in a polyethylene block. The first ring has four 3 He detectors,
and the two other rings each have eight. The
polyethylene acts as a neutron moderator. The
beam profile was measured at each beam energy
using a high-purity Ge detector. The flux was
monitored online using a large volume NaI detector. The beam has a width of ∼5-10%, with
a typical flux of 104 to 105 γ/s, depending on
beam energy and tune.
Nd142(g,n) Cross Section
Cross Section [mb]
Most elements heavier than Fe are produced
via successive neutron capture. There are, however, 35 nuclei in the region between Fe and Pb
which cannot be created via either the s- or rprocesses. They are known as the p-nuclides, and
they lie on the proton-rich side of the valley of
β-stability. They are produced by photodisintegration reactions on existing heavy seed nuclei
generated earlier via the s- or r-process. This
process is referred to as the p-process.
Present
Carlos et.al.
QRPA
Lorentzian
100
10
9.5
10 10.5 11 11.5 12 12.5 13 13.5
Energy [MeV]
Figure 6.15: (Color online) The results of the
142 Nd (γ, n) cross-section measurements compared with values from
[Car71]. Theoretical calculations for
the Lorentzian and QRPA models
are also shown, with the QRPA
clearly under-predicting the cross
section near threshold.
Cross Section [mb]
Nd150(g,n) Cross Section
Present
Carlos et.al.
QRPA
Lorentzian
100
10
7.5
8
8.5
9
9.5
10
10.5
11
11.5
Energy [MeV]
Figure 6.16: (Color online) The results of the
150 Nd(γ, n)
cross-section measurements compared with values from
[Car71]. Theoretical calculations for
the Lorentzian and QRPA models
are also shown.
For 142 Nd, the (γ, n) cross section was measured at 15 energies, ranging from 9.9 MeV to
13.3 MeV. For 150 Nd, 18 energies were measured,
ranging from 7.4 MeV to 11.3 MeV. For 150 Nd,
this is the first time that the (γ, n) cross section has been measured near the neutron separation energy. The measurements for 142 Nd
improve on the precision of previous measurements. The analysis of the data has been completed, and theoretical calculations were done us-
93
ing the QRPA [Gor07]. For 142 Nd, the results
compared to theory can be seen in Fig. 6.15, and
for 150 Nd in Fig. 6.16. The phenomenological
Lorentzian model for the γSF agrees well with
the measurements for both nuclei, but the microscopic QRPA calculations under-predict the
strength near the neutron separation energy for
142
Nd. It has been proposed that an additional
resonance is needed to account for the additional
strength, as was recently proposed for the Zr isotopes [Uts08]. This possibility is being explored,
and measurements of the γSF below the neutron
separation energy have been recently performed
at HIγS (see Sect. 7.3.3).
[Ang07] C. T. Angell et al., TUNL Progress Report, XLVI, 42 (2007).
[Arn03] M. Arnould and S. Goriely, Phys. Rep.,
384, 1 (2003).
[Car71] P. Carlos et al., Nucl. Phys., A172, 437
(1971).
[Gor07] S. Goriely,
2007.
Private communication,
[Uts06] H. Utsunomiya et al.,
A777, 459 (2006).
Nucl. Phys.,
[Uts08] H. Utsunomiya et al., Phys. Rev. Lett.,
100, 162502 (2008).
94
6.6
6.6.1
Radioactive Decays
Attempt to Manipulate the Decay Rate of Radioactive Nuclei
B. Fallin, B. Grabow, W. Tornow, TUNL
We measured the decay rate of 64 Cu at room temperature and at 12K and did not observe
any difference between the two half-life times within an uncertainty of 0.3%.
Ever since Rutherford and Soddy [Rut02]
published their theory of radioactive decay in
1902 it was widely assumed that the decay rate
of radioactive nuclei cannot be changed by external, non-nuclear processes. It was not until 1947
that this assumption was questioned by Segrè
[Seg47]. However, during the past 60 years, only
very small changes in the decay rate have been
observed in a very specific radioactive decay, the
electron capture (EC) process on a nucleus. The
most studied example is 7 Be (see Ref. [Ray99]
and references therein).
The situation has recently changed dramatically. Limata et al. [Lim06] observed an increase
of (1.2 ± 0.2)% in the positron (β + ) decay rate
(i.e., a shorter half-life) for 22 Na implanted in a
metallic environment of Pd at a temperature of
T = 12 K. Spillane et al. [Spi07] found that the
half-life for the β − decay of 198 Au in a metallic
environment was longer by (4.0 ± 0.7)% when
the metal was cooled to T = 12 K compared to
a room-temperature measurement. Even more
strikingly, Raiola et al. [Rai07] reported a (6.3
± 1.4%) decrease in half-life for the α-decay of
210
Po embedded in a Cu environment at T = 12
K. The results of [Lim06, Spi07, Rai07] are quite
controversial and they are seriously questioned
by both theoreticians [Lan07, Zin07] and experimentalists [Cze06, Goo07, Sto07, Rup08].
It has been known for 20 years that electron screening strongly changes nuclear reaction
rates at sub-Coulomb projectile energies [Ass87].
About five years ago it was observed that the
screening energy can be increased considerably
if the target atoms are implanted in a metallic environment and cooled to low temperature
[Rai02]. An explanation of the large screening in
metals can be obtained from the Debye-Hückel
plasma model [Deb23] applied to the quasi-free
metallic electrons. If time reversed, this model
implies that the half-life of radioactive nuclei im-
planted in a metallic host at low temperature
can be manipulated by orders of magnitude. For
α and β + decay, one expects a shorter half-life,
due to the accelerating action of the Debye electrons; while for β − decay and EC, one expects
a longer half-life. If true, this conjecture has far
reaching consequences, from cosmo-chronometers
to transuranic radioactive waste disposal [Rol06].
In an attempt to contribute to the present debate
and to provide accurate data, we decided to focus
on the β/EC reaction of 64 Cu with an accepted
half-life (T1/2 ) of 12.701 ± 0.002 hours.
Figure 6.17: Decay scheme of
64 Cu.
The 64 Cu sample in our experiment was produced via the 63 Cu(d, p)64 Cu reaction using a
1 mm thick natural, high-purity copper disk.
We used an incident deuteron beam of Ed =4.40
MeV and about 50 µA·h of exposure to produce
the sample. Because we started off with copper, there was no need to implant our radioactive 64 Cu nuclei in a metallic environment. Figure 6.17 shows the decay scheme of 64 Cu. In
addition to the β − and EC branches, which add
up to 82.14%, there is a 17.86% β + branch. According to the Debye-Hückel model, the latter
should decay faster while the former should decay more slowly, resulting in a longer effective
half-life. Following the calculations contained in
Refs. [Lim06, Rup08], one would expect an in-
crease in half-life of about 20% when the sample
is cooled to 12 K.
We focused on the detection of the 511 keV γ
rays from the β + decay of 64 Cu. Due to the small
branching ratio of 0.471%, the 1346 keV γ-rays
from the decay of the 2+ state of 64 Ni were not
considered in the present work. After cutting the
2 mm x 2 mm x 1 mm sample into two pieces,
we used two identical, low-background γ-ray detection systems for the decay-rate measurements.
Similar to Ref. [Lim06], our experimental setup
for the “cold measurement” consisted of a cryopump (operating temperature 12 K) and a wellshielded 60% high-purity germanium (HPGe) detector (placed outside of the cryopump) to detect the 511 keV γ rays from the 64 Cu sample
attached to the cold head of the cryopump. In
the “warm measurement”, the 64 Cu sample was
placed in a holder a set distance from the second HPGe detector. The detector-to-sample distances were adjusted to keep dead-time corrections below 10% for both the warm and cold measurements. In addition, two 137 Cs sources (both
at room temperature) were used for normalization purposes. The data-acquisition system consisted of Canberra 2026 amplifiers with pileup
rejection hardware and a Canberra Multiport II
dual multi-channel analyzer controlled by Canberra’s genie software. We took consecutive onehour runs over the course of three days. During a
span of 10 months we took three separate threeday runs of cold and warm measurements using
three different (and newly produced) 64 Cu samples. Background measurements determined the
natural 511 keV background to be insignificant.
To calculate the half-life, we normalized the yield
of the 511 keV line to that of the 661 keV line
of 137 Cs (the correction for its 30.17 yr half-life
was statistically insignificant). Finally, we plotted the logarithm of the normalized yield versus
time and performed a linear fit with the program
topfit, which also provided the value for the
half-life and its uncertainty, based on counting
statistics and the quality of the fit. Our three independent runs have provided consistent results.
The first set of measurements was a test run. The
second set yielded the following results: warm
measurement, T1/2 = (12.669±0.037) hours; cold
measurement, T1/2 = (12.708 ± 0.033) hours.
Preliminary results from the most recent run
95
indicate a measured half-life of 12.706 h for both
the warm and cold setups with similar error
bounds. As can be seen, within the overall uncertainty of ≈ 0.3% we do not observe a significant difference between the half-lives measured
for 64 Cu at room temperature and at 12 K. Our
results are also consistent with the accepted value
of T1/2 = (12.701 ± 0.002) hours.
[Ass87] H. J. Assenbaum et al., Z. Phys., A327,
461 (1987).
[Cze06] K. Czerski et al., Eur. Phys. J., A27,
83 (2006).
[Deb23] W. Debye and E. Hückel, Phys. Z., 24,
185 (1923).
[Goo07] J. R. Goodwin et al., Eur. Phys. J.,
A34, 271 (2007).
[Lan07] K. Langanke and Y. E. Kim, Private
communication, 2007.
[Lim06] B. Limata et al., Eur. Phys. J., A28,
251 (2006).
[Rai02] F. Railoa et al., Eur. Phys. J., A13, 377
(2002).
[Rai07] F. Raiola et al., Eur. Phys. J., A32, 51
(2007).
[Ray99] A. Ray et al., Phys. Lett., B455, 69
(1999).
[Rol06] C. Rolfs,
(2006).
Nucl. Phys. News, 16, 9
[Rup08] G. Ruprecht et al., Phys. Rev., C77
(2008).
[Rut02] E. Rutherford and F. Soddy, Phil. Mag.,
5, 582 (1902).
[Seg47] E. Segrè, Phys. Rev., 71, 274 (1947).
[Spi07] T. Spillane et al., Eur. Phys. J., A31,
251 (2007).
[Sto07] N. J. Stone et al., Nucl. Phys., A793, 1
(2007).
[Zin07] N. Zinner,
(2007).
FirstPhysics EventO-TPC atHIaS
16O(a_)12C
(Run1750,event18,M arch31,2008,15:06:33)
560
120
b-electron
Time Projection
(PM T Signal)
12
C
540
12
C
100
CCD -Y Pixel
Counts/40ns
_
520
500
480
460
_>
a
440
50mm
260
280
300
320
340
60
20
360
CCD -X Pixel
First in-beam dissociation
80
40
420
240
_
16
380
400
5
5.5
6
Time(+ s)
6.5
7
O(γ, α) physics event measured with the Optical
Time Projection Chamber (O-TPC) detector at HIγS. A CCD picture of a track
of the reaction (left) and its time projection (right) are shown. The inverse αcapture process in the helium burning cycle is an important reaction in nuclear
astrophysics.
Photonuclear Reactions at HIγS
Chapter 7
•
•
Photodisintegration of the Deuteron
γ-3He Interaction
•
Study of Many-Body Systems Using NRF Techniques
•
Nuclear Astrophysics at HIγS
•
Interrogation of Special Nuclear Materials
•
HIγS Intrumentation
98
7.1
7.1.1
Photodisintegration of the Deuteron
An Indirect Determination of the Gerasimov-Drell-Hearn (GDH)
Sum Rule and Forward Spin Polarizability (γ0 ) for the Deuteron at
Low Energies
M.W. Ahmed, W. Tornow, H.R. Weller, TUNL
An analysis of d(~γ ,n)p reaction to extract the Gerasimov-Drell-Hearn (GDH) sum rule integrand for the deuteron and the sum rule integrand for the forward spin polarizability (γ 0 )
near photodisintegration threshold is presented.
An analysis of deuteron photodisintegration
data from HIγS was performed to obtain the
Gerasimov-Drell-Hearn (GDH) sum rule integrand and the sum rule integrand for the forward spin polarizability (γ0 ). The GDH sum
rule relates the helicity-dependent photoabsorption cross-section difference to the anomalous
magnetic moment of the target and states that
Z ∞
dω
(σP (ω) − σA (ω))
I GDH =
ω
ωth
experiments is presented. A fit to the world data
analyzed in this manner gives a GDH integral
value of -603±43 µb between the photodisintegration threshold and 6 MeV. This result is the
first confirmation of the large contribution of the
1
S0 (M1) transition predicted [Are97] for the
deuteron near photodisintegration threshold. In
addition, a sum rule value of 3.75±0.18 fm4 for γ0
is obtained between photodisintegration threshold and 6 MeV. This is a first indirect confirmation of the leading-order effective field theory
2
2 2 κ
= 4π e
S,
(7.1) prediction for the forward spin-polarizability of
M2
the deuteron. Fig. 7.1 shows the fit to world
where σP/A is the photoabsorption cross sec- data to extract the GDH sum rule integrand and
tion with photon and target spins parallel/anti- Fig. 7.2 shows the extraction of γ0 , which is comparallel, ωth is the threshold energy for the in- pared to the theoretical predictions. A detailed
elastic process [Dre66], and κ is the anoma- description of this work can be found in [Ahm08].
lous magnetic moment of the target with ground
state mass M , and spin S. The forward spinpolarizability for the case of the deuteron is given [Ahm08] M. W. Ahmed et al., Phys. Rev., C77,
044005 (2008).
by [Ji04]
Z ∞
1
dω
γ0 = − 2
(σP (ω) − σA (ω)) 3 (. 7.2) [Are97] H. Arenhövel et al., Phys. Lett., B407,
8π ωth
ω
1 (1997).
It is shown that a measurement of analyzing power obtained with linearly polarized γ-rays [Dre66] S. D. Drell and A. C. Hearn, Phys. Rev.
Lett., 16, 908 (1966).
and an unpolarized target can provide an indirect determination of these two physical quantiX. Ji and Y. Li, Phys. Lett., B591,
ties near photodisintegration threshold. An anal- [Ji04]
2004 (2004).
ysis of data for the d(~γ ,n)p reaction and other
σP-σA (µ b)
99
0
-200
-400
Ahmed et al.
Tornow et al. [13]
Schreiber et al. [12]
Sawatzky et al. [14]
Soderstrum et al. [26]
Del Bianco et al. [28]
Holt et al. [25]
Drooks et al. [27]
Thermal n-Capture [33]
Fit to Data
Arenhovel et al. [6]
3 σM1 Approximation [6]
-600
-800
-1000
-1200
-1400
-1600
-1800
0
2
4
6
8
10
12
Eγ (MeV)
0
γ (fm4)
Figure 7.1: (Color online) The prediction for the GDH and forward spin polarizability sum rule integrand and indirect measurements. References for the data can be found in [Ahm08].
10
9
Experiment
8
Running Integral of γ Calculated Using Arenhovel et. al. [6]
7
γ LO
0
6
γ LO+NLO Full Integral Prediction by Ji et al. [2]
0
Full Integral Prediction by Ji et al. [2]
0
5
4
3
2
1
0
2
3
4
5
6
7
8
9
10
Eγ (MeV)
Figure 7.2: Running integral of γ0 calculated using σP − σA predictions of Arenhövel et al. [Are97].
Also shown are the values of the full integral predictions of Ji and Li [Ji04] for γ0LO and
γ0LO+N LO . The predictions are compared to the experimental result integrated up to 6
MeV.
100
7.1.2
Measurement of d(γ,n)p Reaction Cross Section
M.W. Ahmed, E.E. Clinton, S.S. Henshaw, B.A. Perdue, P.-N. Seo, S. Stave, H.R.
Weller, TUNL
Data on the photodisintegration of the deuteron at Eγ = 2.44, 3.5, and 4 MeV were collected
at three scattering angles to measure the fore-aft asymmetry in this reaction cross section as
well as the absolute cross section. A preliminary analysis of the data is presented.
One of the motivations for this study was an
earlier report [Saw05] indicating a significant
fore-aft asymmetry in the cross section at 4.0
MeV. Such an asymmetry would indicate a large
E1-E2 interference, not predicted by any theory.
If the cross section is expanded in terms of Legendre polynomials, then the fore-aft asymmetry
(as ) can be written as
as
0.5
d = 9"
122
T = 0.4"
55
o
o
Li−Glass Detectors
2" x 10 mm
o
85
(7.3)
Sawatzky, 1999
0.3
2
0.577a1 − 0.3849a3
1 − 0.392a4
Ahmed, 2008
0.4
D O Target
=
where a1 , a3 , and a4 are coefficients of the Legendre polynomials. Restricting only to M1(swaves), E1(p-waves), and E2(d-waves) at these
low energies, a1 is proportional to the E1-E2
(p-d wave) interference, a4 is proportional to
the total E2 strength (small at these energies),
and a1 = - a3 . These relations simplify the
above equation to the fact that as =0.9622 a1 .
Therefore, a measure of the fore-aft asymmetry is indeed a measurement of the E1-E2 interference. The a1 coefficient extracted from our
data is shown in Fig.7.4 and compared to the
earlier measurements and theory. Our result is
consistent with almost no E1-E2 interference at
these low energies, as predicted by the theory.
a1
A measurement of the absolute cross section
for the photodisintegration of the deuteron was
carried out at the HIγS facility. The inverse of
this reaction (the radiative n−p capture process)
holds a key position in the Big-Bang nucleosynthesis framework in predicting the light element
abundances in the universe. The cross section
of the capture reaction is related to the photodisintegration cross section via the principle of
detailed balance.
The experimental setup consisted of three
thin (10 mm thick) Li-glass detectors placed at
θlab = 55◦ , 85◦ , and 122◦ . The detectors were
placed at a nominal distance of 10 in. from a thin
(0.4 in.) heavy water target. The target was oriented to minimize the scattering of neutrons in
the target material. The γ-ray beam was polarized in the plane of the detectors, and the typical
flux on target was ∼106 γ/s. Data for time-offlight (ToF) and pulse height were collected at Eγ
= 2.44, 3.5, and 4.0 MeV. Total yields in each detector were obtained by integrating the ToF spectrum gated by the pulse height. Data were then
corrected for Li-glass efficiencies, finite geometry
corrections, and neutron multiple scattering in
the target. The data are not yet normalized for
the total γ-ray flux.
Arenhovel
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
γ −ray Beam
Figure 7.3: (Color online) Experimental setup for
measuring deuteron photodisintegration cross section and fore-aft asymmetry.
-0.5
0
1
2
3
4
5
6
Eγ
Figure 7.4: (Color Online) The a1 coefficient from
an expension of the cross section in
Legendre polynomials as a function of
energy.
The angular distributions at these three energies were studied by normalizing the θCM =90◦
experimental yield to the theoretical predictions
at each energy, since we have not normalized the
data to the absolute flux. The angular behaviour
of the data is then compared to the theoretical
behaviour. The data and theoretical angular distributions are in agreement as shown in Fig. 7.5
σ (mb)
101
5
4.5
Data
4
Theoretical Prediction
3.5
3
dσ/dΩ (mb/sr)
2.5
0.4
0.35
0.3
4 MeV
3.5 MeV
2.44 MeV
2
Theorectical Predication
1.5
1
0.5
0.25
0
1
0.2
0.15
3
4
5
6
7
Eγ (MeV)
Figure 7.6: The measured total cross section is
normalized to the theory at 4 MeV
and the measured cross section behaviour as a function of energy is compared to theory [Are97].
0.1
0.05
0
0
2
20
40
60
80
100 120 140 160 180
θ CM (deg)
Figure 7.5: (Color Online) The measured differential cross section is normalized
to the theoretical value at θCM =
90◦ for each energy, and the angular behaviour is compared to theory
[Are97].
The angular distributions were then fitted to
an unnormalized differential cross section of the
form
dσ
dΩ
= a + bsin2 (θ)
(7.4)
where a and b are arbitrary coefficients. The
fitted differential cross sections were then integrated to obtain unnormalized total cross sections. The cross section at 4 MeV was then normalized to the theoretical value and the energy
evolution of the total cross section as a function
of energy was studied. Figure 7.6 shows the behaviour of the total cross section obtained in this
experiment compared to theoretical predictions
[Are97].
As it is seen in Fig.7.6, the error bars on the
data are large. Most of this error comes from the
fit to the differential cross section. Since the angular coverage of this experiment was limited, the
fit is unable to constrain the behaviour of the differential cross section at extreme angles. It was
however realized that other data, collected earlier
to measure the photon analyzing power [Ahm08],
can be used to obtain the θCM =0◦ yield via the
measurement of the 1 S0 M1(s-wave) transition.
Initial analysis of data indicates a successful reduction of the errors. The data are still under
analysis but will provide results with reduced uncertanities. In addition, we are analyzing data for
the flux determination in order to obtain absolute
cross sections.
These data will provide a measurement of the
photodisintegration cross section near and at the
region of Big Bang nucleosynthesis, where there
is little or no world data.
[Ahm08] M. W. Ahmed et al., Phys. Rev., C77,
044005 (2008).
[Are97]
H. Arenhövel et al., Phys. Lett., B407,
1 (1997).
[Saw05] B. Sawatzky, Ph.D. thesis, University
of Virginia, 2005.
102
7.2
7.2.1
γ-3He Interaction
Total Cross-Section Measurements of the Two-Body Photodisintegration of 3 He at Low Energies
W. Tornow, H.J. Karwowski, J.H. Kelley, G. Rusev, A.P. Tonchev, TUNL
Precision data have been obtained of the total cross section for the two-body photodisintegration of 3 He in the 9 to 13 MeV energy range using a high-pressure 3 He/Xe gas scintillator
as both target and detector. NaI and HPGe detectors were used for absolute γ-ray flux
determinations.
After a three-year break caused by the upgrade of the HIγS facility, we returned to our
studies of the two-body breakup of 3 He. Our
goal is to determine the total cross section for
the reaction 3 He(γ,p)2 H in the incident γ-ray energy range, 7 - 20 MeV. During the past three
years, considerable progress has been made by
the Lisbon group in calculating this observable
[Del07]. For the first time the Coulomb interaction has been included in the calculations, resulting in a reduced cross section for γ-ray energies
below 15 MeV compared to the previous calculations of the Krakow group [Wit03]. Furthermore,
the Japanese group of Naito et al. [Nai06] published data at 10.2 and 16 MeV. While their datum at 16 MeV is in fair agreement with the theoretical calculations by the Lisbon and Krakow
groups and with other existing data, their datum
at 10.2 MeV is considerably lower than the Lisbon prediction but in agreement with previous
data from Wölfli et al. [Wöl66].
In June 2008 we took data in the 9 to 13 MeV
γ-ray energy range in 1 MeV increments. The
OK-5 undulator was used for FEL photon production instead of the OK-4 undulator employed
during our initial work in 2004-2005. The electron energy was changed between 475 MeV and
572 MeV, and the FEL photon wavelength was
varied between 453 nm and 463 nm to produce
the γ-ray energies of interest. Using the booster
injector, the electron beam current in the storage ring was kept constant at either 60 mA (at
Eγ = 9 - 12 MeV) or 90 mA (at Eγ = 13 MeV).
In addition, the FEL power was kept constant to
guarantee constant γ-ray flux at each individual
energy.
Our target and detector system consists of
a high-pressure helium-xenon gas scintillator to
detect the protons and deuterons from the two-
body breakup reaction (Q = −5.49 MeV) of 3 He.
For a given total pressure, the xenon contribution
can be optimized to provide sufficient stopping
power for protons (to minimize wall effects) and
at the same time to yield sufficient pulse height
separation between pulses produced by protons
and deuterons and those generated by electrons
through Compton scattering of the intense incident γ-ray flux in the gas and its stainless steel
container of 1 mm wall thickness. The protons
and deuterons are emitted preferentially into the
angular range of 50◦ to 130◦ relative to the incident γ-ray momentum. For a total pressure of
750 psi, a 4:1 ratio of 3 He to Xe gas is optimal
for 10 to 13 MeV γ rays, while a 3:2 ratio is more
suitable for energies above 13 MeV. A xenon pressure of about 75 psi is the optimum pressure for
γ-ray energies of 9 MeV and below. At 13 MeV
we took data with two 3 He/Xe gas scintillators
of different gas mixtures to check on the consistency of our results. Figure 9.1-1 of [Est04]
shows a typical pulse-height spectrum obtained
with a 3 He/Xe gas scintillator exposed to 10 MeV
γ rays. To correct for γ-ray-induced chargedparticle reactions on xenon, we used an identical
gas scintillator with the 3 He gas replaced by 4 He
(with a breakup threshold of 19.81 MeV). The
gain matching of these two types of gas scintillators can easily be accomplished by determining
the photopeak locations of standard γ-ray test
sources.
Due to the up-right cylindrical shape of our
gas scintillator with inner diameter of 52 mm, our
approach is sensitive to the horizontal location of
the γ-ray beam formed by the 3/8 in. collimator
located about 4 m upstream of the helium/xenon
gas scintillator in a well shielded collimator hut.
We determined the location and spatial dimensions of the γ-ray beam at the location of the
3
He/Xe gas scintillator using a recently developed γ-ray beam imager [Sun07].
The γ-ray flux measurements were accomplished using two flux monitors. Because of the
high γ-ray flux (5 ×105 γ/s for the present 3/8
in. collimator size), a NaI or high purity Ge
(HPGe) detector cannot be placed directly into
the γ-ray beam. Therefore, as in our previous
studies, we placed a 123% HPGe detector at 22◦
relative to the γ-ray beam direction to detect the
Compton γ rays scattered off of a copper plate
positioned directly in the γ-ray beam, approximately 5 m downstream of the 3 He/Xe gas scintillator [Ton05]. In addition, we calibrated our
standard γ-ray detector paddle system, located
right after the collimator in the HIγS collimator hut. For this purpose we inserted three copper blocks, each of which attenuated the γ-ray
beam flux by about a factor of 10, in the γray beam. These well characterized attenuators
[Ahm07] are positioned about 30 m upstream of
the 3 He/Xe gas scintillator, inside of the storage ring shielding wall. With attenuators #1, 2
& 3 inserted the γ-ray flux in the HIγS target
room is low enough to place our standard HPGe
detector and a 8 in. diameter and 12 in. long
NaI detector directly at 0◦ (one at a time) to
cross calibrate the paddle. Due to its (purposely)
low efficiency, the paddle under these conditions
counts at a rate of 1 to 2 Hz (depending on γray energy). From the yields measured with the
HPGe and NaI detectors at 0◦ and their known
efficiencies, the paddle-rate/γ-ray conversion factors were determined for all of our energies using
two different detectors. However, because the actual cross-section measurements were performed
with all three attenuators taken out of the beam,
the γ-ray flux in this case can be reliably inferred
from the paddle rate only if the paddle rate is linear with γ-ray flux. Assuming linearity is a reasonable assumption but has never been carefully
checked over a wide range of γ-ray fluxes. Therefore, we placed our 3 He/Xe gas scintillator at 0◦
and, at the same time, positioned the HPGe detector off-axis at 22◦ . We also removed all three
attenuators and then cross calibrated the paddle
rate at Eγ =11 MeV relative to the 3 He/Xe as
well as to the HPGe detector rates. In the case of
the 3 He/Xe gas scintillator, we recorded the twobody breakup events and therefore have an excellent handle on possible slight PMT gain changes
due to the change in γ-ray flux. Next we inserted
attenuator #1 and repeated the measurement to
1% statistical accuracy in the 3 He/Xe scintillator yield. Of course, the statistical accuracy of
the yield recorded with the HPGe detector was
considerably better. Finally, we inserted attenuators #1 and #2. With the γ-ray flux reduced
by a factor of about 100, the yield in the 3 He/Xe
103
gas scintillator was low, resulting in a statistical
significance of only 2.5%. However, the statistical accuracy in the HPGe detector yield was still
excellent. The preliminary analysis of these data
reveals excellent linearity of the HIγS scintillator
paddle system with count rate.
The data taken with the 4 He/Xe gas scintillator confirmed our conjecture that the background
due to γ-ray-induced charged-particle reactions
in Xe, the MgO reflector and wavelength shifter
deposited on the inner wall of the gas scintillator
housing is very small. In the pulse height region
of interest this background accounts for less than
1% at all energies investigated.
At Eγ = 13 MeV we accumulated data with
both the 600 psi 3 He/150 psi Xe and 450 psi
3
He/300 psi Xe gas scintillators to check on
consistency and on our corrections for edge effects. Without edge effect corrections applied,
the yields normalized to the paddle counts and
corrected for the different amounts of 3 He agree
within 3%. The cell containing the higher xenon
density gives the larger number (as expected).
Once the data are completely analyzed and
corrected for subtle effects not discussed in this
report and once we are convinced that we understand all the issues related to absolute crosssection measurements at HIγS, we plan to extend
our present measurements to lower and higher γray energies. We hope to provide theoreticians
with an accurate set of total-cross-section data
in the γ-ray energy range from 7 to 20 MeV. We
also hope that appropriately shaped 3 He/Xe gas
scintillators eventually will become robust, easy
to use and accurate γ-ray flux monitors which
are not count rate limited and therefore can be
operated in the intense 0◦ γ-ray flux at HIγS.
[Ahm07] M. W. Ahmed et al., TUNL Progress
Report, XLVI (2007).
[Del07]
A. Deltuva and A. Fonseca, 2007, private communication.
[Est04]
J. H. Esterline et al., TUNL Progress
Report, XLIII, 110 (2004).
[Nai06]
S. Naito et al.,
034003 (2006).
Phys. Rev., C73,
[Sun07] C. Sun and Y. Wu, 2007, Private communication.
[Ton05] A. P. Tonchev et al., TUNL Progress
Report, XLIV (2005).
[Wit03] H. Witala, 2003, Private communication.
[Wöl66] W. Wölfli et al., Phys. Lett., 22, 75
(1966).
104
7.2.2
Gerasimov-Drell-Hearn Integral on 3 He Part I: 3-body Measurement for Eγ = 14.7 MeV and 11.4 MeV
H. Gao, H.R. Weller, C.W. Arnold, M.W. Ahmed, M.A. Blackston, M. Busch, W. Chen,
T.B. Clegg, D. Dutta, M. Emamian, J.H. Kelley, J. Li, R. Lu, S. Mikhailov, B.A. Perdue, X. Qian, P.-N. Seo, S. Stave, C. Sun, P. Wang, Y.K. Wu, Q. Ye, W.Z. Zheng, X.F.
Zhu, X. Zong, TUNL
In April and May 2008, we have successfully carried out a first measurement of double polarized three-body photodisintegration of 3 He at incident photon energies of 11.4 and 14.7
MeV. This is a very first study towards the ultimate determination of the GDH integral on
3 He from the two-body breakup threshold to the pion production threshold. The experiment
employed a circularly polarized photon beam, a high-pressure, longitudinally polarized 3 He
target and 7 liquid scintillator neutron detectors.
At Q2 = 0, an important sum rule
known as the Gerasimov-Drell-Hearn (GDH) sum
rule [Dre66, Ger66] is expressed using the total
helicity-dependent nucleon real photoabsorption
P
cross sections σN
(nucleon spin parallel to the
A
spin of the photon) and σN
(nucleon spin antiparallel to the spin of the photon):
I GDH =
Z
∞
νthr
A
P
− σN
)
(σN
dν
4π 2 e2 2
=
κ I, (7.5)
ν
M2 N
where κN is the anomalous magnetic moment of
the nucleon, M is the mass of the nucleon, and
I is the spin. This sum rule is based on low
energy theorems and the validity of the unsubtracted dispersion relation for the spin-flip amplitude f2 . The GDH sum rule also applies to
nuclei. It relates the total cross section of circularly polarized photons on a longitudinally polarized nucleus to the anomalous magnetic moment
of the nucleus. Equation 7.5 can be applied directly, with N representing the nucleus instead
of the nucleon in this case, I being the spin of
the nucleus, and M being the mass of the target
nucleus. The lower limit of the integration is the
photo-nuclear disintegration threshold.
The experimental determination of the GDH
integral on 3 He from the two-body breakup
threshold to the pion production threshold is particularly interesting due to the fact that the polarized 3 He nucleus is commonly used as an effective neutron target. We estimated the part
of the integral from two-body breakup threshold
up to pion production threshold to be about 212
µb, based on experimental data [Ama02, Bia99]
within the plane-wave impulse approximation
picture. The 3 He ground state wave function
used in this estimation is from [Fri90]. On the
other hand, state-of-the-art three-body calculations give a value for this part of the integral
between 16µb and 136µb [Del04, Gol05].
The HIγS facility, located at the Duke Free
Electron Laser Laboratory (DFELL), is an ideal
place for such a measurement. We carried out a
very preliminary measurement on the three-body
photodisintegration of 3 He at incident photon energies of 11.4 MeV and 14.7 MeV in April and
May 2008. A high-pressure polarized 3 He target
and a circularly polarized photon beam were used
in these measurements. The polarized 3 He-target
apparatus has been modified for the longitudinal
polarization configuration. A neutron detection
system including 7 detectors has been designed
and used for this experiment. The 3 He nuclear
spin was aligned parallel and anti-parallel to the
incident photon momentum direction. The threebody photodisintegration process can be studied
by detecting the neutrons from the three-body
breakup channel. This experiment has two goals:
a first measurement of the spin-dependent asymmetry from three-body breakup of longitudinally
polarized 3 He nucleus, and a measurement to
provide part of the first data on the GDH integral of 3 He. The entire experimental setup is
shown in Fig. 7.7, which is enclosed by a 12 ft.
× 12 ft. laser curtain.
We started data taking with a 14.7 MeV photon beam. The 7 neutron detectors were placed
at 50, 75, 90, 105, 130, 145, and 160 degrees. The
overall detector rates were too high to make measurements at more forward angles. Each neutron
counter was 5.3 inches in diameter and the distance between the neutron counter and the cen-
cell made of GE180 glass. This is likely due to
the different chemical compositions of these two
glass types, a subject for further investigations.
Currently, these data are being analyzed, and we
expect to present preliminary results by the end
of the calendar year.
Counts
ter of the target was 15.75 inches. Data were
taken continuously at this energy for a total running time of 36 hours and then another 36 hours
at the incident γ-ray energy of 11.4 MeV. The
target performance was stable throughout both
run periods, with an average polarization around
35%. There was a downtime of about a week between runs at these two energies for replacement
a broken RF amplifier in the storage ring.
105
9000
8000
He3 SPIN A
7000
He3 SPIN B
6000
N2 (GE180)
N2 100 torr (Pyrex)
5000
4000
3000
2000
1000
Figure 7.7: (Color online) The polarized 3 He experimental setup at DFELL inside the
HIγS target room.
Since it takes several hours to flip the γ-ray
beam helicity at HIγS, we flipped the target spin
direction instead, in order to accumulate data for
both target spin directions. The target spin was
flipped once every three hours, and each spin flip
required an access to the γ vault to physically
rotate the quarter-wave plate. In the future, the
target spin flip will be automated.
The polarized 3 He target is based on the spinexchange optical pumping technique. The target cell contains a very small amount of nitrogen
gas in addition to the 3 He gas and a negligible
amount of a rubidium and potassium mixture.
Details of the target and its performance can be
found in [Kra07]. Data for background subtraction were also taken during the experiment, using a nitrogen reference cell constructed from the
same type of GE180 glass and of the same geometry as that of the polarized 3 He target cell,
and some data were also taken from another nitrogen cell made of regular Pyrex glass for comparison. Fig. 7.8 shows the normalized neutron
time-of-flight spectra measured with the neutron
detector located at 90◦ for the GE180 glass target
cell filled with polarized 3 He gas in spin states A
and B, the GE180 glass reference cell filled with
N2 gas, and a Pyrex cell filled with N2 . One
sees clearly that significantly more backgrounds
events were observed from the reference nitrogen
0
0
20 40 60 80 100 120 140 160 180 200
Timing (ns)
Figure 7.8: (Color online) Spectra from the neutron detector located at 90◦ for target spin state A (black), B (red), a
nitrogen-filled reference cell (GE180,
green), and a nitrogen-filled reference
cell (Pyrex, blue). These data were
normalized to the incident γ-ray flux.
[Ama02] M. Amarian et al., Phys. Rev. Lett.,
89, 242301 (2002).
[Bia99]
N. Bianchi and E. Thomas, Phys. Lett.,
B450, 439 (1999).
[Del04]
A. Deltuva et al., Phys. Rev., C69,
034004 (2004).
[Dre66] S. Drell and A. Hearn, Phys. Rev. Lett.,
16, 908 (1966).
[Fri90]
J. Friar et al., Phys. Rev., C42, 2310
(1990).
[Ger66] S. Gerasimov, Sov. J. Nucl. Phys., 2,
430 (1966).
[Gol05] J. Golak et al., Phys. Rep., 415, 89
(2005).
[Kra07] K. Kramer et al.,
Nucl. Instrum.
Methods in Phys. Research, A582, 318
(2007).
106
7.2.3
Photodisintegration of 3 He at Eγ = 12.8 MeV at HIγS
B.A. Perdue, M.W. Ahmed, M.A. Blackston, H.R. Weller, TUNL; P. Wang, Y.K. Wu,
Duke University Free Electron Laser Laboratory, Durham, NC ; G. Feldman, George Washington
University, Washington, DC ; R. Igarashi, Canadian Light Source, Inc., Saskatoon, Saskatchewan,
Canada; N.R. Kolb, Science Applications International Corporation, San Diego, CA; B.E. Norum,
University of Virginia, Charlottesville, VA; B.D. Sawatzky, Temple University, Philadelphia, PA
Measurements of neutron polarization asymmetries and relative cross sections of the 3 He(~γ ,n)pp
reaction using a 100% linearly polarized 12.8 MeV γ-ray beam have been completed using
the Blowfish detector array. Analysis to extract neutron polarization asymmetries and relative cross sections as functions of neutron scattering angle and energy is near completion.
Disagreements between the energy distributions of a theoretical prediction [Del05] and the
experimental data are shown.
This experiment was conducted at the HIγS
facility located on the campus of Duke University. A pulsed beam of 12.8 MeV γ rays, collimated to a 1 inch diameter, was made incident on a high-pressure 3 He gas target. Photodisintegration events resulting in the expulsion
of neutrons were counted using the Blowfish detector array [Saw04]. The target was a 400 mL
cylindrical aluminum bottle filled with 170 bar of
3
He. Blowfish is a spherical array of 88 liquidscintillator neutron detectors mounted on the
surface of an imaginary sphere with a radius of
40.6 cm.
θlab
n = 90°
Experimental Data
Deltuva Prediction
Phase Space Simulation
Deltuva Theory Simulation
80
70
60
50
40
30
20
10
0
0
1
2
Elab
n (MeV)
3
4
5
Figure 7.9: (Color online) Neutron energy speclab = 90◦ .
trum for one detector at θn
Experimental data is plotted against
two sets of simulated results, one using a phase-space cross section and the
other using the Deltuva theory [Del05]
as the seed cross section. The theoretical prediction of the cross section is
also shown.
The process of filtering the experimental data
(or data reduction) is complete. The neutron energy distribution resulting from the filtering process is shown in Fig. 7.9 for a detector positioned
at θn = 90◦ in the lab. The experimental energy
distribution in the figure was derived from the observed time of flight (TOF) of the neutron from
the target to the detector. The experimental result is compared with a theoretical prediction by
Deltuva et al. [Del05] and with two simulated
distributions. The theoretical prediction is the
dot-dashed black curve in Fig. 7.9. The solid red
histogram is the result of a geant4 simulation
with the distribution of outgoing neutrons given
by a phase-space-only cross section. The dashed
blue histogram is the simulated result using the
Deltuva cross section to describe the outgoing
neutron field.
As can be seen in Fig. 7.9, the experimental neutron energy spectrum agrees with the
phase-space simulation results and disagrees with
the Deltuva simulation results. The sources of
this disagreement are being investigated. Future
work will be focused on this investigation and on
extracting and interpreting the polarization observables from the experiment.
[Del05] A. Deltuva, A. C. Fonseca, and P. U.
Sauer, Phys. Rev., C72, 054004 (2005).
[Saw04] B. D. Sawatzky, Ph.D. thesis, University
of Virginia, 2004.
108
7.3
7.3.1
Study of Many-Body Systems Using NRF Techniques
Fine Structure of Nuclear Dipole Excitations below Particle Emission Threshold
A.P. Tonchev, C.T. Angell, S.L. Hammond, C.R. Howell, A. Hutcheson, H.J. Karwowski, J.H. Kelley, E. Kwan, G. Rusev, W. Tornow, TUNL
High-sensitivity studies of E1 and M1 excitations observed in 138 Ba(γ,γ0) reaction at energies below the neutron emission threshold have been performed. The electric dipole character
of the so-called “pygmy” mode was experimentally verified for excitations from 4.0–8.6 MeV.
Fine structure of the M1 “spin-flip” mode was observed for the first time in N = 82 nuclei.
Missing dipole strength has been revealed in our photon-scattering measurements.
During the year 2008, experimental activities
at the HIγS facility have focused on investigations of a special collective excited mode, commonly referred to as the “pygmy” dipole resonance (PDR), which is observed as a clustering
of states close to the neutron threshold. Theoretical calculations indicate a correlation between
the observed total B(E1) strength of the PDR
and the neutron-to-proton ratio N/Z [Tso04].
Although carrying only a small fraction of the
full dipole strength, these states are of particular
interest because they are interpreted as a motion of the neutron skin against the core. The
observation of this collective dipole mode near
the neutron threshold may have important astrophysical implications. For example, it has
been advocated that the nucleosynthesis of certain neutron-deficient nuclei—the so called pnuclei—are strongly influenced by PDR structures [Arn06].
The purpose of our work at HIγS is three
fold. First we want to determine the character
of the pygmy resonance and to find out whether
it is an E1 or M1 mode of excitation. Second, we want to investigate the decay pattern
of these collective states below the particle separation energy. Finally, our goal is to determine
the strength, energy distribution, and nature of
these collective phenomena. To pursue these issues we have focused our experimental activity on
the 138 Ba(γ,γ0) reaction below the neutron separation energy (Bn = 8.61 MeV).
The HIγS facility is used to produce highintensity and nearly monoenergetic photon
beams by intracavity Compton backscattering.
The backscattered photons are collimated by a
cylindrical lead collimator with a diameter of
1.27 cm, located 60 m downstream of the col-
lision point. These γ rays are highly polarized
and result from the Compton scattering process
of ≈100% polarized photons.
The scattering target consisted of a BaCO3
powder of natural isotopic abundance packed into
a Lucite container 2.81 cm in diameter and 1.0 cm
in height. Sample spectra are shown in Fig. 7.10.
Figure 7.10: (Color online) The top panel shows
the γ-ray spectrum at Eγ = 6.5 ±
0.2 MeV from the vertical detectors
along with the background spectrum
from the “blank” measurement. The
lower panel has the corresponding
spectra for the horizontal detectors.
The arrows in the insets indicate the
horizontal polarization of the incident γ rays.
Figure 7.11 shows the experimental asymmetry of 148 dipole states observed in 138 Ba in the
energy range from 4.0 to 8.6 MeV. Most of them
(140) de-excite via an E1 transition, i.e. the levels are J π =1− . In addition, eight M1 dipole
states have been observed for the first time in
N = 82 nuclei. They are concentrated around
6.5 MeV, which is the region of the M1 spin-flip
mode. It should be noted that in heavier nuclei, searches for the fine structure of the M1
strength are very difficult because of the high
level density in the excitation-energy range where
the shell model predicts the M1 states. Furthermore, there are the well-known difficulties in assigning parity to these states. However, this situation has changed drastically with the availability
and high quality of the HIγS beam.
Most likely these M1 spin-flip transitions
occur between the spin-orbit-partner subshells
1h11/2 → 1h9/2 and 1d5/2 → 1d3/2 for neutrons
and protons, respectively. The centroid energy
of these spin-flip states in 138 Ba corresponds to
E ≈ 35 × A−1/3 MeV.
109
E1 transitions to the ground state, due to the
PDR or GDR component in the structure of their
state vectors. Recent analysis of transition densities [Tso04] has shown that these excitations are
related to the neutron PDR mode. The singlephonon component of these dipole states, which
is responsible for direct transitions to the ground
state, is only a few percent of the total state. The
analysis of dipole transition densities for different
excitation energy regions in 138 Ba is presented in
Fig. 7.12.
Figure 7.12: (Color online) One-phonon neutron
(solid curves) and proton (dashed
curves) transition densities in 138 Ba.
In summary, the systematic parity measurements on 138 Ba at HIγS have verified for the
first time that the observed dipole strength below the particle threshold is predominantly electric dipole. Our findings are in agreement with
other QPM predictions for the character of this
dipole mode of excitation. The high level density
at the particle separation energy leads to many
weak transitions which are difficult to observe directly. In addition, these weak transitions decay
Figure 7.11: (Color online) Asymmetry of the
low-lying dipole states in 138 Ba us- via cascades to many intermediate levels. Thereing the 100% linearly polarized γ-ray fore much of the information on dipole strength is
beam at HIγS. The upward-pointing missing from observation of de-excitations to the
triangles represent the observed M1 ground state alone. These transitions are weak
states, while the downward-pointing
triangles represent the distribution but so numerous that their superposition can be
observed as a continuum of unresolved strength.
of E1 states.
Despite the fact that the PDR exhibits wellpronounced collective structure below the neuMicroscopic calculations for the dipoletron separation energy, the inelastic transitions
strength distribution were performed for 138 Ba
will mostly dictate the reaction rate in this enwithin the framework of the quasiparticle-phonon
ergy region. The present measurements confirm
model (QPM). The results show that the structhat the monoenergetic and pulsed beam from
ture of these states contain a large neutron conthe HIγS facility opens up a new opportunity for
tribution (more than 90%). These states corprecision measurements of the nuclear dipole rerespond mainly to oscillations of weakly bound
sponse below the particle emission threshold.
neutrons from the s- and p-shells. Microscopic
studies of the 1− states close to the particle
threshold revealed that their structure is mostly [Arn06] M. Arnould et al., Nucl. Phys., A777,
an admixture of complex configurations. As a
157 (2006).
result, direct transitions from these states to the
ground state are strongly hindered. Very few [Tso04] N. Tsoneva et al., Phys. Lett., B586,
dipole states in this energy region decay by strong
213 (2004).
110
7.3.2
Population of h11/2 Isomers in N = 81 Isotones Using the (γ,n) Reaction
A.P. Tonchev, A. Hutcheson, J.H. Kelley, E. Kwan, G. Rusev, W. Tornow, TUNL
We present experimental results for the population of h11/2 isomers in N = 81 isotones
using the (γ,n) reaction. The giant dipole resonance (GDR) was excited, and, after emission
of one neutron, the nucleus has an excitation energy of a few MeV. The ensuing γ decay by
direct or cascade transitions deexcites the nucleus into an isomeric or ground state. Many
states were found connected to the ground and isomeric states by E1, E2, and M1 transitions.
Statistical and microscopic calculations will be applied to explain the isomer excitation in
N = 81 nuclei.
The study of the electromagnetic transitions coupling low-lying states with those having intermediate energy reveals a delicate interplay between the two main excitation modes in
atomic nuclei: single-particle modes and collective modes. Nuclear isomers are an ideal tool for
these studies, since they have relatively low excitation energy and a total angular momentum Jiso
that might be very different from the angular momentum of the ground state Jgs . Due to these
specific properties, their electromagnetic decay
into the ground state is strongly hindered, and
they are characterized by half-lives ranging from
milliseconds to years, depending on the value of
| Jiso - Jgs |.
In this study, nuclei were excited into their
GDR state, from which their main mechanism
of decay is neutron emission. The residual nuclei with excitation energy of 5 to 7 MeV decay by γ transitions into the isomeric and ground
states. Since the excitation energy of the intermediate states populated in this reaction is relatively high, a statistical approach is appropriate
for the theoretical interpretation of the γ-decay
process.
The N = 81 nuclei have identical spins and
parities in their isomeric and in their ground
states. Therefore, the isomeric ratio should not
be affected by such factors as the difference between the spin of the isomeric (J π = 11/2−) and
ground states (J π = 3/2− ). Moreover, the isomeric levels for the isotopes 139 Ce, 141 Nd, and
143
Sm have an energy of the order of 755 keV
while that for 137 Ba has an energy of 661 keV.
The structure of the nuclear states at intermediate excitation energy is rather complex. The interplay of the simple collective and the more complex modes leads to the fragmentation of the simple excitations [Tso00]. The experimental data as
well as theoretical calculations reveal the spreading of the simple modes into a wide energy region,
where large fluctuations in strength are obtained.
We report the experimental results for the
population of h11/2 isomers in 137 Ba, 139 Ce,
141
Nd, and 143 Sm following the (γ,n) reaction
of energies between 11 and 16 MeV. Previous
measurements have been performed on the corresponding N = 82 target nuclei [Ang06]. The
present measurements significantly increase the
number of observed transitions, mainly due to
two factors. First, the γ-ray flux after the 2007
HIγS upgrade was increased by more than a factor of 10 and now can reach values of more than
1 × 108 s−1 . Second, significant improvements
in the detector shielding system were accomplished, greatly decreasing non-resonant scattering. Figure 7.13 shows the experimental setup
consisting of a quartet of 60% high-purity germanium (HPGe) detectors mounted on an aluminum wheel.
Figure 7.13: (Color online) Quartet of 60% HPGe
detectors used in the isomer excitation experiments at HIγS. The N =
82 targets were centered in the middle of the evacuated plastic tube.
Figure 7.14: (Color online) Spectrum of 141 Nd
measured with one of the 60% detectors. Incident γ energy was 15.9
MeV
The beam was colimated to 1.27 cm, and its
intensity distribution at the target position was
monitored by a γ-ray beam imager at each en-
111
ergy. The energy spread of the γ-ray beam was
3.5%. On average, three hours of beam time were
used per incident energy per isotone. This was
sufficient to obtain statistical uncertainties of 1–
3% for most of the observed γ transitions. A
total of 47, 20, 58, and 5 transitions have been
observed in 137 Ba, 139 Ce, 141 Nd, and 143 Sm, respectively. Figure 7.14 shows part of the γ spectrum measured in 141 Nd at Eγ = 15.8 MeV. The
ground state transitions are labeled with the letters g, while the isomeric transition is labeled
with the letter m. Background peaks are labeled
with ∗.
Future plans include calculating the partial
cross section in N = 81 isotones. These data
will provide benchmarks for testing the statistical model.
[Ang06] C. Angell et al., In APS Conf. Proc.,
volume CP819, p. 363, 2006.
[Tso00] N. Tsoneva et al.,
044303 (2000).
Phys. Rev., C11,
112
7.3.3
Nuclear Resonance Fluorescence Measurements on 142,150 Nd to Determine the γ-Ray Strength Function for p-process Nucleosynthesis
Calculations
C.T. Angell, S.L. Hammond, H.J. Karwowski, J.H. Kelley, E. Kwan, G. Rusev, A.P.
Tonchev, TUNL; A. Makinaga, H. Utsunomiya, Konan University, Kobe, Japan
The γ-ray strength function (γSF) is a key component for calculating the photodisintegration
reaction rates used for the p-process. During the p-process, the nucleus can be thermally
excited lowering the threshold of photodisintegration. To calculate the reaction rates for excited states, the γSF is taken from an extrapolation of the low-energy tail of the giant dipole
resonance. A new technique for determining the γSF using nuclear resonance fluorescence
was developed, and measurements were taken for 142 Nd and 150 Nd at Eγ = 4–9 MeV. The experiment was done at the HIγS facility using a polarimeter consisting of four clover detectors.
Most elements heavier than Fe are produced
via successive neutron capture. There are, however, 35 nuclei in the region between Fe and Pb
which cannot be created via either the s- or rprocess. They are known as the p-nuclides, and
they lie on the proton-rich side of the valley of βstability. They are produced by photodisintegration reactions on existing heavy seed nuclei generated earlier via the s- or r- processes. This process is referred to as the p-process. The p-process
occurs in an environment that is hot enough to
thermally produce high energy γ rays which can
photodisintegrate nuclei.
Determining reaction rates for the p-process
is complicated by two issues. First, most cross
sections have not been measured and cannot be
measured, because most nuclei involved are unstable. Second, because of the hot environment
in which the p-process takes place, the nucleus
can become thermally excited, which will greatly
increase the reaction rates, and the photodisintegration cross sections on excited states need
to be calculated. The statistical model is used
to calculate cross sections to determine the reaction rates, and a key input parameter is the
γ-ray strength function (γSF) [Arn03]. The γSF
describes the average reduced radiative width of
a nucleus as a function of energy. An extrapolation of the tail of the giant dipole resonance
is typically used as the form for the low-energy
γSF. To calculate the (γ, n) reaction rate on excited states, the γSF needs to be known below
the neutron separation energy according to the
Brink-Axel hypothesis [Axe62]. The potential
presence of a pygmy resonance can add additional
strength at low energies. If it is near the neutron separation energy, this resonance can further
enhance the reaction rate during the p-process,
and its systematics need to be understood. Microscopic models are needed to reliably calculate the γSF for nuclei far from stability. One
such model, the quasi-particle random phase approximation (QRPA), predicts an order of magnitude increase in the relative low-energy γSF from
142
Nd to 150 Nd [Gor07]. By measuring the lowenergy γSF on these two nuclei, a key test can be
made of the QRPA. Additionally, the systematics
of the pygmy dipole resonance can be studied as
a function of neutron number. These measurements of the γSF below the neutron separation
energy complement previous experiments done to
measure the (γ,n) cross section near threshold
[Ang07]. The final results of that experiment are
shown in a separate progress report contribution
(see Sect. 6.5.1).
The γSF below the neutron separation energy can be measured using nuclear resonance
fluorescence (NRF) by obtaining the total (γ, γ 0 )
cross section [Rus06]. Since no other channels are
open in NRF measurements, the γSF can be obtained directly from the total cross section. Using a mono-energetic beam simplifies the measurements because all measured transitions are
associated with a narrowly excited region. There
are two ways to determine the total (γ, γ 0 ) cross
section (see Fig. 7.15). The first is to extract
the partial cross section for each state which decays directly to the ground state. In Fig. 7.15,
these are labeled “1” and “3”. These states include both the depopulation of the initially excited states as well as those states which are populated via branchings then decay to the ground
state. The sum of these partial cross sections is
the total (γ, γ 0 ) cross section. This method was
used for 142 Nd. The second way is to integrate
the total counts above the atomic background in
the detectors. This is also referred to as a cascade, and is labeled “4” in Fig. 7.15. Even if
no peaks are observed, the depopulation of the
initial excited states is directly seen as a broad
distribution above the atomic background. This
method was used for 150 Nd.
Probing
GSF
4
2
1
Nd142(g,g’) Cross Section
1000
100
M1
E1
(g,n)
10
1
0.1
0.01
3
G.S.
Excite with
Gamma Rays
Figure 7.15: (Color online) The γSF is probed in
4 ways: (1) decay back to the ground
state, (2) branching to low-lying excited states, (3) de-population of
low-lying excited states, and (4) the
cascade of γ rays moving through
multiple states.
NRF measurements were made at HIγS on
142
Nd from 3.4 to 9.7 MeV and on 150 Nd from
5.6 to 7.2 MeV. They comprise the first NRF
measurements of 142 Nd and 150 Nd near the neutron separation energy, and the first attempt to
determine the γSF below the neutron separation
energy in both nuclei. The measurements were
made using a quad clover high-purity germanium
detector array (see Sect. 9.5.4). A 30 g 142 Nd
target, and a 1.3 g 150 Nd target were used, both
of which were enriched to 96%. The beam profile was measured using a high-purity Ge detector, and the flux was monitored by measuring the
Compton scattering off of a Cu plate.
The preliminary total cross sections deduced
for 142 Nd are shown in Fig. 7.16, and those for
150
Nd in Fig. 7.17. More work is needed on flux
normalizations to reduce the uncertainties. Evidence for the pygmy resonance can be seen close
to threshold for 142 Nd. A resonance this close
6
7
8
9
Energy [MeV]
10
11
Figure 7.16: (Color online) The preliminary
142 Nd(γ, γ 0 ) total cross section.
Nd150(g,g’) Cross Section
100
Cross Section [mb]
1st E.S.
113
to threshold can significantly enhance the reaction rates for the p-process, and alter the flow
of the process. The resonance is much larger in
150
Nd, but lower in energy. If the pygmy resonance of this nature is seen in even-odd or oddodd nuclei, then the neutron separation energy
will be even lower than in 142,150 Nd, so that the
pygmy resonance even at lower energies could
play an important role in calculations for those
nuclei. More theoretical work is needed to understand the nature of the pygmy resonances seen
in 142,150 Nd, how these resonances will vary systematically, and, finally, their potential effect on
p-process nucleosynthesis.
Cross Section [mb]
M1
E1
(g,n)
10
1
6
7
8
9
Energy [MeV]
10
11
Figure 7.17: (Color online) The preliminary
150 Nd(γ, γ 0 ) total cross section.
[Ang07] C. T. Angell et al., TUNL Progress Report, XLVI, 42 (2007).
[Arn03] M. Arnould and S. Goriely, Phys. Rep.,
384, 1 (2003).
[Axe62] P. Axel, Phys. Rev., 126, 671 (1962).
[Gor07] S. Goriely,
2007.
Private communication,
[Rus06] G. Rusev et al., Eur. Phys. J. A, 27,
171 (2006).
114
7.3.4
Fine Structure of the M1 Resonance in
90
Zr
G. Rusev, C.T. Angell, S.L. Hammond, A. Hutcheson, H.J. Karwowski, J.H. Kelley, E.
Kwan, A.P. Tonchev, W. Tornow, TUNL; R. Schwengner, A. Wagner, Research Center
Dresden-Rossendorf, Germany
Photon-scattering experiments on
90 Zr
carried out at the HIγS facility with 100% linearly
polarized beams revealed a significant M1 strength around 9 MeV. We observed more than
30 transitions in the plane of polarization. The transitions correspond mainly to M1 deexcitations and represent the fine structure of the M1 resonance in
present work are compared with predictions from the shell model.
The M1 resonance in 90 Zr has attracted the
interest of experimentalists because of the predicted strong M1 spin-flip excitations at excitation energies around 7 to 9 MeV. The nuclide
90
Zr is a semi-magic nucleus with a closed g9/2
shell of 50 neutrons and a closed p1/2 subshell
of 40 protons. The spherical shape of 90 Zr enhances the M1 strength concentrated in spin-flip
excitations from neutron g7/2 to g9/2 orbitals.
Experiments searching for M1 excitations in
90
Zr were performed with different probes. A
broad resonance structure at Ex = 8.9 MeV has
been observed in inelastic proton scattering at
Ep = 200 MeV [Dja82]. High-resolution electronscattering experiments revealed that a significant part of the observed structures in heavy
nuclei are due to M2 excitations [Meu81]. For
the case of 90 Zr, the total M1 strength
P from
(e, e0 ) experiments was found to be
B(M1)
↑≤ 2.5µ2N , where µN is the nuclear magneton, and is distributed over three states at excitation energies Ex = 8.233, 9.000, and 9.371
MeV [Meu80]. Due to lack of detector resolution, neither type of experiment, proton- or
electron-scattering, was able to probe any fine
structure in 90 Zr. Photon-scattering experiments with partly-polarized, tagged photons determined
the strength of the M1 resonance to be
P
B(M1)↑∼
= 6.7µ2N , located between 8.1 and 10.5
MeV [Las87]. The scattered γ rays were registered with a NaI detector that did not allow
them to identify the individual excitations of the
M1 resonance.
We performed high-resolution photonscattering experiments on 90 Zr at the HIγS facility for beam energies from 7 to 10 MeV, in
order to determine the fine structure of the M1
resonance. Linearly polarized beams irradiated
a 90 ZrO2 target with a mass of 4054.22 mg and
90 Zr.
The results of the
enriched to 97.7% in 90 Zr. The scattered γ rays
were measured with four high-purity germanium
detectors with 60% efficiency relative to 3 in. ×
3 in. NaI detectors. Two of the detectors were
positioned horizontally, in the plane of polarization, and the other two in the perpendicular
plane. Spectra measured at a beam energy of 8.1
MeV are presented in the top panel of Fig. 7.19.
Figure 7.18: Top panel: Results from the present
work for the M1 strength in 90 Zr
under the assumption that all peaks
observed in the horizontal detectors
are M1 de-excitations. Note, that
the lowest photon-beam energy was
7 MeV. Bottom panel: predictions
from the shell model for the M1 excitations in 90 Zr.
Overall, we observed more than 30 peaks
in the horizontal detectors which correspond to
115
Figure 7.19: (Color online) Measured spectra of 90 Zr at the HIγS facility with a 8.1 MeV linearly
polarized photon beam (top panel) and at the electron accelerator ELBE of the Research
Center Dresden-Rossendorf at Ee = 13.2 MeV (bottom panel). The arrows show the
corresponding peaks assigned as M1 de-excitations.
M1 transitions. Theoretically, they could be
E2 transitions, but following the low-spin transfer in the photon absorption, this is not very
likely. Assuming that all of the peaks are M1 deexcitations, we determined the structure of the
M1 resonance in 90 Zr presented inPFig. 7.18.
The resonance has a strength of
B(M1)↑∼
=
3.8(2)µ2N and a center of gravity at 9 MeV.
The experimental results are compared in Fig.
7.18 to predictions from the shell model. The
calculations were performed for effective g factors of gseff = 0.7gsfree in the model space of
π(0f5/2 , 1p3/2 , 1p1/2 , 0g9/2 ) ν(0g9/2 , 1d5/2 , 0g7/2 )
allowing at most two protons to be lifted from
the f p orbits to the 0g9/2 orbit and up to two
neutrons to be lifted from the 0g9/2 orbit to the
1d5/2 and 0g7/2 orbits. The
P calculations predict a summed strength of
B(M1)↑= 6.37µ2N
and a center of gravity at 7.7 MeV. The main
part of the strength is caused by three states
around 8 MeV which are dominated by the neutron g9/2 → g7/2 spin-flip configuration.
We plan a more detailed analysis of the
present experimental data, including comparison with photon-scattering experiments using
bremsstrahlung on 90 Zr [Sch08], as shown in
Fig. 7.19. These data were obtained with the
superconducting electron accelerator ELBE at
the Research Center Dresden-Rossendorf in Germany. They will help with the assignment of
the multipole order of the transitions observed
at the HIγS facility. In addition to the resolved
transitions, we will analyze the strength of the
continuum formed by many overlapping weak
de-excitations in order to determine the total
strength of the M1 resonance.
[Dja82] C. Djalali et al., Nucl. Phys., A388, 1
(1982).
[Las87] R. M. Laszewski et al., Nucl. Phys., 59,
431 (1987).
[Meu80] D. Meuer et al., Nucl. Phys., A349, 309
(1980).
[Meu81] D. Meuer et al., Phys. Lett., B106, 289
(1981).
[Sch08] R. Schwengner et al., Phys. Rev. C,
(2008), to be submitted.
116
7.3.5
Multipole Mixing Ratios of Transitions in
11
B
G. Rusev, A.P. Tonchev, W. Tornow, TUNL; R. Schwengner, Research Center DresdenRossendorf, Germany; C. Sun, Y.K. Wu, Duke Free Electron Laser Laboratory, Durham, NC
The mixing ratios for M1 and E2 radiations for transitions in 11 B have been determined
by measuring the azimuthal asymmetry of the radiation emitted from levels populated by
resonant absorption of polarized photons. The photon-scattering experiments were carried
out at the Free Electron Laser Laboratory at Duke University using monoenergetic and 100%
linearly polarized photon beams. The mixing ratios were deduced from a comparison of the
measured azimuthal asymmetries with calculations for the angular distributions of mixed
transitions.
Transitions in 11 B are frequently used for flux
calibration purposes in photon-scattering experiments with bremsstrahlung beams because of
their large decay widths. The small number of
strong de-excitations in 11 B, distributed over a
wide energy range, makes this nucleus a convenient calibration standard in experiments aimed
at determining the dipole strength in nuclei up
to the neutron separation energy. Three of the
transitions in 11 B, at 4.444, 5.019, and 8.916
MeV, are known as mixed M1 and E2 radiations
[Bel68, Com68] from comparisons of measurements and distorted-wave Born-approximation
calculations. The angular distribution for these
transitions depends on the relative contributions
of dipole and quadrupole radiation defined by the
mixing ratio δ. Measurements of δ are important
for the correct determination of the photon flux
if 11 B is used as a calibration standard.
We present the results of photon-scattering
experiments on 11 B with linearly polarized and
nearly monoenergetic beams. The measured
asymmetry of the scattered γ rays was used to
derive the mixing ratio δ in a model-independent
way. The measurements were carried out at the
High-Intensity γ-Ray Source (HIγS) facility. The
photon beam was collimated by a lead collimator
with length of 30.5 cm and a cylindrical hole with
diameter of 1.9 cm. The energy distribution of
the incident photon beam, measured with a largevolume high-purity germanium (HPGe) detector
placed in the beam, is presented in Fig. 7.20 (a).
The beam irradiated the 11 B target positioned in
an evacuated plastic tube made of poly-methylmeta acrylic in order to reduce background. The
scattered γ rays were detected with four HPGe
detectors, each with 60% efficiency. The HPGe
detectors were positioned at 90◦ relative to the
beam at azimuthal angles of 0◦ , 90◦ , 180◦ , and
270◦, so that two of the detectors are in the horizontal plane and the other two in the vertical one.
All detectors were equipped with passive shields
made of 3 mm copper and 2 cm thick lead cylinders. Lead and copper absorbers with thicknesses
of 5 mm and 3 mm, respectively, were placed in
front of the detectors. Spectra measured from
11
B and the plastic target container are shown in
Figs. 7.20(b) and 7.20(c), respectively.
The asymmetry of the scattered γ rays was
determined according to the relation:
a=
Ah /th − (εh /εv )Av /tv
,
Ah /th + (εh /εv )Av /tv
(7.6)
where Ah and Av were the registered γ rays for
measuring time th and tv in the horizontal and in
the vertical plane, respectively. The ratio of the
full-energy-peak efficiencies, εh /εv , of the horizontal and vertical detectors was deduced from
the weighted mean of the values for the transitions in 56 Co with energies above 2 MeV. This
eliminated any dependence of εh /εv on the thickness of the lead and copper absorbers. For this
purpose a 56 Co source was placed in the target
position. This measurement resulted in a value
of 1.011(10). The asymmetry is related to the
analyzing power Σ(θ, φ) and the degree of polarization of the photon beam Pγ by:
a = C(θ)Pγ Σ(θ, φ) =
W (90◦ , 0◦ ) − W (90◦ , 90◦ )
,
C(θ)Pγ
W (90◦ , 0◦ ) + W (90◦ , 90◦ )
(7.7)
where W (θ, φ), the angular distribution of the
emitted γ rays, depends on the angular momenta
of the excited and ground states, the type of transitions, and the mixing ratio δ. The correction
factor C(θ) is due to the finite opening angle of
the detectors. We determined the value of C(θ)
by means of geant3 simulations for the detector
setup. In the simulations, the γ rays were emitted
randomly from the target with an energy of 10
MeV and the direction given by W (θ, φ). We obtained values for C(θ) of 0.952(18) and 0.943(15)
for the spin sequences 3/2 → 5/2 → 3/2 and
3/2 → 3/2 → 3/2, respectively. The degree of
polarization of the photon beam at the HI~γ S facility was known to be 100%.
117
ing ratio was determined for the transitions in
B at 4.444, 5.019, and 8.916 MeV by comparing to the measured asymmetry. An example
is shown in Fig. 7.21 for the spin sequence of
3/2 → 5/2 → 3/2 for the transitions at 4.444
and 8.916 MeV. The multipole mixing ratios obtained using the present approach are compared
with those of the literature in Table 7.1. We
found good agreement of δ for the transitions
at 4.444 and 5.019 MeV, while the transition at
8.916 MeV has almost no mixing of M1 and E2
radiation, in contrast to the result obtained in
Ref. [Com68].
11
Table 7.1: Results for the observed asymmetry
and the deduced mixing ratios for transitions in 11 B. The first two literature
values are from [Bel68] and the third
from [Com68].
Eγ
keV
4443.93
5019.08
8916.3
Figure 7.20: Spectra from the measurement on
11 B at 4.44 MeV photon-beam energy. (a) energy distribution of the
incident beam, see text, (b) scattered γ rays from 11 B in the horizontal plane and (c) background from
the plastic container of the 11 B powder. The transition in 12 C at Eγ =
4437 keV, which can contaminate the
peak at Eγ = 4444 keV in 11 B, is
not seen in panel (c). The spectra
presented in panels (b) and (c) were
measured for a similar photon flux.
Using the predicted analyzing power and
varying δ to fulfil Eq. (7.7), the multipole mix-
a
δ
0.026(19)
0.200(16)
0.215(20)
0.158+0.026
−0.022
+0.012
−0.034−0.012
+0.013
0.0−0.012
δ
+0.19(3)
–0.03(5)
+0.11(4)
Figure 7.21: Plot of the predicted asymmetry versus the mixing ratio for M1/E2 radiation calculated for the 3/2 → 5/2 →
3/2 spin sequence (solid curve). The
measured asymmetries for the transitions in 11 B at 4.444 MeV and 8.916
MeV are shown by the data points
on the calculated curve. The statistical uncertainty of the asymmetry
is translated to the uncertainty of δ
which is represented by the horizontal bars.
[Bel68]
R. A. I. Bell et al., Nucl. Phys., A118,
481 (1968).
[Com68] M. N. H. Comsan et al., Z. Phys., 212,
71 (1968).
118
7.4
Nuclear Astrophysics at HIγS
7.4.1
First In-Beam Measurement of the
O-TPC Detector.
16
O(γ, α)12 C Reaction With the
M. Gai, P.-N. Seo, University of Connecticut, Storrs, CT ; M.W. Ahmed, E.R. Clinton, S.S.
Henshaw, C.R. Howell, B.A. Perdue, S. Stave, C. Sun, H.R. Weller, Y.K. Wu, TUNL;
P.P. Martel, University of Massachusetts, Amherst, MA
An intense (approximately 2 × 107
γ/s) 9.55 MeV γ-ray beam extracted from the HIγS
facility was used to study the 16 O(γ, α)12 C reaction with the O-TPC serving as both target
and detector. During this engineering run we studied the 9.585 MeV broad (Γ = 400 keV)
1− resonance state of 16 O. A γ-ray camera used in conjunction with lead absorbers placed in
front and in back of the O-TPC detector allowed us to align the detector with sub-millimeter
precision. More than 1,000 dissociation events of
16 O
and
18 O
were identified using track
reconstruction and the total energy deposited in the O-TPC detector. Background events
from the interaction of cosmic rays and from the 14 N(γ, p)13 C reaction constitute 90% of the
collected data but were excluded by demanding a valid α-particle track with the predicted
dE/dx of an α particle.
We commenced in-beam studies with the optical readout time projection chamber (O-TPC)
detector that we constructed for studying oxygen formation during stellar helium burning (the
12
C(α, γ)16 O reaction) by measuring the timereversed process of the photo-dissociation of 16 O.
An intense 9.55-MeV γ-ray beam of approximately 2 × 107 γ/s was collimated with a 15 cm
long 12 mm diameter lead collimator. The profile of the γ-ray beam was measured with a γ-ray
camera that was developed at HIγS for that purpose. Lead absorbers having a length of 8 mm
and diameters of 4 and 8 mm were placed in front
and back, respectively, of the O-TPC detector.
The shadow image of these absorbers was used
to align the O-TPC detector with sub-millimeter
precision, as shown in Fig. 7.22. Note that the
beam was placed 1 mm to the left and half a mm
below the center of the lead collimator to correct
for an internal misalignment inside the O-TPC
detector that will be corrected in the near future.
Events from the dissociation of oxygen isotopes to α + carbon were easily identified by the
characteristic α particle dE/dx along its trajectory (see Fig. 7.23). The data collected on line
is dominated by cosmic interactions (mostly with
the grids of the O-TPC) and by the dissociation
of 14 N from the 20% N2 gas admixture, due to
the large cross section of the 14 N(γ, p) reaction,
as shown in the top panel of Fig. 7.24. Events
from the dissociation of 16 O and 18 O were iden-
tified by their dE/dx and total energy deposits
of 2.43 and 3.33 MeV, respectively, and the background events were easily rejected as shown in the
bottom panel of Fig. 7.24.
50
100
150
200
250
300
350
400
450
500
550
100
200
300
400
500
600
700
Figure 7.22: (Color online) Image of the beam
with lead absorbers placed in front
and back of the detector.
The correlated light (PMT) and energy
(Grid) signals from the dissociation of oxygen isotopes are shown in Fig. 7.25. The peak width
is dominated by the spread in beam energy of
approximately 300 keV. The detector resolution
of 70 keV, measured with 3.18 MeV α particles
from a 148 Gd source, allows us to bin the data
to smaller energy bins. The tail in the low PMT
signals arises from events at the end of the fiducial viewing area of the detector, defined by a 20cm-diameter ring that was placed on top of the
40-cm-diameter quartz window at the top of the
O-TPC. This window was placed to match the
119
FirstPhysics EventO-TPC atHIaS
16O(a_)12C
(Run1750,event18,M arch31,2008,15:06:33)
560
120
b-electron
Time Projection
(PM T Signal)
12
C
540
12
C
100
CCD -Y Pixel
Counts/40ns
_
520
500
480
460
_>
a
440
50mm
260
280
300
320
340
80
60
40
20
420
240
_
360
380
400
5
CCD -X Pixel
5.5
6
Time(+ s)
6.5
7
Figure 7.23: (Color online) First in-beam dissociation 16 O(γ, α) physics event measured with the OTPC detector. A CCD picture of a track of the reaction (left) and its time projection(right)
fiducial viewing area of the PTB-UV lens which
is considerably smaller than the design lens (with
a fiducial viewing area of 37 cm).
We are analyzing these data to extract detailed
angular distributions of the ∼1000 good events
that were collected in this engineering test run.
14N(ap)
Counts/Bin
CO2(80%)+ N2(20%)
150torr
Ea = 9.
55M eV
(On Line Analysis)
(1Hour)
Cosmic
ADC Channel
ADC Channel
Removed Events With Proton Tracks
(10Hours)
ADC Channel
Counts/Bin
16O(a_)
2.
39M eV
18O(a_)
3.
32M eV
ADC Channel
Charge(~2.
4keV/Channel)
Figure 7.24: (Color online) Energy spectra collected with the O-TPC detector.
Figure 7.25: (Color online) Energy (top left) and
light(top right) spectra and correlation (bottom left) collected with the
O-TPC detector.
120
7.4.2
Measurement of 9 Be(γ,n) at HIγS
C.W. Arnold, T.B. Clegg, G. Rusev, A. Imig, H.J. Karwowski, Y.K. Wu, C.R. Howell,
TUNL
Measurements of the 9 Be(γ,n) cross section were taken at HIγS using a high-efficiency neutron
counter. The experiment was run within the range of Eγ = 1.55 to 5 MeV using the OK-5
circularly polarized beam of 0.5 inch diameter. Details of beam and target parameters are
discussed.
Knowing the astrophysical rate of formation
of 9 Be via the α + α + n reaction is of critical importance for understanding how light nuclei are
synthesized into r-process seed nuclei in the exploding Type-II supernova environment. With
a precise determination of the cross section of
the inverse reaction 9 Be(γ,nα)α, Breit-Wigner
parameters can be extracted and converted via
the detailed balance theorem [Sum02] into a rate
of formation. The γ-ray beam at HIγS is ideal for
making a precision measurement of the 9 Be(γ,n)
cross section.
Figure 7.26: HPGe spectrum at Eγ = 1.695 MeV
with ∆Eγ /Eγ = 0.83% and flux =
5 × 103 γ/sec. Average flux for these
runs was ∼ 5 × 104 γ/sec.
this, HIγS was operated with asymmetric electron bunches in its storage ring. The larger
bunch lases, and the photons produced Compton scatter from electrons in the smaller bunch.
The small bunch is characterized by a smaller
∆E and a tighter spatial extent than the large
bunch. The property of smaller ∆E is transfered
to the Compton scattered γ rays. Using an HPGe
(high-purity germanium) detector, we have seen
∆Eγ /Eγ less than 1% (see Fig. 7.26). Flux measurements were taken both from peak efficiency
measurements of HPGe spectra and from total efficiency measurements of a nearly 100% efficient
NaI detector.
Neutrons were measured with a highefficiency neutron counter. The four target positions for this experiment were designated as
9
Be, graphite, D2 O, and target-out (TO) (see
Fig. 7.27). The graphite, and TO positions are
used to identify sources of background. Counts
measured while running with TO are observed
to be time-dependent in nature, while additional
counts measured with the graphite target in place
appear to be atomically scattered gammas inducing false counts. The D2 O target allows us to
measure the energy-dependent absolute efficiency
of the neutron counter by using the well-known
D(γ,n)p cross section.
Neutron data and γ-ray spectra have been
measured at 36 different energies between 1.55
and 5.0 MeV. Data are currently being analyzed.
For high precision measurements of the narrow resonances that exist near the (γ, n) thresh- [Sum02] K. Sumiyoshi, Nuc. Phys., A709, 467
old, ∆Eγ /Eγ must be minimized. To achieve
(2002).
121
Figure 7.27: (Color online) YZ, XY, and XZ views of the neutron counter as modeled in MCNP. The
γ-ray beam is incident upon the top-most target postion (yellow) pictured in the XY
configuration.
122
7.4.3
Preliminary Measurements of Astrophysically Important States in
26
Mg with HIγS
R. Longland, C. Iliadis, C. Ugalde, A.P. Tonchev, G. Rusev TUNL; R. J. deBoer, M.
Wiescher University of Notre Dame, South Bend, IN
A preliminary measurement of excited states in 26 Mg, the compound nucleus in the 22 Ne(α,n)25 Mg
reaction, was performed at HIγS. A horizontally polarized γ-ray beam at Eγ = 11.3 MeV and
an array of four 60% HPGe detectors arranged in the horizontal and vertical planes were used
to measure the parity of excited states. One state was observed in the horizontal detectors at
Ex = 11154.3(25) keV. This measurement shows the state to be an unnatural parity state, contrary to the common belief that it is J π = 1− . This has significant implications for s-process
nucleosynthesis.
The 22 Ne(α,n)25 Mg reaction is an important
source of neutrons for the s-process in massive
stars and asymptotic giant branch (AGB) stars.
The s-process is the slow capture (in comparison
to the β-decay lifetime) of neutrons onto seed nuclei in the iron peak to produce heavy elements.
In massive stars, the 22 Ne(α,n) reaction is the
main neutron source during the He-shell burning
stage. The high mass of the star leads to high
temperatures in the shell, which can ignite this
reaction readily. Massive stars are thought to be
responsible for the “weak component” of the sprocess, which produces the low mass s-process
elements. In AGB stars, the main source of neutrons is the 13 C(α,n)16 O reaction, which is active
during the inter-pulse stages. During the thermal pulse, however, temperatures at the base of
the inter-shell zone become high enough for the
22
Ne source to produce a brief burst of neutrons,
which can go on to be captured. This short burst
of neutrons affects mainly the branchings in the
s-process path. These branchings can be analyzed in great detail in pre-solar grains with typical uncertainties of a few percent. Consequently,
detailed information about the internal structure
of these stars can be obtained by considering the
nucleosynthesis that occurs. In order for this to
be possible, uncertainties in the neutron producing reaction rates must be reduced as much as
possible.
occur as a result of the current uncertainties in
the 22 Ne+α reaction rates. In the latter study,
the current reaction rate uncertainties produced
significant abundance changes of up to a factor
of 10. It is clear from these studies that the uncertainties in the reaction rates must be reduced.
This preliminary experiment was designed to
look for astrophysically important states in the
26
Mg compound nucleus through excitation with
a γ-ray beam. These states are difficult to measure directly due to the low transmission of the
α particles through the Coulomb barrier at the
low energies corresponding to the temperatures
in these stars. Obtaining indirect information
about the quantum numbers of the states can be
essential in determining resonance parameters.
This helps to reduce reaction rate uncertainties
significantly.
The γ-ray beam used at HIγS has an intensity of Iγ ≈ 1 × 106 γ/s, and is linearly polarized
at Eγ = 11.3 MeV. The polarization of the beam
was in the horizontal plane. The beam was incident on a magnesium target, which was placed
in the center of an array of 60% high-purity germanium (HPGe) detectors.
The sample consisted of a mixture of 153 mg
enriched 26 Mg metal and 134 mg enriched
26
MgO, totaling 287 mg of material. This was
mounted in a plastic target holder, smaller than
the beam diameter, in the center of the beam.
The effects of the current uncertainties in the The detectors were placed in an array around the
◦
22
Ne(α,n)25 Mg reaction and its competing reac- sample, at 90 angles from one another.
For a linearly polarized γ-ray beam exciting
tion, 22 Ne(α,γ)26 Mg, have been studied by Pignatari et al. [Pig05] for AGB stars and by The a J = 0 nucleus to a J = 1 or J = 2 state
et al. [The00] for massive stars. In the former through a pure transition, the angular correlastudy, variations in s-process branchings of 25% tion function W (θ, φ) for the de-excitation γ-ray
is [Bie53]: J = 0 → 1 → 0
1
W (θ, φ) =1 + P2 (cos(θ))−
2
1
σ1 (2)
(−1) P2 (cos(θ)) cos(2φ) , (7.8)
2
AGB stars and massive stars. This result shows
that this state is not as important as was previously believed.
25
Horizontal Detectors
20
J =0→2→0
511 keV
511 keV
11154.3 keV
15
where θ and φ are the detection angles with respect to the beam direction and the vertical plane
respectively; P2 and P4 are the Legendre polyno(2)
(2)
mials, and P2 and P4 are the associated Legendre polynomials; σ1 = 0 for electric transitions,
and σ1 = 1 for magnetic transitions. Placing detectors at θ = 90◦ , φ = 0◦ and φ = 90◦ allows
us to determine the parity of the excitation and,
consequently, that of the excited state.
The detectors were energy-calibrated using the background lines at Eγ = 1460.9 and
2614.5 keV, as well as three points from 90 Zr at
Eγ = 3307.9(2), 3841.9(2) and 9784.0(5) keV.
The γ-ray beam was incident on the 26 Mg
sample for a live time of 39970.6 seconds. In
that time, one peak at Ex = 11154.3(25) keV was
observed, with a total of Nk = 70.9(90) counts
in the horizontal detectors and N⊥ = 7.9(43)
counts in the vertical detectors. This corresponds to a positive parity transition (M1 or E2)
meaning that the observed state is most likely
a J π = 1+ or J π = 2+ state, contrary to the
common belief that this must be a J π = 1−
state [Koe02]. This is a significant result, as this
state corresponds to a laboratory α-particle energy of Eα = 637.6(3.0) keV and falls in the nuclear burning region at temperatures relevant for
10
5
Counts
5
8
P2 (cos(θ)) + P4 (cos(θ))
14
7
(2)
σ1 5
− (−1)
P (cos(θ))
28 2
2 (2)
− P4 (cos(θ)) cos(2φ),
21
(7.9)
W (θ, φ) = 1 +
123
0
10000
10400
10800
11200
25
Vertical Detectors
20
15
10
5
0
10000
10400
10800
Energy (keV)
11200
Figure 7.28: (Color online) Spectra for the horizontal and vertical detectors showing
the Ex = 11154.3(25) keV peak and its
single and double escape peaks. This
spectrum has not been energy corrected for recoil shift.
[Bie53] L. C. Biedenharn and M. E. Rose, Rev.
Mod. Phys., 25, 729 (1953).
[Koe02] P. E. Koehler, Phys. Rev., C66, 055805
(2002).
[Pig05] M. Pignatari et al., Nucl. Phys., A758,
541 (2005).
[The00] L.-S. The, M. F. El Eid, and B. S. Meyer,
Astrophys. J., 533, 998 (2000).
124
7.5
7.5.1
Interrogation of Special Nuclear Materials
Nuclear Resonance Fluorescence from
238
U
S.L. Hammond, C.T. Angell, C.R. Howell, H.J. Karwowski, J.H. Kelley, E. Kwan, G.
Rusev, A.P. Tonchev, W. Tornow, TUNL
Nuclear resonance fluorescence provides unambiguous isotope identification as high-energy γ
rays penetrate protective shielding, acting as an identifier of hidden nuclear materials. Using
the mono-energetic γ-ray source at HIγS to investigate the nucleus 238 U through the (γ, γ0)
reaction, we measured the widths of low-spin states observed at incident γ-ray beam energies
in the range of 2.94 to 4.40 MeV.
We investigated the low-spin levels of 238 U at
excitation energies above 3 MeV, since very little
is known about nuclear structure in this region.
We used γ-ray beams from the High Intensity
γ-Ray Source (HIγS) facility and the nuclear resonance fluorescence (NRF) technique to obtain
new spectroscopic data which can be applied to
material identification. Even though the possible states in this region were found to be weaker
than the known states around 2 MeV [Hei88], the
higher energy γ rays in this region will be more
penetrating than the lower energy ones. Further
identification of γ rays from 238 U will help to reduce the risk of false positives in shipment interrogation.
The following preliminary results are from an
experiment conducted during the last two weeks
in March 2008.
Nearly mono-energetic, high-intensity and
100% polarized γ-ray beams from the HIγS facility were used to search for low-spin states in
238
U at excitation energies of 2.94–3.50, 3.90–
4.20, and 4.40 MeV with beam energy spread
of ∼3–4%. The scattered γ rays were detected
with four 60% high-purity germanium (HPGe)
detectors which were positioned at 90◦ relative
to the beam at azimuthal angles of 0◦ , 90◦ , 180◦,
and 270◦ . Detectors placed in the polarization
plane of the beam at angles of 0◦ and 180◦ are
in the horizontal orientation while the detectors
placed out of the plane of the beam at angles of
90◦ and 270◦ are in the vertical orientation. All
detectors were furnished with passive 2 cm lead
shields. The γ-ray beam energies were measured
at low γ-ray flux with an HPGe detector placed
into the path of the beam. The γ-ray fluxes during the actual experiments were determined to
be about 4.5×106 to 1.4×107 γ/s as measured
by the HPGe detector placed at 11.2(1)◦ to the
beam direction and observing Compton scattering from a copper plate.
The 238 U targets – one with a mass of 12.9 g
and a thickness of 0.159 cm and the other with
a mass of 19.2 g and a thickness of 0.238 cm –
were provided by Lawrence Livermore National
Laboratory. The smaller target was used at energies of 2.94, 3.02, 3.10, and 3.18 MeV but then
was replaced with the larger target for the other
energies. The target was placed in an evacuated
plastic tube in order to reduce the background in
the measured spectra. The γ-ray beam irradiated
the target for 6 to 8 hours per energy.
Data collected at γ-ray beam energies of 2.94,
3.02, and 3.10 MeV have been analyzed so far.
In addition to the de-excitation to the ground
state, we also observed strong transitions to the
first excited state located at an excitation energy
of 45 keV. Figure 7.29 shows the low-energy part
of the level scheme in 238 U. The cross section of
elastic scattered 238 U is 0.011(1) mb.
Spin
Energy
4+
148.4 keV
2+
44.9 keV
0+
0.0 keV
Figure 7.29: A low-energy level scheme for
238 U.
125
Excited levels were observed for the first time
This work is supported by NSF/DHS grant
in 238 U(γ,γ0) with resonance widths shown in CBET-0736123.
Fig. 7.30. The parity of these new states has
been determined by comparing spectra in the
horizontally-oriented and the vertically-oriented
detectors. Figure 7.31 shows a comparison between spectra collected from the detectors in
both orientations at Eγ = 3.10(0.13) MeV with
the beam profile overlaid to designate the peaks
of interest.
Figure 7.30: Resonance widths Γ20 /Γ with statistical uncertainty for levels in 238 U.
We plan to complete the scanning of the region from Eγ = 3.0 to 6.0 MeV for 238 U. Further
investigation of states in other nuclear materials as well as in shielding materials will be performed using NRF. These new data will improve
the database on excited states in the actinides
and reduce the risk of an incorrect material identification during cargo interrogation.
Figure 7.31: (Color online) Spectra obtained at
Eγ = 3.10(0.13) MeV with detectors
in horizontal (top panel) and vertical
orientation (bottom panel) with the
beam profile (dotted curve) overlaid
in both.
[Hei88] R. D. Heil et al., Nucl. Phys., A476, 39
(1988).
126
7.5.2
Photodisintegration Cross Section of
241
Am
A.P. Tonchev, S.L. Hammond, C. Huibregtse, A. Hutcheson, C.R. Howell, H.J. Karwowski, J.H. Kelley, E. Kwan, G. Rusev, W. Tornow, TUNL; D.J. Vieira, J.B. Wilhelmy,
Los Alamos National Laboratory, Los Alamos, NM ; M.A. Stoyer, Lawrence Livermore National
Laboratory, Livermore, CA
Improved γ-ray-induced neutron measurements on minor actinides are desired for the
Global Nuclear Energy Partnership’s efforts toward the transmutation of long-lived radioactive waste with advanced high-neutron-energy reactors, and for establishing improved diagnostics for nuclear device performance. Here we concentrate on γ-ray induced reactions on
the minor actinide 241 Am.
The photodisintegration cross section on a radioactive 241 Am target has been measured for the
first time using monoenergetic γ-ray beams from
the HIγS facility. Induced activity from 240 Am,
produced via the (γ,n) reaction, was measured
by the activation technique using high-resolution
high-purity germanium (HPGe) detectors. The
(γ,n) cross section was determined both by measuring the absolute γ-ray flux and by comparison to the 197 Au(γ,n) and 58 Ni(γ,n) cross sections used as standards. Previous (n,2n) measurements on the same radioactive 241 Am targets
using mononergetic neutron beams from TUNL
were performed from threshold (Bn = 6.6 MeV)
to 14.5 MeV [Ton08]. The (γ,n) reaction on
241
Am leads to the same residual nucleus as
the (n,2n) reaction, however the reaction mechanisms are very different. In the case of the
(γ,n) reaction, the GDR is excited first, and neutron emission is the main mechanism of its decay.
The process is mostly compound, and the preequilibrium part is not more than 10–15%.
In the following, we report new data for the
excitation function of the 241 Am(γ,n) reaction
from near threshold to 16 MeV incident γ-ray
energy. The first preliminary results from the
(γ,n) cross section measurements at HIγS are
shown in Fig. 7.32. They are compared to statistical nuclear-model calculations performed with
the gnash code. In general, our cross-section
data are lower then the model prediction. Further measurements will be conducted to determine the shape and the amplitude of the photodisintegration cross section of 241 Am.
Figure 7.32: (Color online) Preliminary experimental results of 241 Am(γ,n) crosssection measurements at HIγS.
This work was supported by the National Nuclear Security Administration under the Stewardship Science Academic Alliances Program
through Department of Energy grant DE-FG5206NA26155 and the auspices of the U.S. Department of Energy at Los Alamos National Laboratory by the Los Alamos National Security, LLC
under Contract No. DE-AC52-06NA25396 and
at Lawrence Livermore National Laboratory by
Lawrence Livermore National Security, LLC under Contract No. DE-AC52-07NA27344.
[Ton08] A. P. Tonchev et al., Phys. Rev., C77,
054610 (2008).
128
7.6
7.6.1
HIγS Intrumentation
Precise Determination of Total Absolute γ-Ray Intensity at HIγS
M.W. Ahmed, M.A. Blackston, M.D. Busch, A.S. Crowell, S.S. Henshaw, C.R. Howell, J. Li, P. Kingsberry, B.A. Perdue, S. Stave, H.R. Weller, TUNL; M. Emamian, S.
Mikhailov, G. Swift, Y.K. Wu, Duke University Free Electron Laser Laboratory, Durham, NC ;
R.M. Prior, M.C. Spraker, North Georgia College and State University, Dahlonega, GA
Precision determination of cross sections requires precise knowledge of the incident γ-ray
intensity. A direct measurement is not possible due to the large beam intensities at the
High Intensity γ-Ray Source (HIγS). Six precision-machined copper attenuators have been
installed in the beamline. The Cu attenuation coefficients were determined for several γ-ray
energies between 2.3 MeV and 40 MeV, and then the attenuated beam intensity was measured
in a 10 × 14 inch NaI detector. The total, unattenuated intensity was calculated using the
deadtime-corrected integrated peak counts from the NaI detector along with the measured Cu
attenuation coefficients. The results for attenuation coefficients and total intensity are in good
agreement with existing attenuation data, intensity calculations, and known cross sections.
An evaluation of the accuracy of these measurements is underway. Future experiments in the
γ-ray vault will use this system to perform flux measurements.
The precision determination of cross sections
for upcoming experiments at HIγS requires precise knowledge of the incident γ-ray intensity. A
method of measuring the γ-ray intensity was successfully tested during Spring 2007. The γ-ray
intensity from the upgraded HIγS facility is so
high (> 108 γ/s) that a direct measurement is
impossible. The rates would be more than a NaI
detector, for example, can withstand. However,
NaI detectors are very good at significantly lower
rates (< 10kHz). In order to bring the beam
intensity down to directly measurable rates, six
precision-machined copper attenuators have been
installed in the HIγS beamline. Each of the attenuators is 8.00 ± 0.08 cm thick, individually
mounted, and controlled remotely. Previous attempts using attenuators at HIγS have caused
problems by introducing large backgrounds in the
γ vault. To decrease the background, the attenuator system is located near the exit of the storage
ring, about 50 meters from the location of the target. In addition to the increased distance, the extra collimation and shielding which resulted from
being so far upstream helped to keep the backgrounds low in the γ vault.
Extrapolating from the directly measured but
attenuated beam requires that the attenuation
coefficients be known. The coefficients are also a
function of energy, so they must be determined
for each energy that will be used during an experiment. The copper attenuation coefficients were
determined for several γ-ray energies from near
deuteron breakup at 2.3 MeV up to 40 MeV.
They were found by using a 10x14 inch NaI detector in the beam and integrating the counts in
the detector when one attenuator is inserted and
removed from the beam. (To keep the rates manageable in the NaI detector, several attenuators
were usually inserted upstream of the attenuator being measured.) A pulser system was used
with the NaI detector to accurately determine
the deadtime. The deadtime-corrected ratio of
the yields then gave the attenuation factor. The
results are shown in Fig. 7.33 and agree very well
with the results from Ref. [Ber98] over a wide
energy range. Different combinations of the attenuators were used to test their effect on the
γ-ray beam, looking for differences in counts and
resolution. The addition of extra collimators did
not affect the measured energy spectrum of the
beam at 8.0 MeV as viewed by a high-purity germanium detector.
With the attenuation factors accurately determined, the total unattenuated intensity can
be calculated using the combined attenuation factors of all the attenuators used in a given run and
the deadtime-corrected integrated peak counts
from the NaI detector.
Attenuation Factor
The total intensity at the various energies is
shown in Table 7.2. The upstream collimator
was changed for certain measurements. In addition, different optical klystron (OK) systems
were used. OK-4 provides linearly polarized photons, but higher energy photons from higher order modes are present on-axis, which leads to
degradation of the mirrors at high intensities.
OK-5 produces circularly polarized light, and its
higher order modes are farther from the center
of the mirror leading to less mirror damage at a
given intensity.
30
25
20
15
10
5
0
5
10
15
20
25
30
35
40
45
Energy [MeV]
Figure 7.33: (Color online) The attenuation factor of copper for different energy
photons from [Ber98] plotted with
the values measured at HIγS. The errors are statistical only and equal to
the size of the points.
As a cross-check of the attenuated beam calculations, an off-axis Compton scattering experiment was performed [Ton07]. Its results are also
included in Table 7.2 and agree very well with
the NaI results.
129
paddle system consists of a thin plastic scintillator veto paddle viewed by two photomultipler
tubes (PMTs), a thin converter foil, then two
more paddles. The second paddle has two PMTs
as well, while the third paddle uses only one
PMT. The paddle system gives a signal when the
second and third paddle are in coincidence and
the first paddle did not fire. This system uses
pair production and Compton scattering to produce a signal that is proportional to the beam
intensity, but it is also very thin so as to not attenuate the beam significantly. However, both
production mechanisms are energy dependent so
the efficiency of the paddle varies with energy.
The paddle was calibrated using the absolute intensity measurements, and the resulting conversion factors are listed in Table 7.4. The factors
were found by taking the measured beam intensity and dividing by the rate seen in the paddle
system. The conversion factors are expected to
be rather large since the very thin nautre of the
converter foil in the paddle means that the paddle is not very efficient (by design).
Table 7.3: Summary of total γ-ray intensity calculations at various energies.
Eγ
[MeV]
2.0
2.3
3.5
5.0
15.0
40.0
Itot
[γ/s]
6.2 × 106
3.8 × 107
3.7 × 108
1.5 × 109
4.5 × 109
1.5 × 108
Table 7.4: Summary of paddle conversion factors.
Table 7.2: Summary of γ-ray intensity measurements at various energies. The result
in boldface is from an off-axis Compton
scattering experiment [Ton07].
Eγ
[MeV]
2.0
2.3
3.5
5.0
5.0
15.0
40.0
System
OK-4
OK-4
OK-4
OK-4
OK-4
OK-5
OK-5
Colli.
[in]
1.5
1.5
1.5
1.5
1.5
0.5
1.5
Iring
[mA]
31
39
55
60
60
66
36
Itarget
[γ/s]
6.5 × 105
4.0 × 106
3.9 × 107
1.18 × 108
1.2 × 108
1.7 × 108
3.6 × 107
Next, the total uncollimated intensity can be
calculated using an empirical formula. Applying the formula gives the intensities shown in Table 7.3. These results agreed with the expectations of the accelerator physicists at DFELL.
Finally, there is an existing system for measuring relative beam intensities at HIγS. The
Eγ [MeV]
2.3
3.5
5.0
15.0
40.0
Paddle
Conversion
Factor
850
760
560
650
650
This test of the attenuator system at HIγS
went very well, and the results all agree with expectations and previous data. This system will
be used in the near future as an integral part of
the upcoming experiments in the HIγS γ vault.
[Ber98] M. J. Berger et al., In XCOM: Photon
Cross Sections Database. NIST, 1998.
[Ton07] A. P. Tonchev, 2007, Private communication.
130
7.6.2
Beam Intensity Determination at HIγS Using the 2 H(γ,n)p Reaction
S. Stave, M.W. Ahmed, H.R. Weller, TUNL
Determination of the γ-ray beam intensity is critical for the accurate measurement of cross
sections at the High Intensity γ-Ray Source (HIγS). Previous determinations have relied on
a NaI detector and a high-purity germanium detector (HPGe). The NaI requires careful
deadtime corrections and the HPGe requires extensive simulations and neither are capable of
remaining in the beam during full intensity running. New measurements have been performed
over the last year using liquid scintillator neutron detectors and the 2 H(γ,n)p reaction, whose
cross section is well known in the 10 to 15 MeV energy range.
Determination of the γ-ray intensity at the
High Intensity γ-Ray Source (HIγS) is critical for
the accurate measurement of cross sections. Previous determinations have relied upon a nearly
100% efficient NaI detector and a high purity germanium (HPGe) detector. The NaI detector was
placed directly in the γ-ray beam and so required
several attenuators (see Sect.7.6.1). The HPGe
detector was placed off-axis and used the known
Compton scattering cross section off of copper.
The NaI detector requires careful deadtime measurements and cannot run in the beam during
full intensity. The HPGe detector requires careful simulation of the detector response and also
cannot typically be used during full intensity running even off of the beam axis. The only method
currently employed all of the time to determine
the γ-ray intensity is a three paddle system which
remains in the beam. The three paddles are thin
scintillators with a converter foil after the first
paddle. The first paddle acts as a charged particle veto while the next two paddles are taken
in coincidence to create a signal which is proportional to the γ-ray beam intensity. However, the
cross section for interaction with the foil is energy dependent and so the paddle system must
be calibrated against the NaI or HPGe detector
for each energy.
A new system has been developed which relies upon the well-determined cross section for
2
H(γ,n)p. A table of total experimental cross
sections for photon laboratory energies of 2.754
to 74 MeV is given in Ref. [Jau96]. The uncertainties in the 10 to 15 MeV region are about
3%. A D2 O target of known thickness was placed
in the beam. By further assuming that the angular distribution of the neutrons is pure sin2 (θ),
the differential cross section can be determined at
any angle. Liquid scintillator neutron detectors
filled with BC-501A [Cro01] were then placed at
90◦ where the 2 H(γ,n)p cross section is largest,
and the distance from the target to the detectors
was carefully measured. The photomultiplier signals were then sent into MPD-4 pulse shape discrimination modules. These modules can be operated in a nearly-deadtime-free mode that puts
out a logic pulse when a neutron is detected. The
neutron detectors were shielded with lead to cut
down on additional γ rays that would add background to the neutron rate. The neutron detectors were placed between 50 and 100 cm from the
D2 O target, which typically limited the neutron
rate to tens of Hz. Monitoring of the neutron detector rate gives a relative intensity measurement
from run to run, and the known cross section for
2
H(γ,n)p is then used to determine the intensity
of the γ-ray beam given the known neutron detector efficiency, solid angle and target thickness.
This method of determining the γ-ray beam
intensity was used during part of the HIγS
optical-time-projection-chamber engineering run
in March 2008 and during the entire 3 He
Gerasimov-Drell-Hearn (GDH) sum-rule experiment in May 2008. The results agreed with the
intensity estimate from the paddle system at the
10% to 20% level. Further cross calibrations will
be performed using the NaI and HPGe detectors,
but the very different counting rates will make
these comparisons a challenge.
[Cro01] A. S. Crowell, Ph.D. thesis, Duke University, 2001.
[Jau96] W. Jaus and W. S. Woolcock,
Phys., A608, 399 (1996).
Nucl.
t = 25 − 30 min.
60
40
40
Y (mm)
50
Y (mm)
50
30
30
20
20
10
10
0
0
10
20
30
X (mm)
40
50
0
0
60
t = 95 − 100 min.
60
60
40
40
10
20
30
X (mm)
40
50
60
t = 145 − 150 min.
Y (mm)
50
Y (mm)
50
30
30
20
20
10
10
0
0
t = 45 − 50 min.
60
10
Images of
20
11
30
X (mm)
40
50
60
0
0
10
20
30
X (mm)
40
50
60
C accumulation in barley shoot taken with the Versatile Imager
for Positron Emitting Radiotracers (VIPER). The VIPER image pixels are integrated to determine the accumulation of 11 C within the shoot. Each image was
made with a 5-minute exposure time. Note the transport out of the upper shoot as
time progressed. Accumulation in an adjacent leaf was evident late in the labeling
period. All images were corrected for background radiation, the relative detection efficiency across the image plane, and the radioactive decay of 11 C. The color
scale indicates the radiotracer activity in each image pixel, with decreasing activity from red to violet. Images such as these were used to investigate the effects
of elevated atmospheric CO2 concentration and root nutrient availability on the
transport and allocation of recently fixed carbon including that released from
the roots via exudation or respiration, in two grass species.
Applied Research
Chapter 8
•
Plant Physiology Studies Using Nuclear Physics Techniques
•
Novel Techniques for Special Materials Identification
134
Applied Research
8.1
Plant Physiology Studies Using Nuclear Physics Techniques
8.1.1
Measurement of Carbon Allocation Responses of Plants to Changes
in Nutrient Availability Under Ambient and Elevated CO2
M.R. Kiser, C.R. Howell, A.S. Crowell, TUNL; C.D. Reid, R.P. Phillips, Duke University
Biology Department, Durham, NC
The primary focus of the collaboration of TUNL and the Duke University Phytotron is to
investigate the impact of elevated levels of atmospheric carbon dioxide (CO2 ) on carbon and
nitrogen dynamics in plants. A radiotracer labeling system was constructed this year and
used to perform an initial set of measurements on plants with
11 CO
2.
These experiments on
two grass species investigated the effects of elevated atmospheric CO2 concentration and root
nutrient availability on the transport and allocation of recently fixed carbon, including that
released from the roots via exudation or respiration. The real-time distribution of 11 C-labeled
photoassimilates was measured in vivo throughout the plant and root environment.
Root exudation is the release of soluble organic compounds (both passively and actively)
into the rhizosphere, the narrow zone of soil
immediately surrounding the root system. Although root exudation accounts for only a small
fraction (< 10%) of total plant carbon allocation,
it may significantly influence nutrient availability
in soil and consequently impact plant productivity. As the photosynthesis rate increases due to
the rising concentration of atmospheric CO2 , it
is not clear whether a larger fraction of the additional assimilated carbon will be allocated belowground and released into the rhizosphere by root
exudation. Previous studies using long-lived or
stable carbon isotopes have not resulted in consistent findings.
γ rays from electron-positron annihilation were
detected in a versatile detection system that included single and coincidence γ-ray counting and
2D positron-emission imaging.
The radiotracer accumulation data were analyzed with an input-output statistical model
[Min80] to determine transport and partitioning
quantities of recently fixed carbon in each plant
studied. The model fit to our data gave the mean
photoassimilate transport velocities from the uptake leaf through the shoot and into the root,
as well as the carbon allocation in the uptake
leaf, shoot and root, including the contributions
of root exudation and respiration. Our preliminary findings are discussed below.
The newly installed radioactive gas-handling
system and labeling loop enabled us to investigate the real-time carbon allocation patterns
of two plant species in response to short-term
changes in nutrient availability under ambient
and elevated CO2 conditions. An annual crop
grass (barley) and a perennial native grass (scirpus) were labeled with 11 CO2 under both ambient (400 ppm) and elevated (600 ppm) CO2 concentrations in the controlled-environment growth
chamber at the Duke University Phytotron. Two
sequential measurements were performed per day
on each plant: a measurement with the root in
a control nutrient solution immediately followed
by a measurement with a 10-fold increase or decrease in nutrient concentration. The 511-keV
8.1.1.1
Influence of Elevated CO2
We found that the average velocity of photoassimilates in the phloem from the shoot to the
root is higher for both grass species grown in elevated CO2 than those grown in ambient conditions. Specifically, the mean velocity of photoassimilates in the phloem from the shoot to root
in the barley plants studied was (10.6 ± 1.0)
mm/min for plants grown at the elevated CO2
level and (6.4 ± 1.5) mm/min for those grown
at the ambient level. Similarly, for scirpus the
mean velocity of photoassimilates in the phloem
from the shoot to root was (7.9 ± 2.2) mm/min
for plants grown at the elevated CO2 level and
(4.4 ± 1.5) mm/min for those grown at the am-
Applied Research
bient level. The uncertainties in the data are
dominated by the biological variance in the three
plants used in each measurement set.
Because root growth is directly proportional
to belowground carbon allocation, the fraction
of belowground carbon allocated to root exudation is an indicator of the amount of root exudation per unit root biomass. As shown in Fig. 8.1,
our preliminary data indicate a strong correlation
between atmospheric CO2 concentration and the
fraction of belowground carbon used for root exudation in both species. The measured barley root
exudation fraction at the elevated CO2 concentration was about two times that in plants grown
at the ambient CO2 level. Whereas, the scirpus
root exudation fraction in the elevated CO2 atmosphere was about 1/3 that in plants grown at
ambient CO2 .
Percent Root Exudation
7
6
Barley
Scirpus
b
5
4
3
a
2
c
1
0
Ambient Elevated
[CO2] [CO2]
60
50
40
30
20
10
0
-10
-20
-30
-40
-50
-60
Ambient Elevated
[CO2]
[CO2]
Figure 8.1: The root exudation fraction of
recently-fixed
carbon
allocated
belowground that was released as soluble organic compounds. Significant
differences (p < 0.05) are indicated
by different postscripts.
In addition to the correlation of the amount
of exudation per unit root biomass with atmospheric CO2 concentration, the fraction of the
total recently-fixed carbon allocated to root exudation provides clues about the differences in resource prioritization in the two grass species. In
the barley plants studied, (1.0 ± 0.2)% and (1.2
± 0.3)% of the total fixed carbon was released
as root exudation in plants grown at the ambient
and elevated CO2 levels, respectively. This apparent lack of dependence on atmospheric CO2
concentration might be an indication that root
exudation is tightly limited by the plant. On
the other hand, the absolute root exudation in
scirpus seems strongly correlated with the atmospheric CO2 concentration. In the scirpus, (0.6
± 0.1)% and (0.3 ± 0.1)% of the total fixed carbon was exuded in plants grown at the ambient
and elevated CO2 levels, respectively. This trend
might be an indication that the higher concentration of CO2 triggers the scirpus to store carbohy-
Ambient [CO2]
Elevated [CO2]
b
a
a
H to L L to H
c
H to L L to H
Figure 8.2: Effect of switching nutrient treatments on root exudation for both ambient and elevated CO2 . The percent
change in root exudation is shown
for plants grown at high nutrient and
switched to low (H→L) and vice versa
(L→H). Significant differences (p <
0.05) in the magnitude of percent
difference are indicated by different
postscripts.
8.1.1.2
a
135
drate in the root for survival beyond the growing
season.
% Change in Exudation
Root Exudation Response to Nutrient Availability
As shown in Fig. 8.2, our data indicate a rapid
response in root exudation to short-term changes
in nutrient availability. These results are the first
observation of a short-time scale (i.e., less than
4 hours) response in root exudation to external
stimuli. We found no significant difference in the
responses of the two grasses. The data plotted
in Fig. 8.2 are the combined results of the measurements made on both grass samples. As before, the uncertainties in these data are mostly
due to the biological variance in the plants. In
these studies plants were grown in a root solution
with either a “high” or “low” nutrient concentration, with the “high” being 10 times more concentrated than the “low”. Each plant was first
labeled for about 4 hours with the nutrient concentration in which it had acclimated and then
switched to the other nutrient concentration and
labeled immediately afterward. The short time
scale of the response suggests that the mechanism for adjusting the root exudation is active.
We also found that the magnitude of the change
in the root exudation due to the altered nutrient
availability was dependent on the elevated atmospheric CO2 level. These results suggest that barley and scirpus plants in nutrient-deficient conditions can respond quickly by reducing root exudation losses when nutrients become available.
[Min80] P. E. H. Minchin and J. H. Troughton,
Ann. Rev. Plant Physiology, 31, 191
(1980).
136
Applied Research
8.1.2
Radioactive Gas-Handling System for Supplying Pulses of
Plants in a Controlled-Environment Chamber
11
CO2 to
M.R. Kiser, C.R. Howell, A.S. Crowell, TUNL; C.D. Reid, R.P. Phillips, Duke University
Biology Department, Durham, NC
The radioactive gas-handling system for our plant research program was installed this year.
The system enables the transfer of 11 CO2 gas via underground lines from the tandem accelerator laboratory at TUNL, where it is produced, to the Phytotron, where it is loaded
into a labeling loop for uptake by plants during photosynthesis. The 11 CO2 is introduced to
plants in the controlled-environment growth chamber that is dedicated for these studies with
short-lived radiotracers. The integrated gas-handling system and labeling loop were approved
by the Duke Radiation Safety Office for operation as engineered. This initial phase of the
gas-handling system has pulse-loading capability. The system will be upgraded in the coming
year to have both pulse and continuous loading capabilities. In addition, a versatile γ-ray
detection system was developed this year to carry out the first set of experiments using this
new radiotracer system.
In the past year, a radioactive gas-handling
system was installed to transfer 11 CO2 gas from
the radioisotope production area in the tandem
accelerator laboratory at TUNL to the dedicated
labeling chamber at the Phytotron. The radioactive gas transport system is contained inside 4 in.
diameter conduits that run between the time-offlight (TOF) target room in the tandem lab and
the mechanical room under Greenhouse 6 (G6) at
the Phytotron. The installation of these conduits
was included as part of the construction of the
French Family Science Center, located adjacent
to both TUNL and the Phytotron. Inside the
Phytotron, smaller conduits extend from the G6
mechanical room across the ceiling of the main
mechanical areas and then up into the research
area directly behind the labeling chamber. The
total distance from the target cell in the TOF target room along the conduit to the labeling chamber at the Phytotron is ∼350 feet.
A schematic of the radioactive gas-handling
system and labeling loop is shown in Fig. 8.3.
The tubing that connects the vacuum pump at
TUNL to the CO2 trap at the Phytotron has 3/8
in. outer diameter. A tube with 1/8 in. outer diameter connects the target gas cell at TUNL to
the cryogenic CO2 trap at the Phytotron. The
radioactive gas is delivered to the Phytotron by
evacuating the transfer lines through a molecular
sieve trap using a vacuum pump located in the
TOF target room at TUNL.
The labeling loop is a closed system that
is used to expose the studied plant to 11 CO2
by flowing the air in the loop across a specific leaf of the plant. The loop is composed
of 1/8 in. Teflon tubing and includes diagnostic instrumentation and control components, as
well as a temperature-controlled leaf cuvette.
Mass
Flow Pressure
Regulator Gauge
1%
CO2
gas
10 MeV protons
Humidifier/Condenser
CO 2
Analyzer
V7
V9
V8
Leaf
Cuvette
V3
LN 2
Trap
Gas Cell
V1
Phytotron
N
2
gas
V5
TUNL
V4
V10
V6
V2
Vacuum
Pump
V11
Labeling Chamber
Flow
Meter
Circulation
Pump
Flow
Figure 8.3: (Color online) Diagram of the radioactive gas-handling system that is used
to transfer 11 CO2 from the production
area at TUNL to the labeling system
at the Phytotron. Valves V1-V4 and
V7-V10 are solenoid valves that are
controlled remotely to minimize personell radiation exposure. The labeling loop on the left is used to introduce 11 CO2 to the leaf of a plant that
is sealed into the leaf cuvette.
After evacuating the transfer line, the trap is
cooled with liquid nitrogen and solenoid valve V1
is opened to empty the contents of the gas cell
Line Detector
Lead
Shielding
&%&% &%&% &%&% $#$# $#$# $#$#
&%&%&%&% &%&% $#$# $#$# $#$#
&%&% &%&% &%&% $#$# $#$# $#$#
&%('&% ('&% (' (' (' $# (' $# (' $#
(''( (''( (''( (''( (''( (''( (''(
Leaf
Cuvette
Applied Research
Leaf Detector −
One on each side
of leaf cuvette
))))))))))
*)*)*)*) *)*) *)*) *)*)
*)**)**)**)*"!"! *)* "!"! "!"! "!"! "!"! "!"! "!"! "!"!
****"!"! * "!"! "!"! "!"! "!"! "!"! "!"! "!"!
"!"! "!"! "!"! "!"! "!"! "!"! "!"! "!"!
"! "! "! "! "! "! "! "!
Root Detector −
One on each side
PMT
137
Gas In
Gas Out
2D PET Imager −
One module on each
side of shoot
Nutrient Solution
Pump
Exhaust
of root cuvette
PMT
Root Cuvette
CO2 −Free Air
,+,+ ,+,+
,+,+,+,+
,+,+
Exudate
Detector #1
PMT
Exudate
Detector #2
Nutrient Solution Flask
PMT
.-.- .-..-.-.-..-.-
Respired CO2
Detector #1
0/0/ 0/0/ 0/0/
0/0/0/0/ 0/0/
0/0/ 0/
PMT
Respired CO2
Detector #2
Soda Lime
Granules
Figure 8.4: (Color online) Experiment setup used to measure 11 C transport and allocation within a
plant and its root environment, including carbon released from the roots as root exudation and root respiration. Coincidence counting is used to measure the accumulation of
11 C in the uptake leaf, root tissue, root exudation, and root respiration. A prototype 2D
positron emission imager monitors the distribution of 11 C in the shoot region of the plant
with about 2.5 mm spatial resolution [Kis07].
through the cooled trap. The 11 CO2 is frozen
out of the gas mixture and stored in the trap for
later loading into the labeling loop. The other
gases from the target cell (mostly nitrogen and
carbon monoxide) are pumped through the trap
and back to the TUNL high speed exhaust system. As a radiation safety precaution, the system
is kept below atmospheric pressure during gas
transfer and trapping to ensure that any leaks
in the tubing result in outside air being pulled
into the system rather than radioactive gas leaking out. With valves V1 through V4 closed, the
trap is warmed to room temperature to return
the 11 CO2 to the gaseous state in preparation of
loading it into the labeling loop. When ready to
load the trapped gas into the labeling loop, valves
V7 and V8 are opened, and the bypass valve (V9)
is closed, thereby putting a pulse of 11 CO2 into
the loop.
This newly developed system provides the
capabilities for production of 11 CO2 at TUNL,
transfer of the 11 CO2 gas from the target area at
TUNL to the labeling chamber at the Phytotron,
labeling of photoassimilates with 11 C, and in vivo
γ-ray detection for real-time measurements of the
radiotracer distribution in small plants. The hy-
brid radiotracer detection system (Fig. 8.4) uses
two detection schemes to measure the distribution of 11 C-labeled photoassimilates throughout
the plant and root environment: (1) coincidence
counting to monitor 11 C accumulation in the uptake leaf, root tissue, root exudation, and root
respiration; and (2) 2D positron emission imaging to measure the accumulation of 11 C as a function of location in the shoot region of the plant.
Additionally, collimated single detector counting
is used to monitor the 11 C activity in the labeling loop as a function of time. The radiotracer
data are analyzed with an input-output statistical model [Min80] to determine the relevant
physical quantities to parameterize the real-time
transport and allocation of recently-fixed carbon.
This model gives the transport velocity of radiotracer within the plant and the fractional allocation of carbon to different parts of the plant.
[Kis07] M. R. Kiser et al., TUNL Progress Report, XLVI, 122 (2007).
[Min80] P. E. H. Minchin and J. H. Troughton,
Ann. Rev. Plant Physiology, 31, 191
(1980).
138
Applied Research
8.2
8.2.1
Novel Techniques for Special Materials Identification
Detecting Specific Material with Nuclear Resonance Fluorescence
D.P. McNabb, M.S. Johnson, C.A. Hagmann, Lawrence Livermore National Laboratory, Livermore, CA; A.P. Tonchev, C.T. Angell, S.L. Hammond, C.R. Howell, A. Hutcheson, H.J.
Karwowski, J.H. Kelley, E. Kwan, G. Rusev, W. Tornow, TUNL
Photons with energies above 1 MeV can be used to detect small amounts of nuclear material inside of large cargo containers. The method consists of using an intense and monoenergetic beam of high-energy photons in order to excite specific levels of interest associated with
certain isotopic compositions. Initiated by Homeland Security programs, LLNL is developing
a program FINDER (Fluorescence Imaging in the Nuclear Domain with Extreme Radiation)
that uses nuclear resonance fluorescence (NRF) to isotopically map a container. Initial feasibility tests of FINDER were performed at the HIγS facility. The preliminary results are
discussed.
The proposed FINDER system works by im- set an experimental upper limit on the amount
pinging a tunable mono-energetic γ-ray beam of notch refilling in our experimental setup. A
onto a container under investigation. The pho- schematic of the setup is shown in Fig. 8.5.
tons in the beam pass through the container
and a fraction of them scatter off of its interior components through various electromagnetic
processes. If the incident beam interacts with
special nuclear material (SNM), the beam photons will be absorbed and scattered into all directions, depleting the spectrum at the resonant
energy. When this occurs, the transmitted γ-ray
beam from the container accrues a notch a few
eV in width after passing through the material of
Figure 8.5: (Color online) Schematic of the
interest. This notched spectrum will impinge on
FINDER feasibility test setup at
a witness foil placed on the opposite side of the
HIγS.
container relative to the γ-ray source.
The witness foil will be made of material identical to the one being sought after. If there is
a notch in the spectrum, then there will be no
scattered photons with the corresponding energy
from the witness foil. The corollary is that if
there is no notch in the transmitted spectrum
then there will be scattered photons from the witness foil at that energy. A simple arrangement of
γ-ray detectors focused on the witness foil are
used to measure the NRF photons.
The main goals for these measurements were
to demonstrate the concept of transmission detection and to perform initial validation of models of the FINDER concept. In particular, previous work [Pru06] indicated that it may be possible for the notch be refilled by small-angle scattering if the interrogating photon source is too
broad in energy. While we did not expect notch
refilling to be significant at HIγS, we were able to
For these measurements cargo types of tungsten (W) and depleted uranium (DU) were assembled and had thicknesses of 1.27 cm. The
cargo of DU or DU+W was placed in front of a
0.5 in. thick lead collimator wall with a 1.0 in. diameter opening. This collimator provided beam
resolution of 3%. The different configurations are
given in Table 8.1.
Since DU was chosen to be the material of interest for this test, it was placed in the witness
foil location. The thickness of the DU foil was 3
mm. Four 60% high-purity germanium (HPGe)
detectors were positioned at backward angles relative to the beam and facing the witness foil location. Two of the detectors were positioned at
100◦ and the remaining two were positioned at
150◦. The detector to foil distance was 20 cm.
Absorbers of 4 mm thick copper were placed on
the front face of the four detectors to reduce back-
Applied Research
139
Table 8.1: Different cargo configurations are listed on in the column. Raw counts and counts normalized to fluence in the dominant resonance peak at 2176 keV are shown.
Cargo material
No cargo
1/2 in. DU
1/2 in. DU +1/2 in. W
Norm. to 238 U foil
1
0.694
0.836
ground, and a passive shield of 2 cm thick lead
was placed around the HPGe’s to reduce background from room scatter.
Raw counts
633(30)
75(24)
62(34)
Norm to 0◦
633(30)
108(35)
74(41)
Time
2.7 hrs
9.3 hrs
25.5 hrs
Exp. counts
633
95
95
Counts
2176 keV
ent cargo materials. The number of counts in the
line at 2176 keV is normalized to the time integrated flux that was recorded by the flux mon700
itor. The spectra in Fig. 8.6 include data from
runs
in which no cargo was present, DU cargo
600
was present, and DU+W cargo was present. For
500
the latter, W was placed downstream of the DU
to produce notch refilling. Clearly evident in the
DU+W
400
null cargo set, is the presence of the 2176-keV line
238
in
U. This is a result of an undisturbed beam
300
impinging on the witness foil and scattering into
DU
the detectors. In the spectra for DU, the 2176200
keV line is depleted significantly. In Table 8.1 for
100
None this data set, the counts in the channel consistent
with 2176 keV have a statistical significance of
2100
2200
2300
2050
2150
2250
3σ which is at our threshold for declaring this a
Eγ (keV)
peak. This deficit resulted in barely significant
NRF scattering from the witness foil, and the
Figure 8.6: (Color online) Spectra from various
peak area is consistent with our expectations for
runs using the indicated cargos.
no refilling of the notch, which accomplishes the
first of our earlier goals.
The γ-ray flux from HIγS was monitored with
Finally, the cargo with DU and W shows
a 123% HPGe located 4.8 m downstream of the a negligible number of counts at 2176 keV. As
setup. Before each run this detector was posi- shown in Table 8.1, the statistical significance of
tioned at 0◦ relative to the incident γ-ray beam. the fitted region was about 2σ, which falls beThe incident γ-ray beam was attenuated with low our criteria for declaring the existence of a
four 20.32 cm thick copper attenuators, posi- peak. This absence of a peak indicates that the
tioned 52 m upstream. This allows us to measure notch was not refilled, and accomplishes our secthe energy spread of the γ-ray beam with the ond goal of the project.
maximum beam current from the FEL storage
This work was performed under the auspices
ring. During the production runs the flux moni- of the U.S. Department of Energy by Lawrence
tor was rotated to 18◦ relative to the beam and Livermore National Laboratory under Contract
measured the Compton scattered photons from a DE-AC52-07NA27344. This work was made pos3 mm thick copper foil positioned 162 cm from sible with funding from DHS/DNDO.
the γ-flux monitor detector.
Spectra from the four 60% HPGe detectors
were summed and are shown in Fig. 8.6. Ta- [Pru06] J. Pruet et al., J. Appl. Phys., 99,
123102 (2006).
ble 8.1 summarizes our observations for 3 differ-
S
uper Conducting Saddle Coil is being wound on a turn table. The HIγS frozen
spin polarized target (HIFROST) requires a saddle coil to produce target spins
which are transverse to the γ-ray polarization axis. Sustaining a ∼ 0.4 T field
in a small target region of 10 cm is a challenge. TUNL and JLab have started a
collaboration to manufacture and test the saddle coils.
Nuclear Instrumentation and
Methods
Chapter 9
•
Accelerator Operations
•
Ion Sources
Development of HIγS Target Room and Associated Equipment
Polarized Targets
•
•
•
Beamlines, Targets, and Facility Development
142
Nuclear Instrumentation and Methods
9.1
Accelerator Operations
9.1.1
Tandem Accelerator Operation
C.R. Westerfeldt, E.P. Carter, J.D. Dunham, R. O’Quinn, TUNL
9.1.1.1
Tandem Operation
Terminal Potential (MV)
The TUNL FN tandem accelerator was operated
184 days for 3340 hours at terminal potentials
ranging from 0.8 MV to 8.8 MV during the period August 1, 2007 to July 30, 2008. Beams
accelerated during this period include polarized
and unpolarized protons and deuterons and also
4
He. The terminal operating potential during the
reporting period is shown in Fig. 9.1.
10
8
6
4
tors were found and replaced during this opening.
Because it had been several years since the foils
were replaced, we decided to break vacuum and
to clean the broken foils from the terminal stripper box. A new set of 75, 3-4 mg/cm2 collodionbacked stripper foils were loaded. Repairs were
made to the foil-changer counter system which
had stopped displaying the correct foil number
some months earlier. The terminal steerer control was repaired, as the linkage was slipping and
the control room readout was inaccurate. A new
set of corona needles was installed as well. The
tank was closed on April 23, 2007 and operations
resumed on April 24.
9.1.1.2
2
Laboratory Projects
Jun. 12, 2008
Apr. 7, 2008
Jan. 31, 2008
Nov. 26, 2007
Sept. 21,2007
July 17,2007
The new large 59-degree general purpose scattering chamber installation was completed in the fall
0
of 2007. This 22 in. ID aluminum chamber was
previously used for fission studies in target room
#4 but had been in storage for the past decade.
The chamber height was increased by adding a 4
Date
in. high extension collar to provide more room for
Figure 9.1: The TUNL FN Tandem Operating
internal collimators and larger detectors. This
Potential.
chamber is pumped by a new 8 in. cryopump
to eliminate hydrocarbon contamination. A oneOne opening of the tandem was made on April ton, wall-mounted crane was installed to permit
7th to perform routine maintenance. Upon en- simple removal of the large heavy chamber lid for
try we discovered that a bolt had fallen from chamber access. The direct extraction negative
the upper high-energy charging-chain idler and ion source was overhauled mechanically and elechad jammed the lower 4 in. idler. The charg- tronically during the year, to improve it’s outing chain had sawed the idler to pieces and there put for new classes of experiments that require
was considerable polyurethane chaff in the tank more beam than we were previously getting. Anand column to clean up. Both charging chains alyzed negative proton and deuteron beams with
were removed, repaired, cleaned and reinstalled. intensities of at least 15 to 20 microamperes are
Because the high-energy charging chain did not now available for acceleration at the low-energy
run well after re-installation, it was decided to Faraday cup. In the low-energy experimental
install the spare chain, which had been run be- area, the small charged-particle scattering chamfore, a number of years ago. This chain worked ber, used previously with the mini-tandem, was
well. A new chain was ordered as a spare and removed, and an end station and detector array
it was received in May. Three bad column resis- was installed by the γ-ray capture group.
9.2
9.2.1
143
Ion Sources
Atomic Beam Polarized Ion Source
J.D. Dunham, T.V. Daniels, T.B. Clegg, TUNL
During the period July 1, 2007 to June 30,
2008, the polarized source provided polarized
(unpolarized) beams for experiments during 19%
(3%) of the calendar days, with an additional 5%
of the calendar days scheduled for source testing and routine maintenance. Experiments using
beams from the source in the low-energy experimental area occupied 5% of the calendar days,
while provision of polarized beams for the tan-
dem accelerator occupied 17% of the days. Detailed use was as follows: 0 (34) days were used
for polarized H+ (H− ) operation; 0 (36) days
were used for polarized D+ (D− ) operation; and
0 (7) (4) days were used for unpolarized H± (D± )
(He) operation, respectively.
The source ran without major difficulty during this period.
144
9.2.2
First ECRIS Beam to Target with Upgraded Accelerator System
in the LENA Laboratory
J.M. Cesaratto, A.E. Champagne, T.B. Clegg, TUNL
A new electron-cyclotron resonance ion source (ECRIS) has been constructed for TUNL’s
Laboratory for Experimental Nuclear Astrophysics (LENA). Building this source required
modeling and building a new magnetic field confinement region, and designing and constructing a new plasma chamber and beam extraction system [Ces07]. Over the course of the last
year, the new ion source and acceleration system have been extensively tested, and various
components of the accelerator system have been upgraded. At an extraction voltage of 13
kV and an accelerating voltage of 100 kV, we have seen beam currents of 3.48 mA before the
magnet and 270 µA on target.
The electron-cyclotron resonance (ECR) ion capability.
source operates with input rf power of 100 to
500 W at 2.45 GHz. The magnetic field for the
source is provided by a cylindrical array of NdFeB permanent magnets, which produce the 87.5
mT field needed for electron-cyclotron resonance.
The array, which resembles that of Chalk River
Laboratory [Wil98], is composed of twelve 25 mm
× 25 mm × 150 mm NdFeB bars around the
plasma chamber, providing a magnetic-mirrorlike region along its axis. With the rf power and
magnetic field, a plasma is created when low pressure H2 is injected into the chamber. The H+
ions produced are initially extracted at voltages
of 10 to 20 kV from a 5 mm aperture by a acceldecel extraction lens system. The beam formed is
then accelerated from the high-voltage platform
through a 24-gap column, with 10 MΩ resistance
across each gap. In all, the ECRIS can produce
H+ beam of energies from ∼20 to 220 keV. Figure 9.2 shows the ECRIS accelerator system at
LENA.
Figure 9.2:
In early tests of the source during summer
2007, the existing 200 kV, 5 mA power supply
could not provide enough current to transport
the intense beams produced by the new source. A
new 200 kV, 36 mA Glassman power supply was
purchased and commissioned in February 2008.
This open stack, master-slave supply is twice the
physical size of the former supply, so its support
platform was redesigned to accommodate the increase. Another upgrade included replacing the
6 W, 10 MΩ resistors of the acceleration column
with new 20 W resistors. The former resistors
had been severely underrated in terms of power
(Color online) ECR ion source acceleration system at LENA.
As tests progressed throughout early 2008,
two major issues developed. First, during beam
extraction and acceleration, divergent ions in the
beam hit surfaces inside and near the acceleration column, knocking off electrons that were
then accelerated back toward the source. The
backstreaming electrons collided with surfaces of
the acceleration column and charged its electrodes. This charging led to excessive current
loading on the 200 kV supply and, eventually, to
sparking between electrodes. The second issue
was the production of very high bremsstrahlung
x-ray fluxes generated when the backstreaming
electrons hit surfaces at the source. At beam energies of 110 keV, we observed x-ray radiation of
0.5 to 1 r/hr at the ion source.
To remedy these problems, a water-cooled
copper plate with a 0.5 in. diameter tantalum
aperture was installed to collimate the beam before it reaches the acceleration column. This
eliminated the most divergent ions in the beam,
thus limiting the number hitting interior surfaces and thereby reducing the number of secondary electrons. Since installing this copper
plate, excessive current loading on the power supply and sparking between accelerating gaps are
no longer problems. However, we still observe
extremely high levels of x-ray radiation, so additional shielding is currently being installed to
facilitate safe, routine occupancy of LENA areas
outside the interlocked accelerator enclosure.
Another step taken to reduce the number of
secondary electrons generated by the source was
to increase the diameter of the beam line as much
as possible at the exit end of the acceleration
column. A 2 in. inside diameter beam stop and
beam viewer after the column were removed, and
the beam stop was installed farther downstream,
past the first quadrupole lens. In their place we
installed a beam profile monitor (BPM) containing a 3 in. acceptance aperture. Incorporating
the BPM to the system allowed us to remove the
beam viewer.
With these improvements, these systems are
nearly ready for routine use for experiments. We
have obtained the currents shown in Table 9.1 at
various stops along the beam line. These results
were obtained by tuning for maximum beam,
without benefit of BPMs, which must still be
interfaced with LabVIEW. While collecting the
data in Table 9.1, the source parameters were 300
W input rf power, 13 kV extraction voltage, 2.68
sccm H2 gas flow, and +100 kV table voltage. We
are confident of obtaining our goal of 1 mA of H+
beam on target once BPMs provide better information about the beam transport through the
analyzing magnet and quadrupole lenses. The
2007 TUNL Progress Report provided an incorrect normalized emittance of beam from the
source as 0.407 π-mm-mrad [Ces07]. The actual
normalized emittance is 0.194 π-mm-mrad.
When running higher gas flows into the ECR
chamber, our H+ mass fraction decreases. Figure 9.3 shows the results of a recent beam mass
145
scan through the analyzing magnet under the
conditions given above. There 44% of the beam
+
is H+ , 41% is H+
2 , and 15% is H3 . This can
be compared with a beam mass scan in [Ces07],
where the H+ mass fraction with lower gas flow
was 87%. We will continue to study how this
mass distribution depends on ECR chamber pressure. However, the fact that the H+ and H+
2 mass
fractions are nearly equal with high gas flow may
prove useful experimentally at lower beam energies where using H+
2 could effectively double the
number of incident nuclei on target. This could
be especially helpful in probing very small, lowenergy capture cross sections.
ECR Ion Source Mass Scan
35
30
Beam Current (µA)
25
20
15
H2+
H+
10
H3+
5
0
2
3
4
5
6
7
8
Analyzing Magnet Current (arb. units)
9
Figure 9.3: (Color online) Mass scan of hydrogen
beam under heavy gas load.
Table 9.1: Beam currents (in µA) obtained at each
beam stop along the LENA beam line.
The target beam current is current
with electron suppression. The others
are unsuppressed.
IECR
3480
IM agnet
1260
IBS3
900
IT arget
270
Further development of the source will consist
of pulsing the beam at several Hertz and ∼10%
duty cycle, by pulsing the bias voltage to the
plasma chamber and/or the input hydrogen gas.
Cosmic-ray-induced backgrounds can be reduced
by maintaining a large average beam current on
target while turning off detector electronics during the beam pulse “downtime.”
[Ces07] J. M. Cesaratto et al., TUNL Progress
Report, 46, 134 (2007).
[Wil98] J. S. C. Wills et al., Rev. Sci. Instrum.,
69, 4 (1998).
146
9.3
9.3.1
Development of the HIγS Target Room and Associated
Equipment
The HIγS Frozen-Spin Target
S.S. Henshaw, M.W. Ahmed, M. Busch, P.-N. Seo, S. Stave, H.R. Weller, TUNL; A.M.
Bernstein, Massachussetts Institute of Technology, Cambridge, MA; D. Crabb, S. Kucuker, B.
Norum, University of Virginia, Charlottesville, VA; R. Miskimen, University of Massachusetts,
Amherst, MA; S. Whisnant, James Madison University, Harrisonburg, VA
The HIγS frozen-spin target is being constructed for use in several planned experiments.
The status and progress of this multi-institutional effort to provide frozen-spin targets with
longitudinal or transverse polarization are described.
The HIγS Frozen-Spin Target (HIFROST) is
currently under construction for use in several
experiments planned for the near future. These
include the measurement of the Gerasimov-DrellHearn (GDH) integrand for the deuteron and
the determination of nucleon polarizabilities and
spin-polarizabilities from Compton scattering off
of protons and deuterons. In addition, when photon energies at HIγS reach the pion-production
threshold, measurements of multipole amplitudes
in pion photoproduction are planned, many for
the first time ever, in order to determine pionnucleon isospin breaking and to test chiral perturbation theory predictions of these quantities.
The HIFROST dilution refrigerator was designed and built by Tapio Niinikoski at CERN
and was operated successfully there before being
disassembled. It has been shipped to the University of Virginia, where HIFROST collaborators
under the direction of Don Crabb are rebuilding
the apparatus for use at HIγS.
The entire 3 He system has been shown to be
leak free, and the final heat exchanger for this
system is being fabricated at the University of
Virginia and at CERN. Once this task is completed, the 3 He system will be soldered to the 4 He
system, and the entire refrigerator will undergo
a final leak test. Then the entire unit, including all the heat shields, will be assembled and
transported to HIγS to be installed in the target
room.
Various ancillary system that are critical to
the infrastructure for operating the target successfully are being developed at HIγS. Both the
3
He and 4 He pumping stations have been built,
and the pumping lines to connect these pumps
to the refrigerator have been purchased and are
ready to be cut to size and installed. The support structure for the target has been designed,
as have the alignment tracks used to align the polarizing magnet and the Blowfish detector array.
The alignment tracks and the Blowfish substructure that mate to these tracks have been installed
and are ready for experiments. Figure 9.4 shows
what the installed system will look like.
The NMR System which is used to measure the target polarization is also being developed. Various items, including Liverpool Qmeters, power supplies, and a water chiller, have
been acquired from David Haase’s polarized target. These are being tested. A cooling box
for the Liverpool Q-meters has been designed by
Matthew Busch and is being constructed at the
Duke Instrument Shop.
Once these tasks are completed and the target is installed in the target room, experiments,
such as the GDH sum-rule measurement, can
proceed, because longitudinal polarization alone
is required. The Compton-scattering and pionphotoproduction measurements, however, will require transverse polarization as well. We must
therefore be able to apply a transverse holding
field, to rotate the longitudinally polarized target spins, and to maintain transverse polarization
in frozen-spin (data-taking) mode. We have designed a modified inner vacuum chamber (IVC)
vessel to support a pair of superconducting “saddle” coils wound on its exterior, as was done for
our solenoid. This is intended to produce a transverse field of approximately 0.35 T.
There is only a 1.16 mm space between the
original IVC vessel and the next-outermost wall
147
Figure 9.4: (Color online) Schematic of the installed HIFROST target system. The neutron detector
array (Blowfish) is also shown.
in the refrigerator, the radiation shield. Beyond (outside) the radiation shield are two more
tightly-spaced bulkheads (the heat shield and the
outer vacuum envelope). As a result, the diameter of the IVC had to be reduced from that of its
original design to allow room for the solenoid, so
that the coil windings would not contact the radiation shield, possibly resulting in a quench of the
coil. As the saddle coils for a transverse holding
field will have a higher and less uniform profile
on the IVC exterior than the solenoid, an IVC
vessel with an even smaller diameter will have to
be fabricated for this purpose.
The high current (∼25 A) required for the
transverse holding field produces strong forces
that can actually distort the thin walls of the
IVC. This again could cause a quench of the coil.
So although the completed IVC assembly will
provide more support for the coil than a simple
tube, we do not wish to risk this expensive component for development and testing of the coils.
We thus obtained 316L tubing of a standard size
as close as possible to the diameter and thickness
required for the downstream (target) portion of
the IVC and plan to wind prototype coils on such
tube segments for coil development and testing.
Pil-Neyo Seo has obtained all of the equipment
and parts needed to wind these coils and plans to
travel to Jefferson Lab in mid-July to wind these
coils under the supervision of Mikel Seely.
These experiments will initially be conducted
using frozen butanol, doped to provide free
electrons for hyperfine coupling.
We plan
to continue the Compton-scattering and pion-
photoproduction experiments with an active,
scintillating target. This will provide information on the target-proton recoil momentum that
will be crucial in identifying background events.
Doped scintillating materials and successful fabrication techniques for HIFROST have been developed and tested at James Madison University
under the direction of Steven Whisnant.
One challenge will be to guide the light from
a scintillating target to small photo-detectors.
These detectors may have to be positioned within
the IVC, the inner wall of which is formed by the
(dilution) mixing chamber containing the target.
Several schemes for positioning the optical transport system have been devised by Rory Miskimen
at the University of Massachusetts, where potential photo-detectors are being tested and characterized. These configurations appear promising,
and have reduced the light-transport footprint in
the IVC, thus allowing a reduction of its radial
width to 4.3 mm. This is sufficient to permit the
reduction of the outer diameter of the IVC to
that which will allow the transverse saddle coils
to fit safely on its exterior.
Design of the gas-flow control system is nearing completion. Many more components required for the system have been procured, including temperature sensors, flowmeters, pressure
gauges, a power supply for driving our microwave
generator, and the remaining pumps and Dewars.
Plans are underway to install the HIFROST target in conjunction with the transfer of control of
the HIγS facility to TUNL in late 2008.
148
9.3.2
Status of the HIγS Frozen-Spin Target Transverse Holding Coil
P.-N. Seo, M.W. Ahmed, S.S. Henshaw, S. Stave, H.R. Weller, TUNL; P.P. Martel, R.
Miskimen, A. Teymurazyan, University of Massachusetts, Amherst, MA; M. Seely Thomas
Jefferson National Accelerator Facility, Newport News, VA
The current HIγS Frozen Spin Target (HIFROST) can produce target spins polarized parallel
(or antiparallel) to the beam helicity. A target spin polarized transerve to the beam helicity is
needed to carry out the spin polarizability measurements for the proton. This work reports on
a study to design and build a super-conducting transverse-magnetic-field holding coil (called
the saddle coil) for HIFROST.
The Compton@HIγS program will determine
the spin-polarizability of the proton by measuring the nuclear Compton scattering cross section
using circularly polarized γ-rays and a polarized
proton target. This measurement requires a target spin direction transverse to the beam helicity.
The target cell is a long cylinder with an existing
longitudinal holding coil to produce target polarizations along the beam helicity. A relatively
strong magnetic field, perpendicular to the target cylinder axis, is required to produce a transverse polarized target. The only known technique to produce such fields is via a set of superconducting coils placed like a saddle around the
target cylinder.
We have completed a design of such a superconducing saddle coil which can produce a 0.4
tesla magnetic field. The holding coil system is
made using a superconducting wire in a superposition of two saddle coils and each saddle coil
is wound in four layers. In order to estimate the
magnetic fields arising from such a set of coils,
the transverse coil was modeled in mathematica as a set of straight segments and circular arcs
as shown in Fig. 9.5. The ends of the coil were
turned on a radius to match the spacing of the
straight sections resulting in a “racetrack”-shape
coil as shown in Fig. 9.6.
Figure 9.5: A model to calculate the transverse
field which is produced by two saddle
coils wrapped around the cylindrical
target.
In this model the diameter of a coil is 0.0178 cm
and the length of the saddle coils is 19.0 cm. The
outer diameter (OD) of a coil carrier in Fig. 9.5
is 4.051 cm. The optimum numbers of turns for
four layers to produce the desired field were found
to be N 1 = 170, N 2 = 170, N 3 = 152, and
N 4 = 60. This model produced a 0.4006 T central field, assuming a magnet current of 25 A (arbitrary). The calculated magnetic field on axis as
a function of z is shown in Fig. 9.7. The field
is very uniform and the uncertainties, ∆B/B0 at
positions (x = 1 cm, y = 0, z = 0), (x = 0,
y = 1 cm, z = 0), and (x = 0, y = 0, z = 1 cm)
around the center region are −0.0016, −0.0016,
and 0.0008 respectively. The peak field at the
conductor (wire) was calculated to be 0.549 T at
25 A.
Figure 9.6: A cross-sectional view of two saddle
coils in 4 layer configuration. (Most
inner layer N 1 = 170 turns, N 2 = 170
turns, N 3 = 152 turns, and N 4 = 60
turns.)
This is the field just outside the inner layer
in the z = 0 plane. The maximum current for
this coil is predicted to be approximately 37.6 A
if the heat shielding is at 1.5 K. This corresponds
to a peak central field of almost 0.56 T.
Figure 9.7: Calculated transverse magnetic field
of two saddle coils in 4 layer configuration.
We have started winding these saddle coils in collaboration with Thomas Jefferson National Accelerator Facility (JLab). As shown in Fig. 9.8,
the transverse coils are wound on a fixture
consisting of three aluminum plates that are
mounted on a rotating table. The upper and
lower plates are fixed to each other by three pins.
The center plate is attached to the lower plate by
tension springs. The center plate rests on screws
which are threaded through each corner of the
lower plate.
Figure 9.8: (Color online) A winding setup on a
rorational table; a spool of Supercon
54S43 wire on the left and fixtures on
the right side.
These adjustment screws allow the gap between
the upper and center plates to be adjusted as
they are placed for the adhesive to be applied
to the finished coil. We did not attempt applying adhesive to the coils as they were being
wound, even though this may have given better
149
results. The center slot runs across both sides of
the plate to allow the completed half section of
the coil to be passed through the plate and thus
avoid the need for a joint between the upper and
lower halves of the coil. As modeled, the four
layer coil was wound using Supercon 54S43 wire,
0.0075 in. diameter with insulation (0.006 in. actual wire diameter) as sketched in Fig. 9.6. The
outer diameter of the coil is defined by the OD
= 1.595 in. of our coil carrier, a stainless steel
heat shielding. The inner diameter is then set
by the number of turns and the diameter of the
wire. Plastic forms are made to match the inner
dimensions of the coil. The thickness of the plastic shims is equal to the wire diameter. These
forms are placed between the center and upper
plates of the winding fixture. By stacking multiple plastic forms together in the winding fixture
which is teflon-coated, multiple layers could be
wound at once. Each half of the coil was wound
in one pass. The coils are glued to a five micron polyester backing to help prevent the layers
from separating when they are wrapped around
the coil carrier.
Figure 9.9: (Color online) One racetrack-shape
saddle coil wound in four layer configuration.
We successfully wound multiple coils at JLab using this method. An effort is now underway to design and build such a winding system at TUNL.
Furthermore, low-temperature tests of the transverse coils, to measure the maximum applied current and B-field, are planned both at JLab and
TUNL.
150
9.3.3
The HIγS NaI Detector Array: Construction and Testing
S. Stave, M.W. Ahmed, S.S. Henshaw, H.R. Weller, TUNL; A. Teymurazyan, P. Martel, University of Massachusetts, Amherst, MA; L.A. Jackson, S.E. O’Brien, G.S. Maust,
C.S. Whisnant, James Madison University, Harrisonburg, VA
The HIγS NaI Detector Array (HINDA) will be used in upcoming Compton scattering experiments at HIγS. The array consists of 8 NaI core detectors, each one surrounded by a
segmented NaI shield. The array is currently under construction, and performance tests are
being conducted on the delivered parts.
Great progress has been made in studies
of the electric, magnetic and spin polarizabilities of the proton through Compton scattering
[Bla01, dL01, Mac95]. Nevertheless, discrepancies remain for the proton polarizabilities, particularly for γπ . The neutron data remain sparse
due to the lack of a free neutron target. The
Compton scattering program at HIγS will address the measurement of the static dipole and
spin-polarizabilities of both the proton and the
neutron.
Figure 9.10: (Color online) Conceptual design
drawing of the detector frame showing the eight NaI core and shield detectors out-of-plane at back angles
and positioned around a central target.
The HIγS NaI Detector Array (HINDA) is being constructed to carry out these Compton scattering measurements. The full array (shown in
Fig. 9.10) will consist of eight 10.5 in. × 10.5 in.
NaI core detectors, each one surrounded by a 3
in. thick segmented (into eight pieces) NaI anticoincidence shield (shown in Fig. 9.11). There
will also be a lead collimator in front of each core
to block line-of-sight from the shield detectors to
the target. The frame for the detectors is still
being designed but will have the ability to position the detectors both in- and out-of-plane (see
Fig. 9.10).
As of June 2008, three of the shield detectors have been delivered with the remaining five
to arrive over the next six months. Four of the
core detectors are ready for the array while the
remaining four are being refurbished at the manufacturer. Their delivery is also expected over
the next six months.
Figure 9.11: (Color online) GEANT4 drawing
showing a NaI detector assembly
with the core detector, shield detector and front lead collimator.
Two complete core and shield systems have
been tested to date, with more tests to come
shortly. The performance tests were carried out
using an AmBe source and specifically looking at
the 4.4 MeV γ-ray line. The source was placed
approximately 1 m in front of the detector, and
the shield detector was shielded from the source
using a lead wall with a collimator for the core
detector. Each of the eight separate shield segments was read out only when the core detector
signal went above the threshold. The results of
the test are shown in Figs. 9.12 and 9.13. It is anticipated that the core-shield system will be run
in one of two modes. Spectra from both modes
are shown in Fig. 9.12. The first mode is adding
all shield events, especially the first and second
escape peaks, back to the core event to recreate
virtually one large NaI detector which detects the
full energy peak. The second mode is to use the
shield as a veto to eliminate any first or second
escape events thus creating a very clean spectrum but with fewer events. The first case can be
seen in Fig. 9.12 as the tall peaks. Note that the
4.4 MeV peak does not have a first escape peak.
Figure 9.12 also shows the anti-coincidence mode
which also has good energy resolution and no first
escape peak.
Figure 9.13 shows the raw, uncut spectrum
from the core detector along with the core spectrum when the shield is used to select 511 keV
events only. The first escape peak that was difficult to see in the uncut spectrum is very clear
in the cut version. This test shows that the
energy resolution of the new shield detectors is
clearly sufficient to perform clean cuts using the
deposited energy.
HINDA Detector Test
4.4 MeV
CORE + Shield as One Detector
CORE AND Shield Anti-Coincidence
6.8 MeV
8000
151
HINDA Detector Test
4.4 MeV
CORE + Shield as One Detector
CORE without Shield
CORE AND Shield 511-Coincidence
6.8 MeV
8000
6000
4000
2000
0
SEP of 4.4
3000
4000
SEP of 6.8
5000
6000
7000
8000 9000
E (KeV)
Figure 9.13: (Color online) Additional results
from a test of the first shield and core
system for HINDA. The uncut core
energy histogram has a small first escape peak. If the shield detectors are
used to select only 511 keV events,
the first escape peak is clearly separated from the other events.
Taken together, these first tests have shown
that the HINDA detectors do perform as expected. Further tests are planned for fall 2008,
when at least two detectors and shields will
be used in the HIγS γ-ray vault for the first
time, along with a scintillating deuterium target.
These tests will determine how well the detectors
perform in the beam environment and will test
methods for rejecting background events. They
will also aid in the design of the detector stand
and any additional shielding that the detectors
may require.
6000
[Bla01] G. Blanpied et al., Phys. Rev., C64,
025203 (2001), 029902(E).
4000
[dL01]
2000
0
3000
4000
5000
6000
7000
8000 9000
E (KeV)
Figure 9.12: (Color online) Results of a test of
the first shield and core system
for HINDA. Note how the first escape peak is successfully added back
to the core spectrum for “CORE
+ Shield as One Detector” and
that the first escape peak is eliminated for “CORE AND Shield AntiCoincidence”.
O. de León et al., Eur. Phys. J., A10,
207 (2001).
[Mac95] B. MacGibbon et al., Phys. Rev., C52,
2097 (1995).
152
9.3.4
GEANT4 Simulation Package for HINDA and HIFROST
A. Teymurazyan, University of Massachusetts, Amherst, MA; M.W. Ahmed, S. Stave, H.R.
Weller, TUNL
The current state and future development of the simulation package for the HIγS NaI detector
array and the HIγS frozen-spin target are discussed.
A simulation package, based on the geant4
toolkit [Ago03] and root data analysis framework [Bru97], for the HIγS NaI detector array (HINDA) and the HIγS frozen-spin target
(HIFROST) is in its advanced stage of development.
The main goals of this simulation package are
to determine the solid angles and the detection
efficiency of the detector array, and to study possible sources of background.
The simulation package is designed around
two requirements: 1) modularity, which allows
direct use of the source code for different components of the package in other simulations and 2)
flexibility, which allows a user to change the geometry of the experimental setup and other parameters of the simulation without any knowledge of geant4 or C++ (no recompilation of
the source code is required).
cm NaI(Tl) crystal packaged into an aluminum
barrel with a wall thickness of 0.32 cm. Surrounding the core is an active “shield” of eight
30.48 cm long NaI(Tl) crystals, each covering 45◦
segments with inner and outer diameters of 27.69
cm and 41.28 cm. The shield crystals, individually wrapped in three layers of 0.025 cm thick
Teflon reflector, are arranged in a ring in an aluminum housing with an outside and inside wall
thicknesses of 0.81 cm and 0.32 cm, respectively.
In addition, the direct line of sight from the target to the shield part of each detector is restricted
by a 15.88 cm thick lead annulus. The scintillation light from the crystals is registered in 5.08
cm photomultiplier tubess (7 for the core crystal
and 1 for each shield segment). Note that the
scintillation light and its readout are not implemented in the simulation.
At the start of the simulation, by default, an
array of eight NaI detectors is installed in the
“XZ” plane at 24, 60, 96, 132, 210, 246, 282
and 318 degrees with respect to the beamline (see
Fig. 9.15).
The energy deposited, positions of start point
and end point, event number, timing information, particle type, detector core/shield segment
ID and detector angle are recorded in a root file
for every track hitting the NaI(Tl) crystal parts
of the detector array. This approach enables one
to study the hits originating in the target separately from the background events.
Features that are currently implemented include the ability to dynamically: change the geometrical configuration of the NaI detector array
(change θ and φ angles of each detector); change
Figure 9.14: (Color online) GEANT4/DAWN the number of detectors in the array (maximum
drawing showing a NaI detector number of detectors is 8); change the lead shield
assembly as implemented in the thickness/remove the lead shielding; define/load
simulation.
new materials; change the target material.
Critical work still needed is: finish modeling
As is shown on Fig. 9.14, the core of the the frozen-spin target geometry; implement the
HINDA detector assembly is a 26.67 cm×26.67 magnetic holding field for the frozen-spin target;
153
Figure 9.15: (Color online) GEANT4/DAWN drawing illustrating the NaI detectors aranged in the
”XZ” plane (top view). The outline of the first stage implemetation of the HIFROST
target is also shown. The HIγS γ-ray beam is incident from the left.
model the NaI detector array support structure;
optimize the list of physics processes; optimize
the cuts and thresholds.
Less critical items are: implementation of the
“run” number tracking; restructuring of the output data root tree format, i.e., adding/reducing
data fields; implementation of the ability to
dynamically load/unload parts of experimental
setup.
Finally, an extremely useful feature would be
the implementation of a graphical user interface.
[Ago03] S. Agostinelli et al., Nucl. Instrum.
Methods, A506, 250 (2003).
[Bru97] R. Brun and F. Rademakers, Nucl. Instrum. Meth., A389, 81 (1997).
154
9.3.5
Commissioning the Optical Readout TPC (O-TPC)
M. Gai, A.H. Young, P.-N. Seo, University of Connecticut, Storrs, CT ; M.W. Ahmed, E.R.
Clinton, S.S. Hinshaw, C.R. Howell, S. Stave, H.R. Weller, TUNL; P.P. Martel, University of Massachusetts, Amherst, MA; B. Bromberger, V. Dangendorf, K. Tittelmeier,
Physikalisch-Technische Bundesanstatt, Braunschweig, Germany
We commissioned the optical readout time projection chamber (O-TPC) detector constructed
at the University of Connecticut’s Laboratory for Nuclear Science at Avery Point by the
UConn-Yale-PTB-Weizmann-UCL-TUNL collaboration. The O-TPC detector was assembled
in a newly constructed clean room. A VME-based data acquisition and CCD-camera control
were developed and employed in this study. We measured a total (charge) energy resolution
as good as 2.6% with 3.183 MeV α particles from a 148 Gd source. Time projection signals
from two quartz-window photomultiplier tubes were measured, and an algorithm for 3D track
reconstruction was developed. Track angle reconstructions with an accuracy of 5◦ or better
were obtained. At the end of the commissioning period, the O-TPC detector system was
transferred to the HIγS beam line for in-beam experiments.
The optical readout time projection chamber
(O-TPC) detector that we constructed will be
used in a new study of oxygen formation during stellar helium burning (the 12 C(α, γ)16 O reaction) by measuring the time reversed process
of the photo-dissociation of 16 O. Prior to using
this detector in the beam, we commissioned it in
a dedicated laboratory at TUNL that included a
newly constructed clean room. The commissioning of the O-TPC detector was carried out using
3.183-MeV α particles from a standard 148 Gd αparticle source and the O-TPC detector operating at 150 torr with a CO2 (80%) + N2 (20%) gas
mixture.
Entries
M ean
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Grid
140
120
h1
717
660.1
379.5
100
2.6 %
80
60
was developed at TUNL. This DAQ system allows us to include pulse-height information, digitized photomultiplier tube (PMT) signals from
the flash ADC, and the content of the track
image contained in the charge-coupled device
(CCD) camera in the same event. Attention
was given to blocking event triggers from reaching the DAQ system during the approximately
two-second-long process of downloading the image data from the camera to the DAQ computer.
This slow process limits the data rate to a maximum of 0.1 Hz.
148G d −SourceC alibration ofO −TPC
CO 2(80% )+ N 2(20% )
150 Torr
40
20
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500
1000
1500
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2500
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Entries
717
M ean
949.7
RM S
220.5
PMT 1
35
3500
h4
717
Entries
M ean x
609.7
M ean y
941.7
RM S x
56.84
RM S y
65.6
Grid Vs. PMT
1600
1400
25
15
10
1200
7.9 %
20
1000
800
1
600
10
400
5
0
2000
1800
30
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500
1000
1500
2000
2500
3000
3500
4000
−1
200
400
600
800
1000
1200
1400
1600
1800 2000
10
Figure 9.16: (Color online) Total energy (Grid)
and light (PMT) signals measured
with 3.183 MeV α particles from a
148 Gd source.
A new VME-based data acquisition (DAQ)
system coupled with a camera control system
Figure 9.17: (Color online) Time projection PMT
signals for a horizontal track and a
track tilted at 45◦ .
The total charge signal, measured from the
155
Figure 9.18: (Color online) Projection of pixel content along the track direction and perpendicular to
it. The longitudinal projection follows the calculated dE/dx along the track.
grid after electron multiplication, was used to determine the total energy deposited in the chamber during each event. The excellent energy resolution (70 keV at 3.1 MeV) shown in Fig. 9.16
is essential for binning the data taken with
the broad HIγS γ-ray beam (approximately 300
keV). A well-collimated α-particle source was
used to measure the time projection using the
PMT signals as shown in Fig. 9.17. Simulation
of the observed PMT signal, taking account drift
and charge collection, allowed us to deduce a drift
velocity of approximately 1 cm/µsec. Automated
algorithms were developed for track projection
perpendicular to the track direction and along
the track as shown in Fig. 9.18. The longitudinal projection follows the calculated dE/dx along
the track as expected.
tering angle. Hence, angular distributions with
angular binning of 10◦ can be measured using
the O-TPC detector.
tan φ = tan β /Si
nα
Cosθ = Cosβ xCosα
_>
The track angle α is deduced from the centroid of the track direction with respect to the
beam direction, and the out of plane angle β is
measured using the time projection. From these
measured angles, the scattering angle θ is deduced as shown in Fig. 9.19. We estimate an
accuracy of 5◦ or better for the constructed scat-
γ
Figure 9.19: (Color online) The transformation of
the measured track angle α and the
out of plane angle β to the usual scattering angle θ and azimuthal angle
(φ).
156
9.3.6
A γ-Ray Beam Imaging System
C. Sun, M. Emamian, J. Li, Y.K. Wu, Duke Free Electron Laser Laboratory, Durham, NC
At the Duke Free Electron Laser Laboratory (DFELL), an optical imaging system to capture
the γ-ray-beam spatial distribution has been developed. This system has been successfully
used to align the γ-ray beam with the lead collimator located in the collimator hut, and to
align the collimated γ-ray beam with the samples and detectors mounted in the target room.
The γ-beam imager can also provide detailed information about the γ-ray beam distribution.
Such information can be used to optimize γ-beam production.
The High Intensity γ-Ray Source (HIγS) is
a nearly-monochromatic, highly polarized, high
flux Compton γ-ray source produced by colliding
the intense free electron laser (FEL) beam inside
an FEL resonator with an intense electron beam
in the Duke storage ring. The γ-ray beam is produced more than 60 meters away from the sample. The spatial matching of the γ-ray beam to
the sample is a very challenging task, especially
without having an imaging system for the γ-ray
beam. Good matching is critical for all nuclear
physics experiments that need either to measure
the on-target γ-ray flux or to reduce the signal
background by avoiding γ-ray beam scattering
against the sample holder.
9.3.6.1
γ-Beam Imager Design
In the past several years, several techniques have
been developed to image the gamma-ray beam at
HIγS. The recent development based on the bismuth germanium oxide (BGO) crystal has been
rather successful. Conceptually, this system consists of a BGO plate which converts the γ-ray
photons to low energy photons, an optics system
to collect the low-energy photons in the visible
spectrum, and a charge-coupled device (CCD)
camera system to form the image. All of these
components are placed in a light-tight box, and
a computer is used to control and interface the
CCD camera. During the design, the MonteCarlo simulation toolkit geant4 [Ago03] was
used to study the performance of the γ-beam imager.
9.3.6.2
γ-Beam Imager Performance
The alignment of the γ-ray beam with the collimator and the subsequent alignment of the collimated γ-ray beam with the sample in the tar-
get room is a critical step before carrying out a
nuclear experiment. A good alignment can maximize the γ-ray flux, reduce the energy spread,
and minimize the background noise due to γray scattering on the sample holder. Before this
imager was developed, an alignment laser was
used to align the collimator and sample. Additional adjustments of the collimator were then
made empirically using a γ-beam energy spectrum scan. This was a time-consuming process
that could not guarantee a good alignment without an extensive scan. By providing a direct visual image of the γ-ray beam, the alignment process can be carried out rapidly and with muchimproved accuracy. A procedure has been developed to take advantage of the γ-ray imager;
this procedure was used successfully for a number of recent experiments including the oxygen
formation in stellar helium burning experiment
(see Sect. 9.3.5) with the optical readout time
projection chamber (O-TPC) and the 3 He GDH
sum rule experiment with long gas cells as sample
targets (see Sect. 7.2.2).
9.3.6.3
Collimator Alignment
The γ-ray beam is first collimated by the fixed
preliminary collimators located inside the storage ring shielding. The final collimation is provided by a movable collimator inside the collimator hut, which is located about 60 meters away
from the collision point. This movable collimator determines the γ-ray beam distribution on
the sample, so that its alignment with respect
to the γ-ray beam is very critical. To align this
collimator, the γ-beam imager is placed directly
downstream from the collimator without any objects in between. By moving the collimator or
scanning the electron-beam angles, the centroid
of the γ-ray beam can be brought to the center
of the collimator. The beam images of the collimator before and after the alignment are shown
in Fig. 9.20.
Before the alignment of a Gamma−ray beam to a 1" collimator
After the alignment of a Gamma−ray beam to a 1" collimator
0
0
100
100
157
50
50
100
100
150
150
200
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250
250
300
300
350
350
400
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450
500
500
550
200
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Y pixel
200
300
400
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300
400
(a)
300
500
600
700
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200
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(b)
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500
500
100
200
300
400
X pixel
500
600
700
100
(a)
200
300
400
X pixel
500
600
700
(b)
Figure 9.20: (Color online) A 9.8 MeV γ-ray beam
is produced by a 454 MeV electron
beam colliding with the OK-5 FEL
lasing at 587 nm. The 1-inch diameter collimator is located 60 m downstream from the collision points. The
γ-beam images are (a) before aligning the γ-ray beam to the collimator;
(b) after aligning the γ-ray beam to
the collimator.
9.3.6.4
550
100
Sample Alignment
After aligning the collimator, the sample needs to
be aligned with the collimated γ-ray beam. This
is done by using two different size lead alignment
targets (absorbers), one upstream and one downstream of the sample. This arrangement allows
us to find both the displacements and angles of
the γ-ray beam with respect to the sample. For
example, for aligning a long gas sample in the
O-TPC experiment, a smaller lead target, 4 mm
in diameter and 8 mm in length, is put in front
of the sample. A bigger lead target, 8 mm in
diameter and 8 mm in length, is placed in back
of the sample. In Fig. 9.21, both alignment targets are clearly seen as yellow and green circular
shadows. The centroid of the targets can be determined and aligned with an accuracy of better
than 0.5 mm.
Figure 9.21: (Color online) (a) Before aligning the
target to the collimator, (b) after
aligning the target to the collimator.
9.3.6.5
Advanced Applications of the γbeam Imager
Other than being a very capable transverse profile monitor for the γ-ray beam, this γ-beam imager is also capable of measuring the γ-flux. The
preliminary test of the system has shown that it
can be used as a reliable relative flux monitor by
calibrating the integrated intensity of the γ-beam
image with another flux monitor. As a flux monitor, the γ-beam imager has a very wide dynamic
range that is selectable by changing the integration time. A dedicated γ-beam imager is being
developed as a device for monitoring both the γray beam motion and flux. This new device is
being integrated with the ultra-high vacuum system of the γ-ray beamline. Progress is also being
made on developing the computer interface for
this system, so that it can be fully integrated
with routine operation of the HIγS accelerators.
In the near term, we are also exploring additional capabilities of the γ-ray imaging system for
radiographic imaging and for polarization monitoring of the γ-ray beam.
[Ago03] S. Agostinelli et al., Nucl. Instrum.
Methods, A506, 250 (2003).
158
9.4
9.4.1
Polarized Targets
Pressure Shift and Broadening of Cesium D1 and D2 Lines in the
Presence of N2 , He, and Xe
A.H. Couture, T.B. Clegg, TUNL; B. Driehuys, Duke Center for In Vivo Microscopy, Durham,
NC
The change in central wavelength and FWHM of the cesium D1 and D2 absoption resonances
have been measured as a function of the density of N2 and He perturbing gases. Spectra
were also taken using Xe as the perturbing gas. However, due to the presence of satellite
lines which distort the Xenon spectrum, only the line shift was obtained. These measurements
were performed using white light spectroscopy with perturbing gas densities up to 10 amagat.
We have been exploring the feasibility of producing hyperpolarized 3 He and 129 Xe by spinexchange optical pumping (SEOP) with polarized cesium vapor. This would require a source of
polarized light tuned to the cesium D1 resonance.
Theoretically, direct spin exchange with polarized cesium vapor should work well for polarizing
129
Xe, while efficient polarization of 3 He would
require hybrid, two-step Cs-K followed by K-3 He
spin-exchange. Both of these processes have total spin-exchange efficiencies similar to those of
traditional rubidium SEOP. However, gains are
expected because the large separation between
the cesium D1 and D2 lines inhibits unwanted
pumping of the D2 line.
cesium D1 absorption line (894.6 nm) were not
available. However, we have obtained a madeto-order, 13 W, spectrally narrowed diode laser
that emits at 894.3 nm, manufactured by Innovative Photonics Solutions. We originally began
examining how the Cs absorption lines shift with
gas pressure with the hope of reconciling the laser
with the D1 line. It turns out that while the pressure needed is far from ideal for optical pumping
with our laser, the pressure shifts we measured
will be important for any future work.
Table 9.2: Lineshift Results
Gas
He
He
N2
N2
Xe
Xe
Cs Line
D1
D2
D1
D2
D1
D2
Shift (nm/amg)
-0.013±0.002
-0.0037±0.0002
0.026±0.002
0.018±0.001
0.029±0.002
0.028±0.002
Figure 9.22: Cesium D1 and D2 line shift and
FWHM versus He density.
Limited work has been performed on this process in the past because inexpensive lasers at the
Figure 9.23: Cesium D1 and D2 line shift and
FWHM versus N2 density.
Figure 9.24: Cesium D1 and D2 line shift versus
Xe density.
To determine the location and width of the
cesium absorption lines, we used white light spectroscopy. A 120 W spotlight was focused through
a series of lenses onto a small pyrex cell filled with
less than a gram of cesium and a known density
of either N2 , He, or Xe. The cell was heated to
120±1◦C. After passing through the cesium vapor, the light was collected by a fiber optic cable
159
and fed into the Ando optical spectrum analyzer,
which allowed measurement of absorption spectra to 0.01 nm resolution. Data were taken from
vacuum to 10 amagat of perturbing gas density,
in 1 amagat steps. The spectra obtained were fit
to an asymmetric Lorentzian line shape, and the
central wavelength and full width at half maximum (FWHM) were recorded. The data show
that He lowers the central wavelength, while N2
and Xe raise it. In all cases, the width of the absorption line is increased with increasing density.
The results are shown in Figs. 9.22, 9.23, and
9.24 for He, N2 , and Xe, respectively. Only lineshift data are included for Xe because the presence of satellite absorption lines in the spectra
severely distort the shape of the absorption resonance, making it difficult to extract the width.
We have submitted these results for publication
in the Journal of Applied Physics.
Table 9.3: FWHM Results
Gas
He
He
N2
N2
Cs Line
D1
D2
D1
D2
Width (nm/amg)
0.063±0.002
0.047±0.002
0.043±0.002
0.062±0.004
160
9.4.2
Production of K-Rb Hybrid Optical Pumping Cells for the Production of Hyperpolarized 3 He
A.H. Couture, T.B. Clegg, T.V. Daniels, C.W. Arnold, TUNL; B. Driehuys, Duke Center
for In Vivo Microscopy, Durham, NC
Methods of producing and testing K-Rb hybrid optical pumping cells for the production of
hyperpolarized 3 He have been developed for use in spin-polarized nuclear-scattering targets.
The advantages and shortcomings of these cells are discussed.
The production of hyperpolarized 3 He has
many uses in scientific research, such as magnetic resonance imaging, spin polarized targets,
neutron spin filters, and precision measurements.
The traditional method of polarizing 3 He utilizes absorption of circularly polarized photons
at the rubidium D1 resonance (794.7 nm). The
polarized Rb atoms then spin exchange with the
3
He nuclei, transferring their atomic polarization
to the 3 He nucleus. The spin-exchange optical
pumping (SEOP) efficiency for this process is
about 2%, meaning it takes about 50 photons
to polarize a single 3 He nucleus [Bab03].
Recently, a method has been developed to
improve this efficiency, thus improving both the
rate of polarization and the maximum polarization. The SEOP efficiency between K and 3 He is
about 23%, thus taking only 4 photons to polarize a He nucleus. However, pumping directly on
the potassium D1 resonance does not realize this
improvement, because of the close spacing (∼3
nm) between the D1 and D2 resonances. This
causes inevitable pumping on the D2 line, which
counteracts the D1 pumping by depopulating the
+ 21 spin state. The solution is to prepare optical pumping cells containing a mixture of K and
Rb at a ratio between 5:1 and 10:1 as a vapor.
When pumping on the D1 line of Rb, the Rb
nearly instantaneously transfers its electronic polarization to K, which may then spin exchange
with the 3 He. This takes advantage of the relatively large separation between the D1 and D2
lines (∼15 nm) in Rb, as well as the SEOP efficiency of K with 3 He.
We have tried several methods for obtaining
the desired ratio of K to Rb in these cells, after seeking the advice of other groups [Ave06].
Initially, we used a Y-shaped manifold to chase
first K and then Rb into the cells. This proved
to be very difficult to control. We then tried
pre-mixing the metals in an argon environment,
but had problems with oxidizing the alkali metals. The method found to be most reliable was
to order ampoules containing less than 0.1 gram
of Rb (just a drop) from Strem Chemicals. We
then put one of these in a manifold with the standard 1 gram ampoule of K and chased the metals into the cells together. We determined the
ratio of K to Rb vapor in the cells using white
light spectroscopy. A standard 120 W spotlight
was focused on the cell whose ratio was to be
determined. The cell was then heated to optical
pumping temperatures (∼230◦ C). Light having
passed through the cell was then collected and
analyzed in a spectrometer. The integral of an
absorption line is proportional to the density of
the absorber as well as the oscillator strength of
the resonance:
Z
σdλ = Cρω
σ
= absorption cross section
ρ = alkali density
ω = oscillator strength
(9.1)
Thus the ratio of K to Rb in the vapor may
be determined by taking the ratio of the integrated absorption lines, weighted by the ratio of
the oscillator strengths, ω, for those lines.
R
ρK
ωRb σK dλ
R
.
=
ρRb
ωK σRb dλ
(9.2)
We obtained K:Rb ratios ranging from 4:5 to
10:1, with the best cells included in the higher
part of that range. We gained a significant increase in the rate of 3 He polarization using the
hybrid cells. With pure Rb cells our decay time
constant was typically 10 hours, taking 35-40
hours to obtain saturation polarization. With
the hybrid cells the time constant was typically
2.5 hours taking about 10 hours to saturate.
However, we were disappointed not to acheive
significant gains in saturation polarization. With
pure Rb we were able to obtain polarizations of
35-40%, which we matched but did not surpass
with the hybrid technology. Another troubling
feature of the cells was that when optical pumping was stopped and the cells were allowed to
depolarize in a holding field, they lost their polarization more quickly than the pure Rb cells.
The characteristic depolarization time constant
161
(T1) for the hybrid cells was usually 20 hrs or
less, while we made pure Rb cells with T1 values
above 30 hrs. For the hybrid cells, the T1 also decreased with continued use, for reasons that are
not understood.
[Ave06] T. Averett and T. Gentile, 2006, Private
communication.
[Bab03] E. Babcock et al., Phys. Rev. Lett., 91
(2003).
162
9.5
9.5.1
Beamlines, Targets, and Facility Development
Sensitivity of the KURF Low-Background Counting Facility
P. Finnerty, R. Henning, H.O. Back, TUNL; A.G. Schubert University of Washington, Seattle, WA; R.B. Vogelaar, Virginia Polytechnic Institute and State University, Blacksburg, VA
Experimental background data and MAGE simulations were combined to calculate the sensitivity of the Melissa HPGe detector, a component of the KURF Low-Background Counting
Facility. A teflon cube with a uniform concentration of 238 U and
lated in order to estimate the sensitivity to a generic sample.
9.5.1.1
Introduction
The Kimballton Underground Research Facility
(KURF) is located about 25 miles from the campus of Virginia Polytechnic and State University in the Chemical Lime Company’s Kimballton mine. The experimental area is located on
the 14th level at a depth of 520 m, or approximately 1400 meters of water equivalent shielding.
The cosmic-ray background reduction in a highpurity germanium (HPGe) detector is shown in
Fig. 9.25.
Count Rate [Counts/Day/keV/kg Ge]
Background Comparison of VT-1
Bkg-91107.Spe
Kimballton
Entries 8191
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10
Mean
RMS
Surface
556.8
585.3
2
10
10
1
-1
10
500
1000
1500
Energy [keV]
2000
2500
Figure 9.25: (Color online) The reduction of background at KURF is apparent.
During October 2007 a 100 ft × 40 ft ×
20 ft dust-exclusion enclosure and a commercial
grade HEPA filter were installed for air handling.
Currently there are two self-contained trailers in
place, one belonging to the U.S. Naval Research
Laboratory (NRL) and the other to TUNL.
9.5.1.2
The Detectors
The NRL trailer is the home of the LowBackground Counting Facility. The facility consists of two HPGe detectors specifically designed
232 Th
daughters was simu-
for low-background assay work. These detectors
were installed and commissioned in the last year.
One detector, named “Melissa”, is a 50% RE
(relative efficiency to NaI), Canberra LB (lowbackground) detector. Melissa is in a vertical
orientation and is cooled via a dipstick cryostat.
The endcap is made of high purity aluminum.
The lead shield for Melissa is 6 in. thick and made
of Doe-run lead. The full width at half maximum (FWHM) at 1.33 MeV is 1.95 keV. The
other detector is a 35% RE Ortec LLB Series detector in a J-type configuration. This detector
is named “VT-1” in honor of its owners, Virginia Tech. VT-1 has a FWHM of 1.8 keV at
1.33 MeV. The VT-1 detector internals are made
from OHFC copper. Activated charcoal is used
as the cryo pump in both detectors, because it
has a lower background than sieve does. VT1’s preamp and HV filter are removed from the
line of sight of the crystal, further reducing the
background. Melissa’s preamp and HV filter are
shielded by 10 cm of Doe-run lead. This report
concerns the sensitivity of the Melissa detector.
9.5.1.3
Monte Carlo Simulations
Sensitivity Calculations
and
A mage/geant4 [Hen05] simulation was performed based on the Melissa detector geometry. We simulated the naturally occurring decay
chains of 238 U and 232 Th. Due to the long halflives and the potential lack of secular equilibrium
of these chains, we simulated a 3 cm3 teflon block
uniformly doped with 214 Bi, 228 Ac, 234 Pa, and
208
Tl. The block was placed directly on top of the
endcap and 4M decays of each isotope were simulated in four separate simulations. The mage
output class MJOutputDetectorEvent was used
to process the energy deposits in the crystal and
produce an energy spectrum. The energy spectrum from the 214 Bi simulation can be seen in
Fig. 9.26.
3
Melissa: 4M 214Bi decays in 3 cm of Teflon
105
The sensitivities to several isotopes are given in
Table 9.4. Sensitivities were computed using a
background spectrum of 7.2 days.
Table 9.4: Predicted sensitivities to various isotopes for the Melissa detector.
Entries 581719
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Energy [keV]
338.0
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3
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10
1
0
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1500 2000 2500 3000
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3500
Isotope (Parent)
228
Ac (Th)
208
Tl (Th)
214
Bi (U)
234
Pa (U)
40
K
214
Bi (U)
208
Tl (Th)
4000
Figure 9.26: A 214 Bi simulated energy spectrum
of energy deposits in the Melissa detector.
To account for the energy resolution seen in
experiment, each peak in Fig. 9.26 must be convoluted with a Gaussian distribution of width σ,
where σ was measured experimentally. According to [Kno05], the energy resolution of a germanium detector is given by
q
2
σ = a2 + b2 EkeV + c2 EkeV
,
(9.3)
where EkeV is the peak energy given in keV. We
measured the σ of several peaks across a wide
energy range of a background run and fit the results according to Eq. (9.3). The values for a, b,
and c are 1.23 ± 0.02, (4.98 ± 0.01) × 10−3 , and
(3.75 ± 0.05) × 10−4 , respectively.
Reference [Doe07] defines a detectable signal
by Eq. (9.4), where the background counts are determined from experimental background spectra
and signal counts are the excess expected from
simulation of the sample. Counts within ±3σ of
the signal-peak energy are included.
√
signal counts
≥3
background counts
163
9.5.1.4
Sensitivity [mBq]
23.59 ± 0.48
2.67 ± 0.06
12.24 ± 0.25
8.34 ± 0.28
3.07 ± 0.07
28.02 ± 0.62
1.53 ± 0.09
Conclusions
This report presents the sensitivity of Melissa to
various isotopes. We will perform these same calculations for the VT-1 detector as well. What
still remains for this calculation is to translate these numbers into an overall sensitivity to
U/Th concentrations if the series were in equilibrium. Unfortunately the lines normally used
(208 Tl and 214 Bi) come well down the chain in
both series; the earlier γ-emitting isotopes (234 Pa
and 228 Ac) only yield low-branching-ratio or lowenergy lines, which are hard to detect. For the
U series, it is usually out of equilibrium, except
for unprocessed mineral samples, and it is really 226 Ra and its decay products that are measured. For Th, it is harder to generalize. However, with Ge spectrometers it should be possible to estimate the parent Th content via 228 Ac
and compare this with ‘equilibrium’ estimates
from 208 Tl [UKD93]. The calculated sensitivities are roughly 4–5 times better than those for
the surface facility at the University of Washington. Sensitivities are expected to improve once
we add a copper shield and Rn-purge to both
Melissa and VT-1.
(9.4)
The mage/geant4 simulation determines the
efficiency for detecting a decay, including geometric effects and branching ratios, so that the number of signal counts in a detector can be related
to the source activity:
signal counts = (source activity rate) ×
(counting live time) × (ef f iciency) (9.5)
The sensitivity is determined by combining
Eqs. (9.4) and (9.5), and solving for the detectable activity rate. This gives the sensitivity,
S:
√
3 background counts
(9.6)
S=
(counting live time) × (ef f iciency)
[Doe07]
P. J. Doe et al., 2007, Sensitivity of
the University of Washington Majorana Counting Facility, University of
Washington internal report.
[Hen05]
R. Henning et al., Nucl. Phys. B143,
544 (2005).
[Kno05] G. F. Knoll, Radiation Detection and
Measurement, Wiley & Sons, 3rd edition, 2005.
[UKD93] U. K. Dark Matter Collaboration,
Radioactivity Data, 1993,
http://hepwww.rl.ac.uk/ukdmc/.
164
9.5.2
Development of an Ultracold Neutron Source at the NC State PULSTAR Reactor Facility
C. Cottrell, S. Dissler, R. Golub, A.T. Holley, P.R. Huffman, E. Korobkina, G.
Palmquist, A.R. Young, TUNL, A. Cook, A. Hawari, B. Wehring, NC State Nuclear Engineering Department, Raleigh, NC
We are finalizing the construction of a world-class, solid deuterium ultracold neutron (UCN)
source at the NC State 1 MW reactor. This source will be comparable in strength to other
facilities being planned in the U.S. and abroad. As part of this project, we have performed
an experiment in collaboration with colleagues at the Research Center for Nuclear Physics
(RCNP) in Osaka, Japan to characterize the energy spectrum of the UCNs generated by a
superfluid helium source located at the RCNP.
Ultracold neutrons play an important role
in nuclear physics investigations that seek to
test the Standard Model of particle physics and
characterize the weak interaction. UCN source
technology is advancing extremely rapidly, and
the numerous techniques presently being implemented should permit several orders of magnitude gain in useful UCN densities. Such higher
densities of UCNs will not only permit improved
measurements of quantities such as the neutron electric dipole moment (EDM) and neutron
β-decay lifetime, where measurements utilizing
UCNs already provide the most precise experimental values, but will also play a role in neutron angular correlation measurements, neutronantineutron oscillation searches, spectroscopic
studies of neutron decay, fundamental tests of
quantum mechanics, and, potentially, cuttingedge neutron scattering applications.
Through a Department of Energy Innovations
in Nuclear Infrastructure and Education (INIE)
grant and National Science Foundation (NSF)
equipment funding, we are in the process of constructing a world-class, solid deuterium UCN
source at the NC State PULSTAR research reactor. Our objective is to establish a cost effective facility, supported primarily by existing nuclear physics and nuclear engineering infrastructure through TUNL and the reactor program at
NC State. We envision a facility that has sufficient UCN intensity to perform fundamental and
applied research, while providing the flexibility
for continued UCN source development.
Our source is well-timed to provide support
for upcoming fundamental neutron physics efforts such as the search for the EDM of the neu-
tron. This source will provide the ideal training
ground for students interested in exploring fundamental and applied physics utilizing UCNs, and
will make possible a range of experiments in areas as widely varied as nuclear physics tests of
the electroweak Standard Model and the dynamics of large biological molecules. Because of the
very small nuclear heat load that we anticipate
on our cryogenic moderators, we can also experiment with UCN source configurations not practical near the core of either a high power reactor or high-flux spallation target. Our proposed
program is interdisciplinary, involving key contributions from physics and nuclear engineering
faculty, and involving both applied and fundamental research.
Figure 9.27: (Color online) MCNP geometry plot
showing the general layout of the
UCN source and guide, nose port,
and reactor core.
Figure 9.28: (Color online) Liquid helium supply
and recovery system.
The concept for our facility is to place the UCN
source in a tank of D2 O positioned in the previous thermal column of the PULSTAR reactor. Neutrons leaving the face of the reactor core are channeled into the D2 O tank by a
45 cm × 45 cm × 70 cm long void in a graphite
assembly called the nose port. The source consists of a UCN converter of solid ortho deuterium,
17 cm diameter × 4.5 cm thick, held at a temperature of < 5 K. The converter is surrounded
by a 1 cm thick, cup-shaped cold source of solid
methane held at a temperature of ≈ 25 K. This
configuration was optimized for maximum coldneutron flux using detailed mcnp calculations.
The basic geometry of the source is shown in
Fig. 9.27, and the liquid He supply and recovery
system, installed and commissioned during 2007,
is depicted in Fig. 9.28.
165
involvement in the neutron electric dipole moment experiment, we used a gravitational spectrometer to characterize the UCN Source at the
RCNP Cyclotron (see Fig. 9.29). The source
generates UCNs in superfluid helium via the superthermal process. These UCNs exit the source
and travel to the gravitational spectrometer that
consists of polished stainless steel tubes connected into a rotatable upside-down U-shape configuration (see Fig. 9.30). By rotating the guide,
thus varying the height of the base of the “U”,
one can measure the energy spectrum of the
UCNs exiting the source; raising the spectrometer limits the number of UCNs with sufficient
energy to overcome the retarding gravitational
potential. A differential spectrum can be extracted from the change in counts between various heights. The spectrometer was adjusted from
being level with the source outlet to a height of
1 m, corresponding to a potential energy range
from 0 neV to 102 neV. Neutrons with sufficient
energy to be transmitted through the spectrometer are counted using a 3 He detector. A shutter
placed in front of the spectrometer was used to
vary the UCN source-holding and emptying times
to characterize any spectral shaping that occurs
in the source. The proton beam current and pulse
length were also varied for this reason.
Figure 9.30: (Color online) The U-shaped gravitational spectrometer shown in the
vertical and horizontal (inset upper
left) configurations.
Figure 9.29: (Color online) The superfluid helium
UCN source located at RCNP. UCNs
exit the source from the horizontal
tube extending through the shielding
wall on the left.
As part of both the source development and our
The source geometry and spectrometer were
simulated using a Monte Carlo UCN transport
code that was developed at NC State. This code
will be used during the data analysis to explore
various source parameters, including wall losses
and specularity in different regions of the apparatus.
166
9.5.3
DEAP/CLEAN Activities
M.C. Akashi-Ronquest, L.T. Evans, R. Henning, S. MacMullin, J. Strain, TUNL; S.
Marquess, University of Maryland, Baltimore County, MD
The DEAP/CLEAN project will search for weakly-ionizing-massive-particle (WIMP) dark
matter via the observation of nuclear recoils in noble liquids. TUNL activities include construction of a calibration system, work on Monte Carlo simulations, and preparation for a
new measurement of the neutron cross section for neon.
The DEAP (dark-matter experiment using argon pulse-shape discrimination)/ CLEAN
(cryogenic low-energy astrophysics with noble
gases) collaboration’s near-term goal is to develop a dark matter detector with a sensitivity to the spin-independent WIMP-nucleon crosssection of 10−46 cm2 for MWIMP ≈ 100 GeV.
The DEAP/CLEAN concept is to observe only
the scintillation light from the noble liquid target, thus removing the need for time projection
chambers, which can lower the light yield and
make larger scale experiments a challenge. The
ratio of the amount of prompt to total scintillation light provides an excellent way to discriminate background electrons from signal nuclear
recoils. Currently the collaboration is running
two smaller R&D detectors: MicroCLEAN and
DEAP-1. MicroCLEAN, situated at Yale University, is used to measure the scintillation characteristics of liquid argon (LAr) [Lip08] and liquid neon (LNe) [Nik08], and to study a number
of engineering issues. DEAP-1 has also been used
to evaluate the pulse-shape discrimination in LAr
[Lid08] and, having been recently moved underground to SNOLab, is about the embark on a
preliminary dark-matter search.
Our involvement centers on the next step
in the DEAP/CLEAN project: MiniCLEAN360. MiniCLEAN-360 will have a sensitivity
to the WIMP-nucleon cross-section of approximately 10−45 cm2 for MWIMP ≈ 100 GeV when
running with a target mass of approximately
360 kg of LAr. In the event of a possible dark
matter signal, the target can be exchanged in favor of LNe, which will allow the expected A2 dependence in cross section to be probed, and will
produce very different intrinsic background characteristics.
In addition to the focus on MiniCLEAN-360,
we are also participating in the design of the
DEAP/CLEAN-3600 detector, which will run
with 3600 kg of LAr. This experiment is expected
to begin taking data in 2010 and should push the
sensitivity down to the 10−46 cm2 level, which
represents the collaboration’s near-term sensitivity goal. The next logical step would then be the
construction of a 10–100 ton scale detector at the
Deep Underground Science and Engineering Laboratory. Due to the presence of 39 Ar in natural
argon, liquid neon, which has no long-lived isotopes, becomes a very attractive candidate for
use as a target/detection material, especially if
sources of argon with depleted levels of 39 Ar are
not feasible. In this way, MiniCLEAN-360 operating in neon mode can be seen as the prototype
for a 10–100 ton dark matter detector.
9.5.3.1
Calibration System
Our main responsibility is the design and
construction of the calibration systems for
MiniCLEAN-360. The calibration system must
be able to demonstrate pulse-shape discrimination down to the level necessary to eliminate
the prodigious 39 Ar background. The calibration system must also provide a way to calibrate
the energy scale and position the reconstruction
algorithms used and to test the effectiveness of
the passive shielding of the detector. Our hope
is that much of this design work can be ported
to DEAP/CLEAN-3600.
We have developed preliminary designs of the
MiniCLEAN-360 calibration system. The focus
so far has been on the most challenging component of the system: an articulated source manipulation system to place a radioactive source anywhere within the liquid cryogen volume. This
allows the location-dependent position and energy resolutions to be mapped. It also allows us
to probe the behavior of events near the edge of
the sensitive volume, thus ensuring that we fully
understand the fraction and nature of events outside the fiducial volume that actually reconstruct
within the fiducial volume.
The source manipulator is an engineering
challenge. It must operate at LNe temperatures
(∼20K), have positional accuracy to within approximately ∼ 10cm, be extremely clean (thus
keeping the liquid cryogen free of contamination), and be mechanically reliable, since failure
of the manipulator inside the detector volume
would make it difficult to extract. We have developed a preliminary design for the manipulator
and are building a prototype as part of a TUNL
Research Experience for Undergraduates project.
We hope that by the end of the summer we will
have demonstrated that the mechanical design is
valid and works reliably down to liquid nitrogen
temperatures. Additionally, it is believed that
the source manipulator’s current design could be
simply modified to allow implementation in the
DEAP/CLEAN-3600 detector, which has similar
calibration requirements.
help guide the design of the calibration system,
the modeling of the TPB wavelength shifter, and
finally involvement in the position reconstruction
code, which is an integral part of the experiment. Another important aspect of our simulation work is the validation of neutron transport within geant4. This is important in understanding MiniCLEAN-360’s background due
to neutron interactions within the liquid cryogen.
9.5.3.3
9.5.3.2
Monte Carlo Simulation
Another thrust of our research program is the further development of the MiniCLEAN-360 Monte
Carlo program, which is based on rat (reactor analysis tool) which is in turn based on
geant4. Recent activity includes work on the
photo-multiplier tube geometry in the model, the
inclusion of the source manipulator in the MiniCLEAN Monte Carlo, various studies used to
Neon-Neutron Cross section measurement
In the course of our validation of the neutron
transport in geant4, we have discovered that
the neutron cross section for neon is not used by
geant4. Instead, the values for argon are used.
This is due to the absence of neon data in the
databases at the National Nuclear Data Center.
As TUNL maintains an excellent neutron beamline, we have decided to make a modern measurement of the neon cross sections. The main
technical challenge at present appears to be the
design of a gas-cell target that can be used to
contain the neon gas while in the neutron beam.
This measurement is the Master’s thesis project
of a PhD candidate, who is currently being assisted by an undergraduate.
9.5.3.4
Figure 9.31: (Color online) The conceptual design
for the MiniCLEAN-360 source manipulator, which consists of a sliding shaft mounted within a rotating
axle assembly (center). The mechanism will be controlled using steel
drive cables (not shown). The length
of the lower shaft is approximately
15 cm. The portion shown will be
inserted through a port with an inner diameter of 2.87 cm. A radioactive source would be mounted on the
front of the sliding shaft. The design
shown allows the source’s radial and
angular positions within the detector
to be scanned.
167
Materials Assay
Our activities at the Kimballton Underground
Research Facility (KURF), described in a separate report, have synergy with all of our experimental pursuits, in that both Majorana and
MiniCLEAN-360 are ultra low background experiments. Priority use of assay facilities is thus
very beneficial. An example is the thermal insulation for MiniCLEAN-360. The original design called for a copper thermal shield, aided by
one particular type of super-insulation. A quick
assay run at KURF revealed that the super insulation was heavily contaminated with radioisotopes, increasing the danger of contamination of
MiniCLEAN-360’s ultra-clean liquid cryogen volume. The design has already been modified in
light of this new information, and the selection
of new thermal insulation candidates will be informed by additional surveys at KURF.
[Lid08] J. J. Lidgard, Ph.D. thesis, Queen’s University, 2008.
[Lip08] W. H. Lippincott et al.,
arXiv:0801.1531v3.
(2008),
[Nik08] J. A. Nikkel et al., Astropart. Phys., 29,
161 (2008).
168
9.5.4
Advanced Detector Array for Sensitive γ-Ray Measurement
C.T. Angell, S.L. Hammond, C.R. Howell, A. Hutcheson, H.J. Karwowski, J.H. Kelley,
G. Rusev, A.P. Tonchev, W. Tornow, TUNL
To meet the evolving needs for advanced nuclear detection systems a quad clover high-purity
germanium (HPGe) detector array has been developed at TUNL. The detectors are arranged
to perform high-sensitivity searches for new low-spin states in actinides using γ-ray fluorescence techniques. The use of this detector array with the nearly monoenergetic, pulsed and
linearly polarized γ-ray beam at the High Intensity Gamma-ray Source (HIγS) gives an improvement in the signal-to-noise ratio in nuclear resonance fluorescence (NRF) measurements
of a factor of 30 over those made at bremsstrahlung γ-ray sources. These capabilities have
successfully been demonstrated in a variety of test experiments using
To guard against the possibility of nuclear
materials crossing our borders, innovative techniques for interrogating cargo containers using
γ-ray beams are being considered. They are limited to using the few known levels which resonantly fluoresce when excited with γ-ray beams.
Exploring a broader energy range will greatly expand the usefulness and capabilities of systems
based on these interrogation techniques.
E1
M1
Figure 9.32: (Color online) Diagram of the quad
clover HPGe detector array.
E1
transitions scatter into the vertical
detectors, and M1 transitions scatter
into the horizontal detectors.
The quad detector array at the HIγS has been
developed to search for and measure the properties of nuclear states in actinide nuclei that are
strongly excited by γ rays. The combination of
142,150 Nd
targets.
the high-intensity, mono-energetic γ-ray beam at
HIγS with the escape-suppressed clover detectors
in the quad array allows for unparalleled sensitivity in the search for new states. Additionally, the gating on prompt γ rays eliminates the
background generated from radioactive targets.
These two features are critical for nuclear fluorescence measurements in actinide targets.
The array consists of four clover HPGe detectors arranged in a cross formation (see Fig. 9.32).
The detectors use segmentation and active shielding to greatly reduce the background. Because of
the beam polarization, states that are resonantly
excited via an E1 transition show up in the vertical detectors, and M1 transitions appear in the
horizontal detectors. The full efficiency of the detector is obtained by adding back together, eventby-event, the energy signals of all four crystals in
the detector. This is termed “add-back” [Jon95].
Figure 9.33: (Color online) Diagram of the electronics developed for the experiment.
All aspects of the detector electronics were
developed to implement add-back of the energy
169
Nd − E beam
= 4.12 MeV
142
80
M1
70
60
50
Parallel
40
Counts
30
20
10
0
80
70
60
50
Perpindicular
E1
40
30
20
10
0
4040
4060
4080
4100
4120
4140
Energy [keV]
4160
4180
4200
Figure 9.34: (Color online) Spectrum from Eγ = 4.2 MeV on 142 Nd. The top spectrum is from the
horizontal detectors, and the bottom is from the vertical detectors. These spectra are
from the same run. Because of the 100% polarization of the beam, the electromagnetic
character of the transition can be clearly distinguished.
signals from the four crystals in a clover detector
(see Fig. 9.33). An electronic sum/invert module adds the four signals together electronically.
Two constant fraction discriminators (CFDs) are
used for each individual crystal signal. The first
CFD uses a high threshold to generate the trigger. A second CFD with a low threshold is used
to generate the timing signal for intra-detector
pulses that may be below the trigger threshold.
A bias amplifier is used to double the resolution
at high energies for the vertical detectors. A replay program using the root framework [Bru97]
was developed for software add-back mode.
The detectors, electronics, and add-back program were all tested during the 142,150 Nd nu-
clear resonance fluorescence (NRF) experiment
done in December 2007 and January 2008 (see
Sect. 7.3.3). The detector was used to measure
NRF states from 3.4 to 9.7 MeV. The results for
Eγ = 4.2 MeV on 142 Nd are shown in Fig. 9.34.
The test runs were a success, and the detector
array was used in March for NRF experiments
on actinides.
[Bru97] R. Brun and F. Rademakers, Nucl. Instrum. Meth., A389, 81 (1997).
[Jon95] P. M. Jones et al., Nucl. Instrum. Meth.,
A362, 556 (1995).
170
9.5.5
Cosmic-Ray Angular Measurement Employing a Reconstructed APEX
for Muon Detection
S. Daigle, A.E. Champagne, TUNL
Cosmic-ray Angular Measurement Employing a Reconstructed APEX (CAMERA), will detect muons in 15◦ azimuthal increments. Progress has been made in successfully connecting
all 24 NaI(Tl) scintillators in the APEX trigger detector and performing background radiation
surveys. New Photonis XP2012B 39 mm photomultiplier tubes (PMTs) were purchased to
replace some of the old Hamamatsu PMTs for improved energy resolution in future experiments.
Cosmic-ray Angular Measurement Employing
a Reconstructed APEX (CAMERA) will utilize
one of the 24-element NaI(Tl) scintillator arrays
originally built for the APEX experiment [Kal93],
which is on loan from Argonne National Laboratory. The APEX array was built as a pair
spectrometer using 24 position-sensitive NaI(Tl)
crystals. Thus, the path of incoming radiation
through the array can be determined by utilizing
the position information of each event through
opposite pairs of scintillator crystals. The main
purpose of CAMERA is to differentiate between
isotropic background events and muons that pass
directly through a small volume in the center of
the array.
Figure 9.35:
trum recorded in a single NaI(Tl) bar is shown in
Fig. 9.35. Those events in the high-energy region
of the spectrum fall into a muon gate that allows
CAMERA to distinguish between background γ
radiation and muons. A coincidence requirement
is then placed on events occurring in opposite detectors in order to select muons that pass through
the central axis of the array. One final position
cut eliminates all muons that do not pass directly
through the center of the scintillator array. The
end result is a histogram of muon flux in 15◦ azimuthal increments for a small volume in the center of the NaI(Tl) annulus. A preliminary angular distribution histogram of muons is shown in
Fig. 9.36. We are currently working to understand the discrepancy between the experimental
data and the theoretical cos2 (θ) distribution of
muons.
(Color online) Preliminary background survey from a single trapezoidal NaI(Tl) bar (crystal 24).
Muons are highlighted in the region
between channels 2500 to 4000.
Figure 9.36:
Twelve pairs of scintillator crystals will separately detect incoming muons in their respective 15◦ azimuthal slices along the top hemisphere of the array. A background energy spec-
(Color online) Preliminary CAMERA histogram comparing the experimental data (background) with a
theoretical cos2 (θ) distribution (foreground). The cause of the dip in the
experimental data is yet to be completely understood.
171
Future improvements include the addition of figure the detector into two concentric rings of 8
new photomultiplier tubes (Photonis XP2012B), and 16 detectors for use as a pair spectrometer.
which are intended to increase energy and position resolution. We presently have enough tubes
on hand to read out 10 elements of the array. [Kal93] N. I. Kaloskamis et al., Nucl. Instrum.
Methods, A330, 447 (1993).
Once the new tubes are in place, we will recon-
172
9.5.6
Absolute Efficiency Characterization of a High-Efficiency Neutron
Counter with a 252 Cf Source and Ionization Chamber
C.W. Arnold, T.B. Clegg, C.R. Howell, TUNL
In preparation for measuring absolute (γ,n) cross sections at HIγS, an absolute efficiency
measurement of a high-efficiency neutron counter was taken using a 252 Cf source placed in
various geometries within the detector, and compared to an MCNP (Monte Carlo neutral
particle) simulation. Maximum efficiency was measured with and without extra neutron
moderator to be 0.442 ± 0.001 and 0.413 ± 0.001 respectively. Our data agreed best with
our MCNP simulation within ± 5 inches of the center of the detector with an uncertainty of
0.33%, and a reduced χ2 = 0.95.
To obtain an absolute calibration of our highefficiency neutron counter, an ionization chamber
containing a 252 Cf film was placed inside the detector in several geometries [Böt92]. The natural
decay of 252 Cf produces alpha particles, fission
fragments, and neutrons. The number of neutrons emitted is directly proportional to the number of fission fragments emitted, with a proportionality constant ν̄. With the source at the center of the detector, simultaneous measurements
of neutrons and fission fragments were taken with
the neutron counter and the ionization chamber
respectively. Measurements were taken with and
without extra neutron moderator in the detector cavity. Subsequent neutron counter measurements were taken by translating this same source
along the axis of the detector for comparison with
mcnp models. The efficiencies from these measurements were normalized to the efficiency measured at the center of the detector during the
simultaneous measurements of neutrons and fission fragments.
Figure 9.37: (Color online) A typical MCA spectrum from the ionization chamber.
The α particles are represented by
the low energy peak, and the fission
fragments by the broad higher energy peak.
Figure 9.38: (Color online)(a) Data compared
with a fit to the MCNP simulation
within ±5 inches of the center of the
detector; χ2 = 0.95. (b) The axial
length of the region of best agreement between measurment and simulation is shown with dashed lines.
The dotted lines designate the edges
of the polyethelene cylinder.
(c)
Data do not agree with simulation
near the edges of the detector giving
an overall χ2 = 148
To operate the ionization chamber, a high
voltage (220 V) is applied to the chamber’s small
parallel plates while methane gas flows through
the chamber into a bubbler that provides backpressure for the gas. The ionization chamber is
assumed 100% efficient. Alpha particles and fission fragments are measured and distinguished
using electronics and a multi-channel analyzer
(MCA), as shown in Fig. 9.37. The number of
fission fragments are multiplied by the neutron
multiplicity ν̄ = 3.7632 ± 0.0054 [Bud88]. The
ratio of neutrons counted to neutrons expected
thus becomes the measured efficiency.
The operation of the neutron counter involves
the application of a high voltage (1780 V) to
18 3 He-gas-filled tubes embedded in a cylinder
of polyethylene. Neutrons thermalized by the
polyethylene (and other moderators) react via
the large 3 He(n,p)3 H cross section (Q = +763
keV). On-board electronics discriminate neutrons
from γ rays, and backgrounds taken with no
sources near the detector are typically less than
0.2% of the source-in count rate.
To test mcnp models of our neutron counter,
data were taken with the source suspended along
the axis of the detector and translated from end
to end at approximately one inch intervals. Further modeling tests included simulating moderator and other materials that will be present in
the detector under experimental conditions. Taking into account only the 0.33% statistical uncertainty of our data, the results from the modeling reproduce the experimental results within ±
5 inches of the center of the detector, with a reduced χ2 = 0.95. The fit is noticably poorer near
the edges of the detector producing an overall
173
χ2 = 148 when all data are included. However,
experimental samples will be no more than ±2
inches from the center of the the detector; well
within the area where the fit is excellent. Figure
9.38 shows comparisons of data with the curve fit
to mcnp simulations as well as a cross-sectional
XZ view of the neutron counter as modeled in
mcnp. Our mcnp model also reproduced features like efficiency increases in the presence of
extra neutron moderators shown in Fig. 9.39.
Figure 9.39: (Color online) Data also agree with
the MCNP simulation when various materials are introduced within
the central region of the neutron
counter. (a) Model of 252 Cf with
graphite, polyethelene, and the targets used for the 9 Be(γ,n) experiment. (b) Model of 252 Cf with a
single graphite plug at one end of
the central region of the neutron
counter.
[Böt92] R. Böttger, 1992, Private communication.
[Bud88] C. Budtz-Jørgensen, Nuc. Phys., A490,
307 (1988).
174
9.5.7
GEANT4 Simulation of SEGA Geometry
M. Boswell, R. Henning, H.O. Back, A.R. Young, TUNL; T. Burritt, University of Washington, Seattle, WA
The new detector mount for SEGA has been simulated in GEANT4. The various components
have been built in the GDML format. This simulation will be used in the near future to
estimate the background reduction of the detector mount.
The segmented enriched germanium array
(SEGA) will soon be remounted in a new lowbackground cryostat. Monte Carlo simulations
are currently underway to estimate the background contributions from various components
of the new cryostat. In addition to background
identification simulation, we are also examining mechanisms for suppressing these cryostatrelated signals.
Ge crystal (see Fig. 9.40). The simulated lowbackground cryostat is shown in Fig. 9.41. The
SEGA crystal has been omitted in this figure to
show the electrical components that have been
included in the simulation. These components include: three insulation spacers to offset the detector from the bottom of the cryostat, a centering
insulation spacer to offset the detector and centrally position it, two central high-voltage lines
that extend down through the center of the detector, an insulation tube surrounding the highvoltage lines, three Kapton signal foils complete
with Cu signal leads and grounding strips, three
Cu shields for protecting the Kapton foils, six
Cu contact foils, and the Cu holder and lid. The
assembly is a little over 10 cm long.
Figure 9.40: (Color online) A close-up view of the
signal cables used in the GEANT4
simulation.
The detector geometry was hard-coded in
C++ using the geometry-description markup
language (GDML) format [Chy01]. GDML, a
specialized form of the more generic extensible
markup language (XML), allows for the direct
conversion of an Autocad stp file to a format
readable by geant4. The conversion program,
fastrad [Beu03], generates each volume as a
tessellated solid in individual GDML files. The
advantage of using GDML is that small complex parts can easily be included in the geometry.
Specifically, in the case of the SEGA detector, we
were able to accurately render and place the copper readout lines in the Kapton cables and the
high voltage lines that are fed down inside the
Figure 9.41: (Color online) The simulated lowbackground cryostat for SEGA.
[Beu03] T. Beutier et al., In Proc. 7th European [Chy01] R.
Chytacek,
In
Conf. on Radiation and Its Effects on
Conf.
Proceedings,
Components and Systems, pp. 181–183,
URL:http://cern.ch/gdml.
2003, URL:http://www.fastrad.net.
175
CHEP01
2001,
176
9.5.8
GEANT4 Simulation of a Highly Segmented n-Type Ge Array.
M. Boswell, R. Henning, TUNL;
A highly segmented n-type detector configuration for use in the Majorana project has been
modeled and simulated in GEANT4 [Ago03]. Detailed simulations of background contributions from the cryostat, the support structures and the cabling are currently underway. These
simulations will be used in the near future to estimate the background contribution as well
as the background suppression capabilities of the current detector design.
Detecting a 0νββ decay from 76 Ge requires generator in the simulation, we also hope to exan unprecedented level of sensitivity in the 0νββ plore the effects of pulse-shape discrimination on
signal region, with background levels on the order suppressing these additional backgrounds.
of one count per year for a next-generation, onetonne experiment [Gai02]. Currently, the Majorana Collaboration is examining several possible
Ge-detector configurations to achieve this level of
sensitivity. One possible design involves several
segmented n-type Ge detectors that effectively
distinguish point-like ββ decays from the distributed interactions of γ-decays. Unfortunately,
such position sensitivity has a price: the additional presence of materials contaminated with
primordial activity; thus, the detectors are exposed to additional backgrounds. The question
then becomes: does the added sensitivity of the
segmented design overcome the additional background from the radioactive contaminants?
Using geant4, we are examining this issue of
background sensitivity in these configurations of
highly segmented detectors. Figure 9.42 shows
the new configuration, as specified to geant4,
for a single crystal of the detector array. The
cryostat comprises 14 such crystals, arranged in
seven strings of two detectors. One of the major
online) A single-crystal of a
enhancements of the current design is the use of Figure 9.42: (Color
14-fold highly segmented n-type Ge
electroformed copper, because electroforming sigdetector. The crystal is simulated as
part of an array currently under connificantly increases the purity of the copper. For
sideration for use in detecting 0ννββ
the simulation, the individual copper components
in 76 Ge. The assembly is a little over
will be contaminated with varying degrees of the
11.2 cm long.
impurities 60 Co, a cosmogenic contaminant, and
208
Tl and 214 Bi, both primordial contaminants
[Gai02]. In addition to examining the response of
individual detectors to these additional radiation [Ago03] S. Agostinelli et al., Nucl. Instrum.
sources, we hope, by simulating the entire conMethods, A506, 250 (2003).
figuration, to understand the collective response
to the radiation, i.e., the effects of self-shielding [Gai02] R. Gaitskell et al., Arxiv preprint nucland multi-crystal vetoes. By including a signal
ex/0201021, (2002).
9.5.9
177
New Differential Pumping System for Use with a Gas Scattering
Chamber
A. Imig, T.B. Clegg, A.H. Couture, TUNL; A. Davis, University of North Carolina, Chapel
Hill, NC
In support of the 2 H(p,pp)n breakup experiment, we developed a differential pumping system
to separate the required 0.1 atm pressure of the target chamber from the high vacuum of the
accelerator and beamline. Our motivation, design details, and experience with this system
are presented and discussed.
We decided, in planning measurements for the
H(p,pp)n breakup reaction (see Sect. 5.2.3), to
employ a gas target to introduce different, and
hopefully less important, sources of experimental
uncertainty than have been present in previous
measurements. This required providing a barrier
to prevent target gas from reaching the beam acceleration system. Our first choice of foils like
Havar to seal the chamber entrance unfortunately
led to high activation by the incident beam. To
avoid this, we introduced a differentially pumped
transition between the chamber filled with target
gas and the high vacuum of the beamline and
accelerator.
The differential pumping system that we used
consists of three successive, coaxial tubes with intervening small chambers backed by three separate vacuum pumps. Each tube contains a series
of apertures with central holes to define the beam
radius while providing resistance to the flow of
gas from the chamber into the beamline. Each
of the three stages contains a variable number of
collinear apertures that slide snugly into a tube
of radius r = 1.905 cm. Each aperture is of
the same thickness t = 0.254 mm, and successive apertures are spaced apart by a distance of
0.3 cm. The first tube stage contains 70 apertures while the second and third stages contain
40 apertures each. The characteristic aperture
radii in the first, second, and third stages are
0.138 cm, 0.150 cm, and 0.175 cm, respectively.
The first stage is backed by a mechanical and a
2
rotary blower pump, the second stage by a turbomolecular pump, and the final stage by an oil
diffusion pump. The smaller the aperture radius,
the smaller the conductance for flowing gas. The
aperture radii get progressively larger as the gas
flows out of the chamber along the beam path,
and therefore most of the impedance exists in
the first stage. However, from the perspective of
the beam traveling into the chamber, the progressively smaller apertures better define the beam
size as it reaches the target. While most of the
apertures are made from Al, the initial, defining
aperture on which the beam is incident in each
stage is made from Ta to reduce beam-related
nuclear activation. Each Ta aperture is thick
enough to stop any beam particles which strike
it.
In our pd-breakup experiment we used a deuterium gas target with a pressure of 75 Torr. The
differential pumping system reduced the pressure from the chamber to 6.8 · 10−1 mbar at the
mechanical pump, 1.9 · 10−3 mbar at the turbo
pump, and 3.0 · 10−6 mbar at the diffusion pump
closest to the accelerator. Furthermore, because
of the high deuterium cost, our design allows Ar
gas to be injected into the first stage tube to
buffer the flow of D2 gas and limit the amount of
expensive gas being pumped away. Even so, the
losses of deuterium gas were high enough that we
finally rejected differential pumping and returned
to using the Havar foil windows.
178
9.5.10
Precise Determination of Detector Solid Angles
A. Imig, TUNL
A digital scanning method has been developed to measure detector apertures for precise
determination of detector solid angles.
In charged-particle scattering and reaction crosssection measurements, such as the ongoing pdbreakup experiment reported elsewhere in this
progress report (see Sect. 5.2.3), one must determine detector solid angles very precisely.
These solid angles are often determined by
measuring a reaction of known cross section, such
as Rutherford scattering. The experimental uncertainty in such a measurement is set by the
accuracy of target thickness, current integration,
and the background in the detector spectra obtained.
Alternatively, these solid angles can be determined by direct physical measurement of the relevant dimensions. When gas targets are used, this
requires knowledge of the radius to, and area of,
both the front and rear detector apertures. The
limiting factor in such measurement is usually a
precise determination of aperture area. This can
be especially difficult when the apertures are not
of simple geometric shape.
An optical measurement with a stereomicroscope determines the aperture and calculates the
area. Rectangular apertures were chosen for the
pd-breakup experiment. However, the rounded
edges lead to a problem. The radius of curvature
can only be estimated assuming that the curvature is circular. This introduces a systematic error which can not be easily determined.
Therefore, we used a different method to minimize the systematic error while determining the
area. A fine pixelated grid with precisely known
spacing was superimposed over an image of each
aperture. A color scanner was the choice because
its pixel resolution of 1.79 · 10−3 mm2 when scanning with a resolution of 600 dpi exceeds by a
factor of two that of a digital camera with six
megapixel. Apertures were filled with modeling
material of a light color (Play-Doh is convenient)
to provide good contrast between material and
opening and to avoid shadows at the edges. Care
was used to assure that the dough filled only the
inside of the aperture and that no reflection on
possible burrs occurred. GIMP (GNU Image Manipulation Program) allows selection of pixels in
one color range so that the image of the aperture could be reduced in steps to two colors. The
number of pixels, which represent the opening,
were then counted (see Fig. 9.43).
Figure 9.43: (Color online) Shows apertures with
color-reduced openings.
For a measure of the error, each pixel which adjoins, in the x- and/or y-direction, a pixel of the
other color, was counted as a pixel at the edge
(see Fig. 9.44).
Figure 9.44: Pixels at the edge of the aperture.
The error is determined from the ratio of points
at the edges (an average of the number of pixels
inside and outside) to the total number of grid
179
points inside the aperture. This method determined the error of the opening area of one of the
apertures used to 2.5%.
The dimensional accuracy was verified simultaneously by digitalizing a caliper opened to
10.00 cm (see Fig. 9.45). For a comparison, an
aperture with a circular 10 mm opening could be
determined with a variance of 0.5%. This method
confirmed the results obtained by elastic scattering and shows a small error for the open area.
Figure 9.45: Caliper and apertures on the scanning bed. The caliper and circular
aperture serve as independent scales.
T
he TUNL Research Experience for Undergraduates (REU) program typically
draws ten students from accross the United States. Apart from working on their
focused projects, the students take part in executing a measurement as a group,
which trains them in all aspects of data taking in low-energy experimental nuclear
physics.
Appendices
•
Graduate Degrees Awarded
•
Publications
Invited Talks, Seminars, and Colloquia
•
•
Professional Service Activities
182
A.1
Appendices
Graduate Degrees Awarded
Ph.D. Degrees
1. John Poole, Dynamically Pumped Polarized Deuterium Target for n-d Scattering,
North Carolina State University, December 2007,
Supervisor: C.R. Gould.
2. Melissa Boswell, Dipole-Strength Distribution below the Particle Emission Threshold in
and 112 Sn,
University of North Carolina at Chapel Hill, December 2007,
Supervisors: H.J. Karwowski and A.P. Tonchev.
124
Sn
3. Matthew Kiser, Development of a System for Real-Time Measurements of Metabolite Transport
in Plants Using Short-Lived Positron-Emitting Radiotracers,
Duke University, July 2008,
Supervisor: C.R. Howell.
4. Anthony Hutcheson, Neutron-Induced Partial Gamma-Ray Cross-Section Measurements on
Uranium,
Duke University, July 2008,
Supervisors: W. Tornow and A.P. Tonchev.
5. Qiang Ye, 3 He Relaxation Time Measurements at Low Temperatures for the Neutron Electric
Dipole Moment (nEDM) Experiment,
Duke University, August 2008,
Supervisor: H. Gao.
M.S. and M.A. Degrees
1. John Cesaratto, ECR Ion Source for the LENA Laboratory,
University of North Carolina at Chapel Hill, October 2007,
Supervisors: T.B. Clegg and A.E. Champagne.
2. William Mohr, An Evaluation of Methods to Image Gamma Beam at HIGS,
University of North Carolina at Chapel Hill, April 2008,
Supervisor: T.B. Clegg.
A.2
Appendices
183
Publications
Journal Articles Published
1. A New Determination of SE1 of the 12 C(α, γ)16 O Reaction, X.D. Tang, K.E. Rehm, I. Ahmad,
C.R. Brune, A.E. Champagne, J.P. Greene, A.A. Hecht, D. Henderson, R.V.F. Janssens, C.L.
Jiang, L. Jisonna, D. Kahl, E.F. Moore, M. Notani, R.C. Pardo, N. Patel, M. Paul, G. Savard,
J.P. Schiffer, R.E. Segel, S. Sinha, B. Shumard, and A.H. Wousmaa, Phys. Rev. Lett. 99,
052502 (2007).
2. Recent Results of Experiments with Radioactive 21 Na and 7 Be Ion Beams, U. Greife et al.
(including A.E. Champagne), Nucl. Instrum. Methods B261, 1089 (2007).
3. Experimental Evidence of a Natural Parity State in 26 Mg and Its Impact on the Production of
Neutrons for the s-process, C. Ugalde, A.E. Champagne, S. Daigle, C. Iliadis, R. Longland, J.
Newton, E. Osenbaugh-Stewart, J.A. Clark, C. Deibel, A. Parikh, P.D. Parker, and C. Wrede,
Phys. Rev. C76, 025802 (2007).
4. Measurement of Nuclear Transparency for the A(e, e0 π + ) Reaction, B. Clasie et al. (including
D. Dutta and H. Gao), Phys. Rev. Lett. 99, 242502 (2007).
5. Investigation of Proton-Proton Short-Range Correlations via the 12 C(e,e’pp) Reaction, R. Shneor et al. (including Y. Qiang), Phys. Rev. Lett. 99, 072501 (2007).
6. The Proton Elastic Form Factor Ratio µp GpE /GpM at Low Momentum Transfer, G. Ron et al.
(including Y. Qiang and X.F. Zhu), Phys. Rev. Lett. 99, 202002 (2007).
7. Precision Measurements of the Nucleon Strange Form Factors at Q2 ∼ 0.1 GeV2 , A. Acha et
al. (including X. Qian), Phys. Rev. Lett. 98, 032301 (2007).
8. Deeply Virtual Compton Scattering off the Neutron, M. Mazouz et al. (including Y. Qiang),
Phys. Rev. Lett. 99, 242501 (2007).
9. Coherent Phi-Meson Photoproduction on the Deuteron at Low Energies, T. Mibe, H. Gao, K.
Hicks, K. Kramer, S. Stepanyan, D. J. Tedeschi et al., Phys. Rev. C76, 052202 (2007).
10. A High-Pressure Polarized 3 He Gas Target for the High Intensity Gamma Source (HIγS) Facility, K. Kramer et al. (including H. Gao, X. Zong, Q. Ye, and X.F. Zhu), Nucl. Instrum.
Methods A582, 318 (2007).
11. Validation of Spallation Neutron Production and Propagation within Geant4, M. Marino et al.
(including R. Henning), Nucl. Instrum. Methods A582 611 (2007).
12. Cold Neutron Energy Dependent Production of Ultracold Neutrons in Solid Deuterium, F.
Atchison, B. Blau, K.Bodek, B. van den Brandt, T. Brys, M. Daum, P. Fierlinger, A. Frei, P.
Geltenbort, P. Hautle, R. Henneck, S. Heule, A. Holley, M. Kasprzak, K. Kirch, A. Knecht, J.
Konter, M. Kuzniak, C.-Y. Liu, C. L. Morris, A. Pichlmaier, C. Plonka, Y. Pokotilovskii, A.
Saunders, Y. Shin, D. Tortorella, M. Wohlmuther, A. R. Young, J. Zejma and G. Zsigmond,
Phys. Rev. Lett. 99, 262502 (2007).
13. Production of Radioactive Nuclides in Inverse Reaction Kinematics, E. Traykov, A. Rogachevskiy,
M. Boswell, U. Dammalapati, P. Dendooven, O. C. Dermois, K. Jungmann, H. W. Wilschut,
and A. R. Young, Nucl. Instrum. Methods A572, 580 (2007).
14. γ-ray Production Cross Sections in Multiple Channels for Neutron-induced Reactions on 48 Ti
for En = 1 to 250 MeV, D. Dashdorj, G.E. Mitchell, P.E. Garrett, U. Agvaanluvsan, J.A.
Becker, L.A. Bernstein, M.B. Chadwick, M. Devlin, N. Fotiades, T. Kawano, R.O. Nelson,
and W. Younes, Nucl. Sci. and Eng., 157 65 (2007).
184
Appendices
15. Neutron-Induced Inelastic Cross Sections of 150 Sm for En = 1 to 35 MeV, D. Dashdorj, G.E.
Mitchell, T. Kawano, J.A. Becker, U. Agvaanluvsan, M. Chadwick, J.R. Cooper, M. Devlin, N.
Fotiades, P.E. Garrett, R.O. Nelson, C.Y. Wu, and W. Younes, Nucl. Instr. Methods B261,
934 (2007).
16. Progress on the Europium Neutron-Capture Study using DANCE, U. Agvaanluvsan, J.A.
Becker, R. Macri, W. Parker, P. Wilk, C.Y. Wu, T. Bredeweg, E. Esch, R. Haight, J. O’Donnell,
R. Reifarth, R. Rundberg, J. Schwantes, J. Ullmann, D. Vieira, J. Wilhelmy, J. Wouters, G.E.
Mitchell, S.A. Sheets, M. Krticka, and F. Becvar, Nucl. Instr. Methods B261, 948 (2007).
17. Bulk Properties of the Iron Isotopes, E. Algin, A. Schiller, A. Voinov, U. Agvaanluvsan, T.
Belgya, L.A. Bernstein, R. Chankova, P.E. Garrett, M. Guttormsen, M. Hjorth-Jensen, M.J.
Hornish, C.W. Johnson, T.W. Massey, G.E. Mitchell, J. Rekstad, S. Siem, and W. Younes,
Phys. At. Nucl. 70, 1634 (2007).
18. A Thermodynamic Analysis of Energy Eigenvalues, J.F. Shriner, Jr., M.P. Pato, G.E. Mitchell,
A.P.B. Tufaile, Nucl. Instr. Methods A581, 831 (2007).
19. Spin and Parity Assignments for 94,95 Mo Neutron Resonances, S.A. Sheets, U. Agvaanluvsan,
J.A. Becker, F. Becvar, T.A. Bredeweg, R. Haight, M. Jandel, M. Krticka, G.E. Mitchell, J.M.
O’Donnell, W. Parker, R. Reifarth, R.S. Rundberg, E.I. Sharapov, I. Tomandl, J.L. Ullmann,
D. Vieira, J.B. Wilhelmy, J.M Wouters, and C.Y. Wu, Phys. Rev. C76, 064317 (2007).
20. An Ultracold Neutron Source at the NC State University PULSTAR Reactor. E. Korobkina,
B. W. Wehring, A. I. Hawari, A. R. Young, P. R. Huffman, R. Golub, Y. Xu, and G. R. Palmquist.
Nucl. Instrum. Methods, A579, 530 (2007).
21. Cross Section Measurements of the
Rev. C77, 054607 (2008).
10
B(d, n0 )11 C Reaction below 160 keV, S. Stave et al., Phys.
22. Reaction Rate Uncertainties and 26 Al in AGB Silicon Carbide Stardust, M.A. van Raai, M.
Lugaro, A.I. Karakas, and C. Iliadis, Astron. Astrophys. 478, 521 (2008).
23. New Reaction Rate for 16 O(p,γ)17 F and Its Influence on the Oxygen Isotopic Ratios in Massive
AGB Stars, C. Iliadis, C. Angulo, P. Descouvemont, M. Lugaro, and P. Mohr, Phys. Rev. C77,
045802, (2008).
24. Relaxation of Spin Polarized 3 He in Mixtures of 3 He and 4 He below the 4 He Lambda Point, Q.
Ye, D. Dutta, H. Gao et al., Phys. Rev. A77, 053408 (2008).
~ e, e0 n)p
25. The Charge Form Factor of the Neutron at Low Momentum Transfer from the 2 H(~
Reaction, E. Geis et al. (including H. Gao), Phys. Rev. Lett. 101, 042501 (2008).
26. 3 He Spin-Dependent Cross Sections and Sum Rules, K. Slifer et al. (including H. Gao), Phys.
Rev. Lett. 101, 022303 (2008).
27. Evaluation of Radioactive Background Rejection in 76 Ge Neutrino-less Double-Beta Decay Experiments Using a Highly Segmented HPGe Detector, D.B. Campbell et al. (including R.
Henning), Nucl. Instrum. Methods A587 60 (2008).
28. Two-Step γ Cascades Following Thermal Neutron Capture in 95 Mo, M. Krticka, F. Becvar, I.
Tomandl, G. Rusev, U. Agvaanluvsan, and G.E. Mitchell, Phys. Rev. C77, 054319 (2008).
29. Pulse-Shape Discrimination with the Counting Test Facility, H. O. Back et al., Nucl. Instrum.
Methods A584, 98 (2008).
30. Study of Phenylxylylethane (PXE) as Scintillator for Low Energy Neutrino Experiments, H.
O. Back, et al., Nucl. Instrum. Methods A585, 48 (2008).
31. Development of High-field Superconducting Ioffe Magnetic Traps. L. Yang, J. M. Doyle, P. R.
Huffman, S. N. Dzhosyuk, C. E. H. Mattoni, C. M. Brome, J. M. Butterworth, R. M. Michniak,
C. M. O’Shaughnessy, E. Korobkina, R. Golub, P.-N. Seo, G. R. Palmquist, H. P. Mumm, A. K.
Thompson, K. Coakley, G. Yang, D. N. McKinsey, and S. K. Lamoreaux. Rev. Sci. Instrum.
79, 031301-1 (2008).
Appendices
185
32. Measurement of the 241 Am(n, 2n) Reaction Cross Section from 7.6 to 14.5 MeV, A. P. Tonchev,
C. T. Angell, M. Boswell, A. S. Crowell, B. Fallin, S. Hammond, C. R. Howell, A. Hutcheson, H.
J. Karwowski, J. H. Kelley, R. S. Pedroni, W. Tornow, J.A. Becker, D. Dashdor, J. Kenneally,
R.A. Macri, M.A. Stoyer, C.Y.Wu, E. Bond, M.B. Chadwick, J. Fitzpatrick, T. Kawano, R.S.
Rundberg, A. Slemmons, D. J. Vieira, and J. B. Wilhelmy. Phys. Rev. C77, 54610 (2008).
33. Neutron-Proton Analyzing Power at 12 MeV and Inconsistencies in Parametrizations of NucleonNucleon Data, R.T. Braun, W. Tornow, C.R. Howell, D.E. González Trotter, C.D. Roper, F.
Salinas, H.R. Setze, R.L. Walter, and G.J. Weisel, Phys. Lett. B660, 161 (2008).
34. Near-Threshold Deuteron Photodisintegration: An Indirect Determination of the GerasimovDrell-Hearn Sum Rule and Forward Spin Polarizability (γ0 ) for the Deuteron at Low Energies,
M.W. Ahmed, M.A. Blackston, B.A. Perdue, W. Tornow, H.R. Weller, B. Norum, B. Sawatzky,
R.M. Prior, and M.C. Spraker, Phys. Rev. C77, 044005 (2008).
35. Measurement of the Neutron-Neutron Scattering Length Using the π − d Capture Reaction, Q.
Chen, C.R. Howell, T.S. Carman, W.R. Gibbs, B.F. Gibson, A. Hussein, M.R. Kiser, G.
Mertens, C.F. Moore, C. Morris, A. Obst, E. Pasyuk, C.D. Roper, F. Salinas, H.R. Setze, I.
Slaus, S. Sterbenz, W. Tornow, R.L. Walter, C.R. Whiteley, and M. Whitton, Phys. Rev.
C77, 054002 (2008).
36. Precision Measurement of Neutrino Oscillation Parameters with KamLAND, S. Abe et al.
(including H.J Karwowski, D.M. Markoff, and W. Tornow), Phys. Rev. Lett., 100, 221803
(2008).
37. Measurements of Thermal Neutron Diffraction and Inelastic Scattering in Reactor-Grade
Graphite, C.D Bowman, D.C. Bowman, T. Hill, J. Long, A.P. Tonchev, W. Tornow, F. Trouw,
S. Vogel, R.L. Walter, S. Wender, and V. Yuan, Nucl. Sci. and Eng, 159 182 (2008).
38. Neutron Stimulated Emission Computed Tomography for Diagnosis of Breast Cancer,A.J. Kapadia, A.C. Sharma, J.E. Bender, G.D. Tourassi, C.R. Howell, A.S. Crowell, M.R. Kiser, B.P.
Harrawood, R.S. Pedroni, and C.E. Floyd, IEEE Trans. on Nucl. Sci. 55, 501 (2008).
39. Neutron Stimulated Emission Computed Tomography of a Multi-Element Phantom,C.E. Floyd,
A.J. Kapadia, J.E. Bender, A.C. Sharma, J.Q. Xia, B.P. Harrawood, G.D. Tourassi, J.Y. Lo,
A.S. Crowell, and C.R. Howell, Physics in Medicine and Biology, 53, 2313 (2008).
40. Experimental Detection of Iron Overload in Liver through Neutron Stimulated Emission Spectroscopy,A.J. Kapadia, G.D. Tourassi, A.C. Sharma, A.S. Crowell, M.R. Kiser, and C.R. Howell, Physics in Medicine and Biology, 53, 2633 (2008).
41. Exploring the Transport of Plant Metabolites Using Positron Emitting Radiotracers, M.R.
Kiser, C.D. Reid, A.S. Crowell, R.P. Phillips,and C.R. Howell, HFSP J. 2, 189 (2008).
42. Low-energy tail of the giant diploe resonance in 98 Mo and 100 Mo deduced from photon-scattering
experiments, G. Rusev, R. Schwengner, F. Dönau, M. Erhard, E. Grosse, A.R. Junghans, K.
Kosev, K.D. Schilling, A. Wagner, F. Bečvář, and M. Krtička, Phys. Rev. C77, 064321 (2008).
43. Measurement of the 3 He mass diffusion coefficient in superfluid 4 He over the 0.45-0.95 K
temperature range, S.K. Lamoreaux, G. Archibald, P.D. Barnes, W.T. Buttler, D.J. Clark,
M.D. Cooper, M. Espy, G.L. Greene, R. Golub, M.E. Hayden, C. Lei, L.J. Marek, J.C. Peng,
S. Penttila, Europhysics Letters, 82, 39901 (2008).
44. Evidence for Neutron Production in Deuterium Gas with a Pyroelectric Crystal without Tip,
W. Tornow, S.M. Shafroth, and J.D. Brownridge, J. Appl. Phys., 104, 034905 (2008).
45. Cross section measurements of the 10 B(d, n0 )11 C reaction below 160 keV, S. Stave, M.W.
Ahmed, A.J. Antolak, M.A. Blackston, A.S. Crowell, B.L. Doyle, S.S. Henshaw and C.R.
Howell, P. Kingsberry, B.A. Perdue, P. Rossi, R.M. Prior, M.C. Spraker, and H.R. Weller,
Phys. Rev. C77, 054607 (2008).
186
Appendices
46. Study of Near-Threshold Deuteron Photodisintegration: An Indirect Determination of the
Gerasimov-Drell-Hearn (GDH) Sum Rule and Forward Spin Polarizability (γ 0 ) for the Deuteron
at Low Energies, M. W. Ahmed, M. A. Blackston, B. A. Perdue, W. Tornow, H. R. Weller, B.
Norum, B. Sawatzky, R. M. Prior, and M. Spraker, Phys. Rev. C 77, 044005 (2008).
Appendices
187
Journal Articles Accepted
1. A Generic Surface Sampler for Monte Carlo Simulations, J.A. Detwiler (LBL, Berkeley &
Washington U., Seattle) , R. Henning (LBL, Berkeley & North Carolina U.) , R.A. Johnson,
M.G. Marino (Washington U., Seattle), to be published in IEEE Trans Nucl. Sci.
2. Measurements of the γ ∗ p → ∆ Reaction at Low Q2 : Probing the Mesonic Contribution, S.
Stave, N. Sparveris, M.O. Distler, I. Nakagawa, et al., to be published in Phys. Rev. C
3. The Effects of Variations in Nuclear Processes on Type I X-Ray Burst Nucleosynthesis, A.
Parikh, J. Jose, F. Moreno, and C. Iliadis, to be published in Astrophys. J. Suppl. Ser.
4. Matching of Experimental and Statistical-Model Thermonuclear Reaction Rates at High Temperatures, J.R. Newton, R. Longland, and C. Iliadis, to be published in Phys. Rev. C.
5. Reducing Parasitic Thermal Neutron Absorption in Graphite Reactors by 30%, C.D Bowman,
E. G. Bilpuch, D.C. Bowman, A.S. Crowell, C.R. Howell, K. McCabe, G.A. Smith, A.P.
Tonchev, W. Tornow, V. Vylet, R.B. Vogelaar, R.L. Walter, and J. Yingling, to be published
in Nucl. Sci. Eng.
6. Neutrons from a Proton-Driven Deuterium Target as a Possible Competitor to Spallation for
Nuclear Energy Applications, C.D Bowman, D.C. Bowman, E.G. Bilpuch, A.S. Crowell, C.R.
Howell, K. McCabe, G.A. Smith, A.P. Tonchev, W. Tornow, V. Vylet, and R.L. Walter, to be
published in Nucl. Sci. and Eng.
188
Appendices
Journal Articles Submitted
1. Quark-Hadron Duality in Neutron (3 He) Spin Structure, P. Solvignon et al. (including H.
Gao), submitted to Phys. Rev. Lett.
2. An Independent Measurement of the Total Active 8B Solar Neutrino Flux Using an Array
of 3 He Proportional Counters at the Sudbury Neutrino Observatory, The SNO Collaboration
(including R. Henning), submitted to Phys. Rev. Lett.
3. Pressure Shifts and Broadening of the Cs D1 and D2 Lines by He, N2 and Xe at Densities
used for Optical Pumping and Spin Exchange Polarization, A.H. Couture, T.B. Clegg, and B.
Driehuys, submitted to J. of Applied Physics.
4. New Results on the Solar Neutrino Fluxes from 192 Days for Borexino Data, C. Arpesella, H.
Back et al., submitted to Nucl. Instrum. Methods A.
5. The Borexino detector at the Laboratori Nazionale del Gran Sasso, G. Alimonti, C. Arpesella,
H. Back et al., submitted to Nucl. Instrum. Methods A.
6. Study of Low-Lying Structure of 94 Zr with the (n, n0 ) Reaction E. Elhami, J.N. Orce, M.
Scheck, S. Mukhopadhyay, S.N. Choudry, M.T. McEllistrem, S.W. Yates, C. T. Angell, M.
Boswell, B. Fallin, C.R. Howell, A. Hutcheson, H. J. Karwowski, J. H. Kelley, Y. Parpottas,
A. P. Tonchev, and W. Tornow, submitted to Phys. Rev. C.
7. Double-Electron Capture on 112 Sn to the Excited 1871 keV State in 112 Cd – A Possible Alternative to Double-Beta Decay, M.F. Kidd, J.H. Esterline, and W. Tornow, submitted to Phys.
Rev. C.
8. Energy Dependence of the Three-Nucleaon Analyzing Power Puzzle, W. Tornow, J.H. Esterline,
and G.J. Weisel, submitted to J. Phys. G: Nucl. Phys.
9. Neutron Detection Efficiency Determinations for the TUNL Neutron-Neutron Scattering Length
Measurements, D.E. González Trotter, F. Salinas Meneses, W. Tornow, A.S. Crowell, C.R.
Howell, D. Schmidt, and R.L. Walter, submitted to Nucl. Instrum. Methods A.
10. Multipole mixing ratios of transitions in 11 B, G. Rusev, A.P. Tonchev, W. Tornow, R. Schwengner,
C. Sun, and Y.K. Wu, submitted to Phys. Rev. C.
11. Experimental Evidence for Deformation Dependence of Electric Dipole Strength below the
Particle-Separation Energy, G. Rusev, R. Schwengner, R. Beyer, M. Erhard, E. Grosse, A.R.
Junghans, K. Kosev, C. Nair, K.D. Schilling, A. Wagner, F. Dönau, and S. Frauendorf, submitted to Phys. Rev. Lett.
12. Photon data shed new light upon the GDR spreading width in heavy nuclei, E. Grosse, A.R.
Junghans, G. Rusev, R. Schwengner, and A. Wagner, submitted to Phys. Lett. B.
13. Attempt to Manipulate the Decay Rate of
mitted to Phys. Rev. C.
64
Cu, B. Fallin, B. Grabow, and W. Tornow, sub-
14. Pygmy Dipole Strength in 90 Zr, R. Schwengner, G. Rusev, N. Tsoneva, N. Benouaret, R. Beyer,
M. Erhard, E. Grosse, A. R. Junghans, J. Klug, K. Kosev, H. Lenske, C. Nair, K. D. Schilling,
and A. Wagner, submitted to Phys. Rev. C.
15. Study of Low-lying Structure of 94 Zr with the (n, n0 γ) Reaction, E. Elhami, J. N.Orce, M.
Scheck, S. Mukhopadhyay, S. N. Choudry, M. T. McEllistrem, S. W. Yates, C. Angell, M.
Boswell, B. Fallin, C. R. Howell, A. Hutcheson, H. J. Karwowski, J. H. Kelley, Y. Parpottas,
A. P. Tonchev, W. Tornow, submitted to Phys. Rev. C.
Appendices
189
Special Reports and Books
1. Thermonuclear Processes, S. Starrfield, C. Iliadis, and W.R. Hix, in Classical Novae (2nd
edition). Eds., M. F. Bode and A. Evans (Cambridge University Press, Cambridge, 2008).
2. Random Matrices and Chaos in Nuclear Physics, H.A. Weidenmüller and G.E. Mitchell, submitted to Rev. of Mod. Phy.
3. Research Opportunities at the Upgraded HIγS Facility, H.R. Weller, M.W. Ahmed, H. Gao,
W. Tornow, Y.K. Wu, M. Gai, and R. Miskimen, Progress in Particle and Nuclear Physics, in
press.
190
Appendices
Conference Reports and Articles in Conference Proceedings
1. Study of Collective Dipole Excitations below the Giant Dipole Resonance at HIGS, A.P. Tonchev,
C. Angell, M. Boswell, A. Chyzh, C.R. Howell, H.J. Karwowski, J.H. Kelley, W. Tornow, N.
Tsoneva, and Y.K. Wu, AIP Conf. Proc. 891, 339, 2007.
2. Missing Dipole Excitation Strength below the Particle Threshold, A.P. Tonchev, C. Angell,
M. Boswell, C.R. Howell, H.J. Karwowski, J.H. Kelley, W. Tornow, N. Tsoneva, Workshop
on Photon Strength Functions and Related Topics, Prague, 2007, Proceedings of Science (on
line).
3. Dipole-strength Functions Below the Giant Dipole Resonance in the Stable Even-mass Molybdenum Isotopes, G. Rusev, R. Schwengner, R. Beyer, F. Dönau, M. Erhard, E. Grosse, A. R.
Junghans, K. Kosev, J. Klug, C. Nair, N. Nankov, K. D. Schilling and A. Wagner, Workshop
line).
4. Spin and Parity Assignments for 94,95 Mo Neutron Resonances Measured with the DANCE
Array, S.A. Sheets, U. Agvaanluvsan, J.A. Becker, F. Becvar, T.A. Bredeweg, R. Haight, M.
Jandel, M. Krticka, G.E. Mitchell, J.M. O’Donnell, W. Parker, R. Reifarth, R.S. Rundberg,
E.I. Sharapov, J.L. Ullmann, D. Vieira, J.B. Wilhelmy, J.M Wouters, and C.Y. Wu, Workshop
line).
5. Is There An Enhancement of Photon Strength at Low γ-ray Energies in Mo Isotopes? M.
Krticka, U. Agvaanluvsan, J.A. Becker, F. Becvar, T.A. Bredeweg, R. Haight, M. Jandel, G.E.
Mitchell, J.M. O’Donnell, W. Parker, R. Reifarth, R.S. Rundberg, E.I. Sharapov, S.A. Sheets,
J.L. Ullmann, D. Vieira, J.B. Wilhelmy, J.M. Wouters, and C.Y. Wu, Workshop on Photon
Strength Functions and Related Topics, Prague, 2007, Proceedings of Science (on line).
6. The Photon Strength Function of Eu Using DANCE, U. Agvaanluvsan, J.A. Becker, F. Becvar, T.A. Bredeweg, A.J. Couture, R. Haight, M. Jandel, M. Krticka, G.E. Mitchell, J.M.
O’Donnell, W.E. Parker, R. Reifarth, R.S. Rundberg, E.I. Sharapov, S.A. Sheets, J.L. Ullmann, D.J. Vieira, J.M. Wouters, C.Y. Wu, Workshop on Photon Strength Functions and
Related Topics, Prague, 2007, Proceedings of Science (on line).
7. Neutron Transversity Measurement at Jefferson Lab with a Polarized He-3 Target, H. Gao, X.
Qian, J.P. Chen, E. Cisbani, X. Jiang, J.C. Peng, and L.Y. Zhu, Few Body Syst. 41, 43, 2007.
8. TUNL Program on Preequilibrium Phenomenology, Constance Kalbach Walker, Proceedings
of the Cross Section Evaluation Working Group and US Nuclear Data Program Meetings,
Brookhaven National Laboratory, 2007.
9. 3N and 4N Systems and the Ay Puzzle, T. B. Clegg, Proceedings of the 5th International
Workshop on Chiral Dynamics, Theory and Experiment, Sept. 18-22, 2006, Durham/Chapel
Hill, NC, USA, Eds. M.W. Ahmed, H. Gao, B. Holstein, and H.R. Weller, World Scientific,
350, 2007.
10. Level Densities and Radiative Strength Functions in 56,57 Fe and 96,97 Mo, E. Algin, A. Schiller,
A. Voinov, U. Agvaanluvsan, T. Belgya, L.A. Bernstein, R. Chankova, P.E. Garrett, M. Guttormsen, M. Hjorth-Jensen, C.W. Johnson, G.E. Mitchell, J. Rekstad, S. Siem, and W. Younes,
Anadolu University Journal of Science and Technology, 8, 247, 2007.
11. Is There a Low Energy Enhancement in the Photon Strength Function in Molybdenum?, S.A.
Sheets, U. Agvaanluvsan, J.A. Becker, T.A. Bredeweg, R.C. Haight, M. Jandel, M. Krticka,
G.E. Mitchell, J.M. O’Donnell, W. Parker, R. Reifarth, R.S. Rundberg, J.L. Ullmann, D.
Vieira, J.B. Wilhelmy, and C.Y. Wu, International Workshop on Compound Nuclear Reactions
and Related Topics, AIP Conf. Proc. 1005, 74, 2008.
Appendices
191
12. Charged Pion Photoproduction and Scaling, H. Gao, Proceedings of the 11th International
Conference on Meson-Nucleon Physics and the Structure of the Nucleon (Menu07), September
10-14, 2007, Forschungszentrum Juelich, Germany, Eds. H. Machner and S. Krewald, eConf
C070910, 115, 2008.
13. Polarized 3 He Targets and Beams, T. B. Clegg, Proceedings of the 12th International Workshop
on Polarized Ion Sources, Targets, and Polarimetry, Sept. 12-14, 2007, Upton, NY., Eds. A.
Kponou, Y. Makdisi, and A. Zelenski, AIP Conf. Proc. 980, 37 (2008).
14. Neutron Capture Cross Section on 151,153 Eu, U. Agvaanluvsan, J.A. Becker, R. Macri, W.
Parker, P. Wilk, C.Y. Wu, T. Bredeweg, E. Esch, R. Haight, J. O’Donnell, R. Reifarth, R.
Rundberg, J. Schwantes, J. Ullmann, D. Vieira, J. Wilhelmy, J. Wouters, G.E. Mitchell, S.A.
Sheets, M. Krticka, and F. Becvar, Lawrence Livermore National Laboratory Technical Report
UCRL-TR-234006.
15. Experimental Overview of Compound Nuclear Resonance Reactions, G.E. Mitchell, Compound
Nuclear-Reactions and Related Topics, Eds. J. Escher, F.S. Dietrich, T. Kawano, and I.J.
Thompson, AIP Conf. Proc. 1005, 10, 2008.
16. Effect of Pre-equilibrium Spin Distribution on Neutron Induced Reaction Cross Sections, D.
Dashdorj, G.E. Mitchell, J.A. Becker, M.D. Chadwick, M. Devlin, N. Fotiades, T. Kawano,
R.O. Nelson, C.Y. Wu, and P. E. Garrett, Compound Nuclear-Reactions and Related Topics,
Eds. J. Escher, F.S. Dietrich, T. Kawano, and I.J. Thompson, AIP Conf. Proc. 1005, 164,
2008.
17. Thermodynamics Analysis of Spectra, G.E. Mitchell, J.F. Shriner, Jr., Nuclei and Mesoscopic
Physics WNMP07, Eds. P. Danielewicz, P. Piecuch, and V. Zelevinsky, AIP Conf. Proc. 995,
185, 2008.
18.
95
Mo Neutron Resonances Measured with the DANCE array, M. Krticka, U. Agvaanluvsan,
J.A. Becker, F. Becvar, T. Bredeweg, R. Haight, M. Jandel, G.E. Mitchell, J. O’Donnell,
W. Parker, R. Reifarth, R. Rundberg, E.I. Sharapov, S.A. Sheets,J. Ullmann, D. Vieira, J.
Wilhelmy, J. Wouters, and C.Y. Wu, Proceedings of the XVth International Seminar on Interactions of Neutrons with Nuclei. JINR Report E3-2008-26 (Joint Institute of Nuclear Research,
Dubna, 2008), p. 84.
19. The Three-Nucleon Analyzing Power Puzzle – The Past 20 Years, W. Tornow, New Facet
of the Three Nucleon Force – 50 Years of Fujita Miyazawa Three Nucleaon Force (FM50),
Proceedings of the International Symposium, Eds. H. Sakai, K, Sekiguchi, and B.F. Gibson,
(AIP Conf. Proc. 1011, 83, 2008.
192
Appendices
Conference Proceedings and Meeting Articles Accepted
1. Effect of Pre-equilibrium Spin Distribution on 150 Sm Cross Sections, D. Dashdorj, T. Kawano,
G.E. Mitchell, J.A. Becker, U. Agvaanluvsan, M.B. Chadwick, J.R. Cooper, M. Devlin, N. Fotiades, P.E. Garrett, R.O. Nelson, C.Y. Wu, and W. Younes, to be published in the proceedings
of the International Conference on Nuclear Data for Science and Technology.
2. Review of Livermore-Led Neutron Capture Studies Using DANCE, W. Parker, S.A. Sheets,
U. Agvaanluvsan, J.A. Becker, F. Becvar, T. Bredeweg, R. Clement, A. Couture, E. Esch,
R. Haight, M. Jandel, M. Krticka, G.E. Mitchell, R. Macri, J. O’Donnell, R. Reifarth, R.
Rundberg, J. Schwantes, J. Ullmann, D. Vieira, J. Wouters, and P.A. Wilk, to be published in
the proceedings of the International Conference on Nuclear Data for Science and Technology.
3. Recent Results from GEANIE at LANSCE, R.O. Nelson, N. Fotiades, M. Devlin, D. Dashdorj,
T. Kawano, S. Cowell, P. Talou, G.E. Mitchell, J.A. Becker, to be published in the proceedings
of the International Conference on Nuclear Data for Science and Technology.
4. The 20th Anniversary of the Three-Nucleon Analyzing Power Puzzle — A Personal Recollection, W. Tornow, to be published in Few-Body Systems.
5. Level Densities and Thermal Properties of 56,57 Fe, E. Algin, U. Agvaanluvsan, M. Guttormsen, G.E. Mitchell, J. Rekstad, A. Schiller, S. Siem, and A. Voinov, submitted to XVIth
International Seminar on Interactions of Neutrons with Nuclei.
6. Study of the Photon Strength Functions for Gadolinium Isotopes with the DANCE Array, D.
Dashdorj, G. E. Mitchell, B. Baramsai, R. Chankova, A. Chyzh, C. Walker and the DANCE
collaboration, submitted to CAARI08.
7. Dipole-strength Distributions Below the Giant Dipole Resonance in the Stable Even-mass Molybdenum Isotopes, G. Rusev, C. Angell, R. Beyer, F. Dönau, M. Erhard, E. Grosse, S. Hammond,
A. Hutcheson, S. Frauendorf, A.R. Junghans, H.J. Karwowski, J.H. Kelley, J. Klug, K. Kosev,
E. Kwan, C. Nair, N. Nikolov, K.-D. Schilling, R. Schwengner, A.P. Tonchev, W. Tornow, and
A. Wagner, submitted to CAARI 2008.
8. Photodisintegration Cross Section of 241 Am, A.P. Tonchev, S. Hammond, C.R. Howell, C.
Huibregtse, A. Hutcheson, H.J. Karwowski, J.H. Kelley, E. Kwan, G. Rusev, W. Tornow, and
J.B. Wilhelmy, submitted to CAARI 2008.
9. Two Experimental Approaches in Nuclear Astrophysics Using Neutrons, Y. Parpottas, C. Iliadis, and H. Tsertos, Proceedings of the 2nd International Conference on Frontiers in Nuclear
Structure, Astrophysics and Reactions, Crete, Greece, 2007. AIP Conf. Proc. (AIP, New
York, 2008) in print.
Appendices
193
Abstracts to Meetings and Conferences
1. Statistical Analysis of Proton and Neutron Resonance Data, D.J. Sissom, J.F. Shriner, Jr., and
G.E. Mitchell,
http://meetings.aps.org/link/BAPS.2007.DNP.DA.75
2. Raman Spectroscopy as a Way to Determine the Ortho-to-Para ratio of Deuterium, P. Z. Wong,
A. R. Young, G. Ribeill, and V. Mehta,
3. Searching for Resonances in the Unbound, K.Y. Chae, D.W. Bardayan, J.C. Blackmon, A.E.
Champagne, J.J. Das, R.P. Fitzgerald, V. Guimaraes, K.L. Jones, M.S. Johnson, R.L. Kozub,
R.J. Livesay, Z. Ma, S.D. Pain, M.S. Smith, J.S. Thomas, and D.W. Visser,
http://meetings.aps.org/link/BAPS.2007.DNP.HD.6
4. Structure of 8 B through 7 Be+p Scattering, J.C. Blackmon, R.J. Livesay, U. Greife, D.W.
Bardayan, K.Y. Chae, A.E. Champagne, C.Diebel, R. Fitzgerald, M.S. Johnson, K.L. Jones,
R.L. Kozub, Z. Ma, C.D. Nesaraja, S.D. Pain, F. Sarazin, J.F. Shriner Jr., D.W. Stracener,
M.S. Smith, J.S. Thoams, P.W. Visser, and C. Wrede,
http://meetings.aps.org/link/BAPS.2007.DNP.HD.7
5. Search for the states in 8 B via 7 Be + d, L. Segen, K.Y. Chae, D. Bardayan, J. Blackmon,
A. Champagne, J.J. Das, R. Fitzgerald, V. Guimarães, K. Jones, M. Johnson, R. Kozub, R.
Livesay, Z. Ma, C. Nesaraja, S. Pain, M. Smith, J. Thomas, and D. Visser,
6. Investigation of Removal of 3 He from Liquid 4 He Solution for the Neutron Electric Dipole
Moment Measurement, D. Haase, R. Golub, and P.P. Huffman,
http://meetings.aps.org/link/BAPS.2007.DNP.ED.5
7. Detector Characterization for the Majorana Project, E. Osenbaugh-Stewart and R. Henning,
http://meetings.aps.org/link/BAPS.2007.DNP.JF.12
8. Neutron-Induced Partial Gamma-Ray Cross-Section Measurements on Uranium at TUNL, A.
Hutcheson, A.S. Crowell, J.H. Esterline, B. Fallin, C.R. Howell, M. Kiser, A.P. Tonchev, W.
Tornow, J.H. Kelley, C.T. Angell, M. Boswell, H.J. Karwowski, R.S. Pedroni, G.J. Weisel, J.A.
Becker, D. Dashdorj, R.A. Macri, and R.O. Nelson,
http://meetings.aps.org/link/BAPS.2007.DNP.BH.6
9. Measurement of the 241 Am(n,2n) Reaction Cross Section from 7.6 to 14.5 MeV, A. Tonchev,
C. Angell, J. Becker, E. Bond, D. Dashdorj, B. Fallin, J. Fitzpatrick, C. Howell, A. Hutcheson,
H. Karwowski, J. Kelley, R. Macri, R. Pedroni, A. Slemmons, M. Stoyer, W. Tornow, D. Vieira,
J. Wilhelmy, and C. Wu,
http://meetings.aps.org/link/BAPS.2007.DNP.BH.7
10. Precise Determination of Total Absolute Gamma Ray Intensity at HIγS, S. Stave, M.W.
Ahmed, M.A. Blackston, M.D. Busch, M. Emamian, S.S. Henshaw, C.R. Howell, J. Li, S.
Mikhailov, B.A. Perdue, G. Swift, H.R. Weller, and Y.K. Wu,
http://meetings.aps.org/link/BAPS.2007.DNP.HH.6
11. Partial (n,n’γ) Cross Section Measurements of Cu, Ge and Pb at 8 and 12 MeV for 0νββ
Decay, E. Kwan, J.H. Esterline, S. Elliot, B. Fallin, S.H. Hilderbrand, A. Hime, C.R. Howell,
A. Hutcheson, H.J. Karwowski, J.H. Kelley, M.F. Kidd, D.B. Masters, D. Mei, A.P. Tonchev,
and W. Tornow,
194
Appendices
12. Direct Measurement of the 1 S0 Neutron-Neutron Scattering Length at the YAGUAR Reactor,
S.L. Stephenson, B.E. Crawford, D. Kawamura, M.R. Schmidt, D.A. Yager-Ebrriaga, C.R.
Howell, W. Tornow, G.E. Mitchell, W.I. Furman, A.R. Krylov, E.V. Lychagin, A.Y. Muzichika,
G.V. Nekhaev, E.I. Sharapov, V.N. Shvetsov, A.V. Streikov, B.G. Levakov, A.E. Lyshin, Y.I.
Chemkhin, and Y.Z. Kandiev,
http://meetings.aps.org/link/BAPS.2007.DNP.JH.1
13. Neutron-Helium-3 Analyzing Powers between 1.60 and 5.54 MeV, J.H. Esterline, A.S. Crowell,
B.A. Fallin, C.R. Howell, A. Hutcheson, M.F. Kidd, M.R. Kiser, R.A. Macri, S.Tajima, W.
Tornow, B.J. Crowe, R.S. Pedroni, and G.J. Weisel,
http://meetings.aps.org/link/BAPS.2007.DNP.JH.3
14. Neutron-Induced Partial Cross-Section Measurements on 76 Ge Motivated by The Majorana
Project 0νββ Decay Search, S. Hilderbrand, E. Kwan, C. Angell, B. Fallin, C.R. Howell,
A. Hutcheson, H.J. Karwowski, J.H. Kelley, A.P. Tonchev, W. Tornow, D.B. Masters, R.S.
Pedroni, and G.J. Weisel,
15. A Scattering Chamber System to Measure Cross Sections of Multiple Star Configurations in
Neutron-Deuteron Breakup at 19 MeV, L. Threatt, B. Crowe, L. Cumberbatch, C. Howell, and
D. Markoff,
16. Measurement in Progress of the Parity-Violating Neutron Spin-Rotation in Liquid 4 He, D.M.
Markoff, C.D. Bass, J.M. Dawkins, T.D. Finley, J.C. Horton, C.R. Huffer, D. Luo, M.G.
Sarsour, W.M. Snow, K. Gan, A.K. Opper, A.M. Micherdzinska, B.R. Heckel, H.E. Swanson,
H.P. Mumm, J.S. Nico, B.E. Crawford, and E.I. Sharapov,
http://meetings.aps.org/link/BAPS.2007.DNP.CD.7
17. Proposed measurement of the neutron spin-rotation through solid-ortho-deuterium, A. Komives,
D.M. Markoff, and B.J. Crowe,
http://meetings.aps.org/link/BAPS.2007.DNP.CD.8
18. Updates for Gadolinium Neutron Capture Measurements at DANCE, D. Dashdorj, G.E. Mitchell,
B. Baramsai, R. Chankova, A. Chzh, C. Walker, U. Agvaanluvsan, J.A. Becker, W. Parker,
C.Y. Wu, T. Bredeweg, A. Couture, R. Haight, M. Jandel, J. O’Donnell, R. Rundberg, J.
Wouters, J. Ullmann, D. Vieira, F. Becvar, and M. Krticka,
19. Double Beta Decay of 150 Nd to Excited Final States, M. Kidd, J. Esterline, and W. Tornow,
20. Calibration and Performance of the UConn-Yale-PTB-Weizmann-UCL-TUNL O-TPC, A.
Young, M. Gai, T. Kading, M. Ahmed, H. Weller, V. Dangendorf, and K. Tittelmeier,
21. Raman Spectroscopy as a Way to Determine Ortho Para Ratio of Deuterium, P. Wong, A.
Young, G. Ribeill, and V. Mehta,
22. A Physical Science Course for Rural Middle School Teachers, D.G. Haase, S. K. Schulze, N.S.
Ragan, and S. Reintjes,
http://meetings.aps.org/Meeting/SES07/Event/73497
23. Neutrinoless Double Beta Decay, R. Henning,
http://meetings.aps.org/Meeting/SES07/Event/73382
24. Initial Asymmetry Results from the UCNA Experiment, R. W. Pattie for the UCNA collaboration,
http://meetings.aps.org/Meeting/APR08/Event/83936
Appendices
195
25. UCN Polarization in the UCNA Experiment, A. T. Holley for the UCNA collaboration,
26. Study of the 13 C(d,n0,1 )14 N Reaction Below Ecm = 400 keV, E. Clinton, M.W. Ahmed, S.S.
Henshaw, B.A. Perdue, P.N. Seo, S. Stave, H.R. Weller, P.P. Martel, R.H. France III, R.M.
Prior, and M. C. Spraker,
27. The commissioning of the O-TPC at TUNL, P.N. Seo, M.W. Ahmed, E.R. Clinton, C.R.
Howell, S.C. Stave, H.R. Weller, A.H. Young, M. Gai, B. Bromberger, V. Dangendorf, and K.
Tittelmeier,
28. Calibration and Installation of the UConn O-TPC at TUNL, A. Young, T. Kading, P. Seo, M.
Gai, C. Howell, E. Clinton, H. Weller, S. Stave, M. Ahmed, V. Dangendorf, and K. Tittelmeier,
29. Nuclear Reaction Dynamics of the 10 B(d, n0 )11 C Reaction Below 160 keV, S. Stave, M.W.
Ahmed, M.A. Blackston, A.S. Crowell, S.S. Henshaw, C.R. Howell, P. Kingsberry, B.A. Perdue,
H.R. Weller, B.L. Doyle, P. Rossi, A.J. Antolak, R.M. Prior, and M.C. Spraker,
30. Study of the 11 B(~
p, α)8 Be Reaction using polarized protons below 5.1 MeV, S. Henshaw, S.
Stave, M. Ahmed, M. Blackston, B. Perdue, H.R. Weller, R. France, R. Lewis, J.P. Metzker,
R. Prior, M. Spraker, and A. Kusnezov,
31. Photodisintegration of Deuterium at Low Energies: Measurements of Cross Section and ForeAft Asymmetries Between Eγ of 2.44 and 4 MeV at the High Intensity γ-Ray Source (HIγS),
M.W. Ahmed, S.S. Henshaw, B.A. Perdue, S. Stave, H.R. Weller, J. Li, S. Mikhailov, and Y.
Wu,
32. TUNL Activities at the Kimballton Underground Research Facility, H. O. Back, A. Champagne,
R. Henning, P. Finnerty, W. Tornow, M. Kidd, and J. Esterline,
33. Systematic Study of Cosmogenic Activation with Low Background Ge Spectroscopy at the Kimballton Underground Research Facility, P. Finnerty, H. Back, and R. Henning,
34. A New Precision Measurement of the Lifetime of 19 Ne, L. Broussard, R. Pattie, H. Back, A.
Young, U. Dammalapat, S. De, P. Dendooven, O. Dermois, A. Rogachevsky, M. Sohani, E.
Traykov, L. Willamnn, and H. Wilschut,
35. Neutron-Induced Partial Cross Section Measurements of Cu, Ge, and Pb at E n = 8 and 12
MeV for Background Radiation in 0νββ Decay Experiments, E. Kwan, J.H. Esterline, B. Fallin,
C.R. Howell, A. Hutcheson, M.F. Kidd, A. Tonchev, W. Tornow, C. Angell, H. Karwowski, J.
Kelley, D. Mei, S. Hilderbrand, D.B. Masters, R.S. Pedroni, and G.J. Weisel,
36. Neutron-Induced Partial Gamma-Ray Cross-Section Measurements on Uranium at TUNL, A.
Hutcheson, A.S. Crowell, B. Fallin, C.R. Howell, M. Kiser, E. Kwan, A.P. Tonchev, W. Tornow,
J.H. Kelley, C.T. Angell, H.J. Karwowski, R.S. Pedroni, G.J. Weisel, J.A. Becker, D. Dashdorj,
R.A. Macri, N. Fotiades, and R.O. Nelson,
37. Determination of the Spectral and Intensity Distribution of the Mono-Energetic Gamma-Ray
Beam at HIγS Using a Large Volume HPGe Detector, G. Rusev, A. Tonchev, A. Hutcheson,
E. Kwan, W. Tornow, C. Angell, H. Karwowski, J. Kelley, C. Sun, and Y. Wu,
196
Appendices
38. Gamma Strength Function for p-process Nucleosynthesis Calculations, C.T. Angell, S. Hammond, H.J. Karwowski, E. Kwan, G. Rusev, A. Tonchev, J.H. Kelley, A. Makinaga, and H.
Utsunomiya,
39. The Astrophysical 187 Re187 Os Ratio: Measurement of the 187 Re(n, 2n γ)186m Re Dectruction
Cross Section, J.H. Kelley, D.B. Masters, S. Hammond, H.J. Karwowski, E. Kwan, A. Hutcheson, A.P. Tonchev, W. Tornow, F.G. Kondev, and S. Zhu,
40. Nuclear Dipole Response of Bound States in N=82 Nuclei Below Particle Threshold, A. Tonchev,
A. Hutcheson, E. Kwan, G. Rusev, W. Tornow, C. Angell, S. Hammond, H. Karwowski, and
J. Kelley,
41. Search for Dipole States in 235,238 U, S. Hammond, C. Angell, H. Karwowski, E. Kwan, G.
Rusev, A. Tonchev, W. Tornow, and J. Kelley,
42. Attempts to Manipulate the Decay Time of Radioactive Nuclei, B. Fallin, B. Grabow, and W.
Tornow,
43.
112
Sn Double-Electron Capture to Excited States - A Possible Alternative to Neutrinoless
Double-Beta Decay, M. Kidd, J. Esterline, W. Tornow,
44. Dipole-strength distributions below the giant dipole resonance in the stable even-mass molybdenum isotopes, G.Rusev, R. Schwengner, A. P. Tonchev, F. Dönau, C. Angell, R. Beyer, M.
Ergard, S. Frauendorf, E. Grosse, S. Hammond, A. Hutcheson, A. R. Junghans, H. J. Karwowski, J. H. Kelley, J. Klug, K. Kosev, C. Nair, N. Nikolov, K.-D. Schilling, A. Wagner,
CAARI 2008, 20th International Conference on the Application of Accelerators in Research
and Industry, August 10 - 15, 2008.
A.3
Appendices
197
1. Plans for Compton scattering from light nuclei at HIGS, M.W. Ahmed, INT workshop on Soft
Photons & Light Nuclei workshop, Seattle, June 16-20, 2008.
2. Nuclear Physics Program at the Upgraded High Intensity γ-ray Source (HIγS), M.W. Ahmed,
74th Annual Meeting of the Southeastern Section of APS, Nov. 9, 2007.
3. Discovering Neutrino Properties: A Low Energy Nuclear Physics Prospective, H.O. Back, Ohio
University Department of Physics and Astronomy colloquium, Athens, OH, January 2008.
4. Discovering Neutrino Properties: A Low Energy Nuclear Physics prospective, H.O. Back, The
University of New Hampshire Physics Department colloquium, Durham, NH, March 2008.
5. KURF, the Kimballton Underground Research Facility, H.O. Back, The University of New
Hampshire Nuclear Physics Seminar, Durham, NH, April 2008.
6. Effect of Pre-equilibrium Spin Distribution on Neutron-Induced Reaction Cross Sections, D.
Dashdorj, International Workshop on Compound Nuclear Reactions and Related Topics, Fish
Camp, CA, 2007.
7. Cross Sections, Level Densities, and Strength Functions, D. Dashdorj, SSAA Symposium,
Washington, D.C., 2008.
8. Study of the Photon Strength Functions for Gadolinium Isotopes at the DANCE Array, D.
Dashdorj, Conference on Applications of Accelerators in Research and Industry, Fort Worth,
TX, 2008.
9. Scaling in Charged Pion Photoproduction from the Nucleon, H. Gao, 11th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, Institute für Kernphysik
Forschungzentrum Jülich, Germany, September 10-14, 2007.
10. The Polarized Compton Sacttering Program at the HIγS Facility at DFELL, H. Gao, Nuclear
Physics Seminar, Ohio University, November 2007.
11. Experimental Search on Color Transparency Effect, H. Gao, Workshop on Nuclear Medium
Effects on the Quark and Gluon Structure of Hadrons, ECT* Trento, Italy, June 3-7, 2008.
12. Neutron Structure Studies Using a Polarized 3 He Target, H. Gao, American Physical Society
Meeting, St. Louis, MO, April 11-April 15, 2008.
13. Measurement of Neutron Transversity Using a Polarized 3 He target at Jefferson Lab, H. Gao,
PKU-BNL Workshop on Transverse Spin Physics, Beijing University, Beijing, China, June
30-July 4, 2008.
14. Powering the Planet, Powering North Carolina: Physics Perspectives on Energy for the 21st
Century, C.R. Gould, Appalachian State University, Physics Department colloquium, September 2007.
15. Energy Today and Tomorrow . US and NC Options, C.R. Gould, Scope Academy, NC State
University, Oct 2007.
16. Powering the Planet, Powering North Carolina: Physics Perspectives on Energy for the 21st
Century, C.R. Gould, UNC-Chapel Hill, Physics Department colloquium, February 2008.
17. Measurements of Few-Nucleon Reactions at HIγS, C.R. Howell., Gordon Conference on Photonuclear Reactions, Tilton, NH, August 2008.
198
Appendices
18. Measuring the Neutron Lifetime with Magnetically Trapped Neutrons, P. Huffman, International Workshop on UCN Sources and Experiments, Vancouver, BC, September 2007.
19. Measuring the Neutron Lifetime with Magnetically Trapped Neutrons, P. Huffman, Physics
Department Colloquium, North Carolina State University, Raleigh, NC, September 2007.
20. Measuring the Neutron Lifetime Using Magnetically Trapped Ultracold Neutrons, P. Huffman,
74th Annual Meeting of the Southeastern Section of the American Physical Society, Nashville,
TN, November 2007.
21. Monte Carlo Reaction Rates, C. Iliadis, Workshop on R-Matrix and Nuclear Reactions in
Stellar Hydrogen and Helium Burning, Santa Fe, New Mexico, April 2008.
22. Low-energy Few-body Experiments - Why do we care about polarization?, A. Imig, Brookhaven
National Laboratory, April 2008.
23. Thermodynamic Analysis of Spectra, G.E. Mitchell, Workshop on Nuclei and Mesoscopic
Physics, East Lansing, MI, 2007.
24. Experimental Overview of Compound Nuclear Resonance Reactions, G.E. Mitchell, International Workshop on Compound Nuclear Reactions and Related Topics, Fish Camp, CA, 2007.
25. Chaos and Symmetry Breaking in Nuclear Physics, G.E. Mitchell, International Workshop on
Chaos and Collectivity in Many-Body Systems, Dresden, Germany, 2008.
26. Neutron Capture Experiments with the DANCE Array, G.E. Mitchell, Technical University
Darmstadt, Darmstadt, Germany, 2008.
27. Neutron Capture Measurements with the DANCE Array, G.E. Mitchell, Forschungzentrum
Dresden-Rossendorf, Rossendorf, Germany, 2008.
28. Purity and Completeness Issues in Nuclear Physics, G.E. Mitchell, Workshop on Statistical
Nuclear Physics and Applications in Astrophysics and Technology, Athens, OH, 2008.
29. Search for phi-N bound state in Jefferson Lab Hall-B, Y. Qiang, Short Range Collaboration
Workshop, JLab, Newport News, VA, October 2007.
30. Polarized 3 He target preparations for Transversity/d2n Experiments, Y. Qiang, Jefferson Lab
Hall A Collaboration Meeting, JLab, Newport News, VA, June 2008.
31. Recent Experimental Results on the 241 Am(n,2n) Cross Section Measurements at TUNL, A.P.
Tonchev, Nuclear Reactions on Americium Workshop, Santa Fe, September 2007.
32. Neutron-Induced Reactions on Actinides Using Pulsed and Monoenergetic Beams at Triangle
Universities Nuclear Laboratory, A.P. Tonchev, 2008 Stewardship Science Academic Alliance,
Washington, DC, February 2008.
33. Nuclear Data Measurements on Actinides Using the High Intensity Gamma-ray Source, A.P.
Tonchev, Academic Research Initiative Grantees Conference, Washington, DC, April 2008.
34. Photodisintegration Cross Section of 241 Am, A.P. Tonchev, CAARI 2008, 20th International
Conference on the Application of Accelerators in Research and Industry, Fort Worth, Texas,
August 2008.
35. The 20th Anniverary of the Three-Nucleon Analyzing Power Puzzle — A Personal Recollection,
W. Tornow, 20th European Few-Body Conference, Pisa, Italy, September 2007.
36. The Three-Nucleon Analyzing Power Puzzle — The Past 20 Years, W. Tornow, International
Symposium on New Facets of the Three-Nucleon Force — 50 years of Fujita Miyazawa ThreeNucleon Force, Toyko, Japan, October 2007.
37. The Three-Nucleon Analyzing Power Puzzle — The Past 20 Years, W. Tornow, American
Physical Society Meeting, St. Louis, MO, April 2008.
Appendices
199
38. Development of an Ultracold Neutron Source at the PULSTAR Reactor, A. R. Young, Workshop on Ultracold Neutrons, TRIUMF, September 2007.
39. NNbar Experiments using UCN, A. R. Young, Workshop on Baryon and Lepton Number
Violation, Lawrence Berkeley Laboratory, September 2007.
40. Progress on the UCNA experiment: Measuring the Beta-Asymmetry using Polarized Ultracold
Neutrons, A. R. Young, NSCL seminar, November 2007.
41. Improved Experimental Limits for Neutron-Antineutron Oscillations, A. R. Young, Los Alanmos P/T Colloqium, Los Alamos National Laboratory, February 2008.
42. The UCNA Experiment: Progress Towards a Measurement of the Beta-Asymmetry with Ultracold Neutrons, A.R. Young, NP08 Symposium, Mito, March 2008.
43. Neutrinoless Double Beta-decay: the Majorana Project, A.R. Young, UNC-Wilmington Colloquium, April 2008.
44. The UCNA Experiment: First Results for a Measurement of the Beta-Asymmetry with Ultracold Neutrons, A.R. Young, High Energy Physics Seminar, Duke University, April 2008.
45. Personal and Biased Summary and Outlook, A.R. Young, Summary talk for the Workshop on
Fundamental Physics with Slow Neutrons, Institut Laue Langevin, May 2008.
46. Development of an Ultracold Neutron Source at the PULSTAR Reactor, A.R. Young, Technical
University of Munich, May 2008.
47. Gerasimov-Drell-Hearn Integral on 3 He Part I: 3-body measurement for Eγ at 11.4 and 14.7
MeV, X. Zong, INT workshop on Soft Photons & Light Nuclei workshop, Seattle, June 16-20,
2008.
200
Appendices
Seminars at TUNL
1. Lisa Whitehead, Stony Brook University, (September 13, 2007)
Neutrino Cross Section Measurements at K2K.
2. Edgardo Browne-Moreno, Lawrence Berkeley National Laboratory, (September 20, 2007)
Nuclear Data for the Skeptic.
3. Michael Ronquest, University of Virginia, (September 27, 2007)
Search for Direct CP Violation in the Decay KL,S → π + π − γ.
4. Kai Vette, Lawrence Livermore National Laboratory, (October 4, 2007)
Gamma-Ray Tracking: Opportunities for Basic and Applied Research
5. Rory Miskimen, University of Massachusetts, (October 18, 2007)
Studies of Proton Spin-Structure at Low-Q: Measuring the Spin-Polarizabilities of the Proton
at HIγS
6. Axel Muller,Technical University of Munich, (October 23, 2007)
Ultra Cold Neutrons: How to Produce Them-How to Use Them
7. Alex Brown, Michigan State University, (October 25, 2007)
New Theoretical Results for sd and pf Shell Nuclei
8. Lee G. Sobotka, Washington University, (November 1, 2007)
Two Seemingly Unrelated Topics: 1. In-medium Correlations as Seen from a Causal Perspective and 2. The Nuclear Caloric Curve T(E*)
9. Markus Berheide, Schlumberger, (November 8, 2007)
Nuclear Applications in the Oil Industry
10. Toshihiko Kawano, Los Alamos National Laboratory, (November 15, 2007)
Nuclear Reaction Theories for Applications
11. Stephan Schlamminger, University of Washington, (November 29, 2007)
High Precision Test of the Equivalence Principle
12. Gianpaolo Carosi, Lawrence Livermore National Laboratory, (December 6, 2007)
Searching for Ghosts: Looking for Dark Matter Axions using the ADMX experiment
13. Larry Phair, Lawrence Berkeley Laboratory, (December 13, 2007)
The Fission Barrier Landscape
14. Ryan Fitzgerald, National Institute of Standards and Technology, (January 10, 2008)
Primary Standardization of Radionuclides
15. Ronald Schwengner, Institut für Strahlenphysik, Research Centre Dresden-Rossendorf, (January 15, 2008)
Dipole-Strength Funtions Studied in Photon-Scattering Experiments at Elbe
16. Earl Babcock, University of Wisconsin, Madison, (January 17, 2008)
Efficient Production of Polarized 3 He for Fundamental Physics with Neutrons
17. Alexander Sibirtsev, Bonn University, Research Centre Juelich and EBAC, (February 14, 2008)
Charged Pion Photoproduction and High Mass Baryons
18. Michael Ramsey-Musolf, University of Wisconsin, Madison, (February 28, 2008)
Baryogenesis, Electric Dipole Moments, and the Higgs Boson
19. Yong-Zhong Qian, University of Minnesota, (March 13, 2008)
New Developments in Flavor Transformation of Supernova Neutrinos
Appendices
201
20. Barry L. Berman, The George Washington University, (March 18, 2008)
Two-Body Photodisintegration of 3 He, 4 He, and 2 He up to 1.5 GeV
21. Steve Nelson, The University of Texas of the Permian Basin, (March 19, 2008)
The High Temperature Teaching and Test Reactor Research Facility
22. Hendrik Schatz, Michigan State University and National Superconducting Cyclotron, (March
25, 2008)
The Origin of the Light ”r-Process” elements
23. Willem Van Oers,University of Manitoba and TRIUMF, (April 3, 2008)
From Hadronic Parity Violation to Parity Violating Electron Scattering and Tests of the Standard Model
24. Kim Lister, Argonne National Laboratory, (April 10, 2008)
An Odyssey Along the N=Z Line Above 56 Ni: Shells, Shapes, Symmetries and Waiting Points
25. Walter Greiner, University of Frankfurt, Germany, (April 22, 2008)
Orientation of Birds and other Animals in Magnetic Field - Discovery of the Sixth Sense
26. Colleen Fitzpatrick, Yeiser and Associates Huntington Beach, CA, (April 24, 2008)
Life after TUNL: Education, Adaptability, Perspiration and Luck
27. Andreas Piepke, University of Alabama at Tuscaloosa, (May 15, 2008)
Status of the EXO Double Beta Decay Experiment
28. Alejandro Garcia, University of Washington, (May 23, 2008)
Searches for Scalar Currents and Determination of Isospin Breaking in
32
Ar decay
29. David McKee, University of Alabama, (July 18, 2008)
Kamland: 210 Po Background and Results
30. C.C. Jewett, Lawrence Berkeley National Laboratory, (July 24, 2008)
Neutron Beam Development at Lawrence Berkeley National Laboratory
31. Peter Mohr, Diakonie-Klinikum, Schwabisch Hall, Germany (July 25, 2008)
K-Isomers in Odd-Odd Nuclei on the s-Process Path: 176 Lu, 180 Ta, and 186 Re
32. William Weintraub, Florida State University (August 1, 2008)
Analyzing Powers for the 12 C( 7 Li,t/d/p) Reactions
202
Appendices
Advances in Physics Lectures and Seminars
1. Richard Prior, North Georgia College and State University (June 3, 2008)
Nuclear Reactions
2. Mohmmad Ahmed, Duke University (June 5, 2008)
Nuclear Electronics
3. Mohammad Ahmed, Duke University (June 5, 2008)
Nuclear Forces
4. Paul Huffman, North Carolina State University, (June 12, 2008)
The Neutron’s 15 Minutes of Fame
5. Thomas Clegg, University of North Carolina-Chapel Hill, (June 19, 2008)
An Introduction to Electron and Ion Sourcery
6. Robert Golub, North Carolina State University, (June 26, 2008)
Conservation laws
7. Reyco Henning, University of North Carolina-Chapel Hill, (July 10, 2008)
The Dark Matter Puzzle and Experimental Solutions
8. Haiyan Gao, Duke University, (July 17, 2008)
The Structure of the Nucleon Through an Electromagnetic Microscope
A.4
Appendices
203
Professional Service Activities
Advisory/Fellowship/Review Committees
1. Nominating Committee, Division of Particles and Fields of the American Physical Society
(APS), T.B. Clegg
2. Chair 2008, JSA/Jefferson Lab Graduate Fellowship Selection Committee, T.B. Clegg
3. Chair 2008, JSA/Jefferson Lab Sabbatical Fellowship Selection Committee, T.B. Clegg
4. Chair and Chair of the Executive Committee, Southeastern Section of the American Physical
Society, 2007, T.B. Clegg
5. Past-Chair and Member of the Executive Committee, Southeastern Section of the American
Physical Society, 2008, T.B. Clegg
6. International Advisory Committee, MENU07, the 11th International Symposium on MesonNucleon Physics and the Structure of the Nucleon, IKP, Forschungzentrum Juelich, Germany,
Sep 10 - 14, 2007, H. Gao.
7. International Advisory Committee, International Spin Physics Symposia, January 1, 2007 December 31, 2010. H. Gao
8. International Advisory Committee, International Conference on Nuclear Physics at Storage
Rings, September 14 -18, 2008, Lanzhou, China, H. Gao.
9. Member, Organizing Committee, SPIN08, Charlottesville, VA, Oct. 2008. H. Gao
10. Member, Communication Committee, Overseas Chinese Physicist Association, January 1, 2006
- December 31, 2007; H. Gao
11. Secretary, Overseas Chinese Physicist Association, January 1, 2007 - December 31, 2008. H.
Gao
12. Member, Scientific Program Committee, Fourth Asia-Pacific Conference on Few-Body Problems in Physics 2008 (APFB08), Depok, Indonesia, from August 19 - 23, 2008, H. Gao
13. Member, International Advisor Committee, NSTAR-2009, Beijing, China, April 2009, H. Gao
14. Member, Hall C Steering Committee, Jefferson Lab, Jan. 1, 2007 - Dec. 31, 2008. H. Gao
15. Member, National Research Council: Panel on Neutron Research (2008) C. Gould
16. Co-Chair, 7th Conference on K-12 Outreach from University Science Departments, RTP, NC,
April 8-10, 2007, D. Haase.
17. Chair-Elect/Chair, Forum on Education of the American Physical Society, D. Haase.
18. Vice-Chair/Chair, Southeastern Section of the American Physical Society, D. Haase.
19. Member, Committee on Education of the American Physical Society, D. Haase.
20. Member, AIP/APS/AAPT Task Force on Physics Teacher Preparation, D. Haase.
21. Chair, Fellowship Committee of the APS Forum on Education, D. Haase.
22. Majorana Senior Advisory Committee, C.R. Howell
23. 2008 Mellon Mays Undergraduate Fellowship Program Dissertation Grant Selection Panel,
C.R. Howell
204
Appendices
24. Nuclear Science Advisory Committee Long-range Plan Writing Group, March - December
2007, C.R. Howell
25. Member of the NSF Site Visit Panel for Review of the National Superconducting Cyclotron
Laboratory, April 16 – 17, 2008, C.R. Howell
26. Member, Executive Committee for the SNS Fundamental Neutron Physics Beam Line Instrument Development Team, 2002-2008, P. Huffman
27. Member, TRIUMF Experimental Evaluation Committee, (2005-2008) C. Iliadis
28. Member, Program Advisory Committee for Fundamental Physics for Los Alamos Neutron
Science Center (LANSCE), G.E. Mitchell
29. Past-Chair, American Physical Society Topical Group on Few-Body Systems and Multiparticle
Dynamics, W. Tornow
30. NNSA Fellowship Committee, W. Tornow
31. Member Executive Committee of KamLAND Collaboration, W. Tornow
32. Member Executive Committee of Majorana Collaboration, W. Tornow
33. Member, Nominating Committee, DNP 2007, A.R. Young
Appendices
205
Conferences, APS Meetings and Workshops
1. Program Committe, 12th International Workshop on Polarized Ion Sources, Targets, and Polarimetry, Brookhaven National Laboratory, Upton, NY, September 10 - 14, 2007, T.B. Clegg
2. Co-Chair, International Conference on Physics Education and Frontier Physics Research - 6th
Joint Meeting of the Chinese Physicists Worldwide, August 2-7, 2009, Lanzhou, China, H.
Gao
3. Co-chair, Workshop on Soft Photons and Light Nuclei, H. Gao
4. Member, APS Panel on Public Affairs (POPA), Jan. 1, 2007 - Dec. 31, 2009. H. Gao
5. Program Committee, March, 2007, APS Meeting in Denver, CO, D. Haase
6. Program Committee, April, 2007, APS Meeting in Jacksonville, FL, D. Haase
7. Program Chair for the 2007 meeting of the Southeastern Section of the APS, at Nashville, TN,
D. Haase
8. International Advisory Committe, 16th International Seminar on Interaction of Neutrons with
Nuclei, Dubna, Russia, 06/08, G.E. Mitchell
9. International Advisory Committee, 13th Capture Gamma Ray and Related Topics Symposium,
Cologne, Germany, 08/08, G.E. Mitchell
10. International Advisory Committe Conference on Nuclear Physics and Related Topics, Ulaanbataar, Mongolia, 09/08, G.E. Mitchell
11. Member, International Advisory Committee, 20th European Conference on Few-Body Problems in Physics, Pisa, Italy, 2007, W. Tornow
12. Member, International Advisory Committee, The Fourth Asia-Pacific Conference on Few-Body
Problems in Physics, Depok, Indonesia, 2008, W. Tornow
13. Member, Scientific Program Committee, 19th International Conference on Few-Body Problems
in Physics, Bonn, Germany, 2009, W. Tornow
14. Co-organizer, Workshop on Baryon and Lepton Number Violation, Lawrence Berkeley Laboratory, September 2007, A.R. Young
206
Appendices
Other Service
1. Field Editor for Experimental Physics of Springer Verlag Journal “Few-Body Systems”,
W. Tornow
2. Associate Editor for The European Physical Journal A, H. Gao (June 1, 2007 – May 31, 2010)
3. Editorial Board, Research Letters in Physics (2007), A. Champagne
4. Visiting professor at Universite Catholique de Louvain, Louvain-la-Neuve, Belgium (July 2007),
C. Iliadis
Index
(Listed by corresponding author)
Ahmed, M. W., 98, 100, 128
Akashi-Ronquest, M. C., 166
Angell, C. T., 92, 112, 168
Arnold, C. W., 104, 120, 172
Boswell, M., 174, 176
Cesaratto, J. M., 144
Chen, W., 50
Couture, A. H., 158, 160
Crowe III, B.J., 60
Daigle, S., 170
Daniels, T. V., 64
Dunham, J. D., 143
Rusev, G., 88, 114, 116
Seo, P.-N., 148
Stave, S., 72, 130, 150
Sun, C., 156
Teymurazyan, A., 152
Tonchev, A. P., 90, 108, 110, 126, 138
Tornow, W., 102
Walker, C. Kalbach, 80, 82
Weisel, G. J., 58
Westerfeldt, C. R., 142
Young, A. R., 16, 18, 28
Esterline, J. H., 66
Fallin, B., 94
Finnerty, P., 162
France III, R. H., 68, 70
Gai, M., 118, 154
Gao, H., 10, 14
Golub, R., 4, 12
Haase, D. G., 6, 8
Hammond, S.L., 124
Henshaw, S., 48, 146
Huffman, P. R., 2
Hutcheson, A., 86
Iliadis, C., 32, 36, 38, 39, 44
Imig, A., 62, 177, 178
Kelley, J. H., 84
Kidd, M. F., 22, 24
Kiser, M. R., 134, 136
Kwan, E., 26
Longland, R., 34, 122
Mitchell, G. E., 54, 56, 76, 78
Newton, J. R., 42
Palmquist, G., 164
Perdue, B. A., 106
Pooser, E. J., 40
207
DENIS II
HELIUM
SOURCE
MAGNET!
#1
MAGNET!
#2
FN TANDEM!
10 MV
10 MV
Shielded!
Source Area
Atomic Beam!
Polarized-Ion!
Source
MINI TANDEM
Enge Split-Pole!
Spectrometer
HIGH VOLTAGE
CHAMBER
BEAM TRANSPORT
MAGNET!
#3
4 Meter Track
TUNL Staff Shop
3He!
Target
Control
Room
Computer
Gas Jet !
Target
Service
Room
Elevator
NCSU
Cryostat
Mechanical Room
Stair!
Well
Toilet
Tandem!
Maintenance!
Shop
TU NL/8 0 1
Work Areas
Polarized!
Target II
Time-of-Flight
Area
6 Meter
Track

Triangle Universities Nuclear Laboratory

Transcription

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