A search for low-mass WIMPS with MALBEK

Transcription

A search for low-mass
WIMPS with MALBEK
(MAJORANA Low-Background BEGe @ KURF)
J.F. Wilkerson
Univ. of North Carolina
Oak Ridge National Laboratory
On behalf of the MAJORANA Collaboration
28 Feb. 2014
Dark Matter 2014, UCLA
1
The MAJORANA Collaboration
Black Hills State University, Spearfish, SD
Kara Keeter
Pacific Northwest National Laboratory, Richland, Washington
Jim Fast, Eric Hoppe, Richard T. Kouzes, Brian LaFerriere, John Orrell, Nicole Overman
Duke University, Durham, North Carolina , and TUNL
Matthew Busch, James Esterline, Gary Swift, Werner Tornow
Shanghai Jiaotong University, Shanghai, China
James Loach
Institute for Theoretical and Experimental Physics, Moscow, Russia
Alexander Barabash, Sergey Konovalov, Vladimir Yumatov
South Dakota School of Mines and Technology, Rapid City, South Dakota
Adam Caldwell, Cabot-Ann Christofferson, Stanley Howard,
Anne-Marie Suriano, Jared Thompson
Joint Institute for Nuclear Research, Dubna, Russia
Viktor Brudanin, Slava Egorov, K. Gusev,
Oleg Kochetov, M. Shirchenko, V. Timkin, E. Yakushev
Lawrence Berkeley National Laboratory, Berkeley, California and
the University of California - Berkeley
Nicolas Abgrall, Mark Amman, Paul Barton, Yuen-Dat Chan,
Paul Luke, Susanne Mertens, Alan Poon, Kai Vetter, Harold Yaver
Los Alamos National Laboratory, Los Alamos, New Mexico
Melissa Boswell, Steven Elliott,, Johnny Goett, Keith Rielage, Larry
Rodriguez, Harry Salazar, Wenqin Xu
North Carolina State University, Raleigh, North Carolina and TUNL
Dustin Combs, Lance Leviner, David G. Phillips II, Albert Young
Oak Ridge National Laboratory, Oak Ridge, Tennessee
Fred Bertrand, Kathy Carney, Alfredo Galindo-Uribarri,
Matthew P. Green, Monty Middlebrook, David Radford, Elisa Romero-Romero,
Robert Varner, Brandon White, Timothy Williams, Chang-Hong Yu
Osaka University, Osaka, Japan
Hiroyasu Ejiri, Ryuta Hazama, Masaharu Nomachi, Shima Tatsuji
28 Feb. 2014
Tennessee Tech University, Cookeville, Tennessee
Mary Kidd
University of Alberta, Edmonton, Alberta
Aksel Hallin
University of North Carolina, Chapel Hill, North Carolina and TUNL
Graham K. Giovanetti, Reyco Henning, Mark Howe, Jacqueline MacMullin, Sam Meijer,
Benjamin Shanks, Christopher O’Shaughnessy, Jamin Rager, Jim Trimble, Kris Vorren,
John F. Wilkerson
University of South Carolina, Columbia, South Carolina
Frank Avignone, Vince Guiseppe, David Tedeschi, Clint Wiseman
University of South Dakota, Vermillion, South Dakota
Dana Byram, Ben Jasinski, Ryan Martin, Nathan Snyder
University of Tennessee, Knoxville, Tennessee
Yuri Efremenko, Sergey Vasilyev
University of Washington, Seattle, Washington
Tom Burritt, Micah Buuck, Clara Cuesta, Jason Detwiler, Peter J. Doe, Julieta Gruszko,
Ian Guinn, Greg Harper, Jonathan Leon, David Peterson, R. G. Hamish Robertson,
Tim Van Wechel
MALBEK - MAJORANA, Dark Matter 2014, UCLA
2
The MAJORANA DEMONSTRATOR (0νββ)
Funded by DOE Office of Nuclear Physics and NSF Particle Astrophysics,
with additional contributions from international collaborators.
Goals: - Demonstrate backgrounds low enough to justify building a tonne scale experiment.
- Establish feasibility to construct & field modular arrays of Ge detectors.
- Searches for additional physics beyond the standard model.
• Located underground at 4850’ Sanford Underground Research Facility
• Background Goal in the 0νββ peak region of interest (4 keV at 2039 keV)
3 counts/ROI/t/y (after analysis cuts)
scales to 1 count/ROI/t/y for a tonne experiment
• 40-kg of Ge detectors
– 30 kg of 86% enriched 76Ge crystals
– 10 kg of natGe
– Detector Technology: P-type,
point-contact.
• 2 independent cryostats
– ultra-clean, electroformed Cu
– 20 kg of detectors per cryostat
– naturally scalable
• Compact Shield
– low-background passive Cu and Pb
shield with active muon veto
28 Feb. 2014
3
MAJORANA Detector R&D - MALBEK
MAJORANA LowBackground BEGe @
†
KURF
 Customized CANBERRA Broad Energy
Germanium Detector
 455g
Ge
 geometry optimized for charge collection and noise.
 Low-background Cu cryostat (J.I. Collar -- UC)
nat
 R&D Goals
 Study optimized point-contact geometry
 Test MJD DAQ
 Understand backgrounds over broad energy range
from sub-keV to 3 MeV (surface events, noise, slowfast, validate MJD background model)
 Explore low-E sensitivity of MJD (68Ge 0νββ
background, Dark Matter)
†
P. Finnerty et al., Nucl. Instr. and Meth. A 652, (2011) 692-695.
P. Finnerty et al. IEEE NSS-MIC, (2010) 671-673.
1450 m.w.e. at the Kimballton Underground Research Facility in Ripplemead, VA
28 Feb. 2014
4
Point Contact Detectors (PPC)
Luke et al., IEEE trans. Nucl. Sci. 36 , 926(1989); P. S. Barbeau, J. I. Collar, and O. Tench, J. Cosm. Astro. Phys. 0709 (2007).
• Ultra-low background requires PSA rejection of multi-site gamma events
• Initially considered coaxial n-type detectors with modest segmentation
• Chose P-type Point-Contact (PPC) detectors
–
–
–
–
–
No deep hole; small point-like central contact
Length is shorter than standard coaxial detector
Simple, cost-effective, low background
Localized weighting potential gives excellent multi-site rejection
Low capacitance (~ 1 pF) gives superb resolution at low energies
n+
p
p+
p
n+
p+
P-type Coaxial
28 Feb. 2014
P-type Point Contact
X
Point Contact Detectors (PPC)
Luke et al., IEEE trans. Nucl. Sci. 36 , 926(1989); P. S. Barbeau, J. I. Collar, and O. Tench, J. Cosm. Astro. Phys. 0709 (2007).
• Ultra-low background requires PSA rejection of multi-site gamma events
• Initially considered coaxial n-type detectors with modest segmentation
• Chose P-type Point-Contact (PPC) detectors
No deep hole; small point-like central contact
Length is shorter than standard coaxial detector
Simple, cost-effective, low background
Localized weighting potential gives excellent multi-site rejection
Low capacitance (~ 1 pF) gives superb resolution at low energies
b) Multi-site
300
200
200
100
100
0
0
80
60
60
40
40
20
20
0
0
200
28 Feb. 2014
400
300 a) Single-site
Current Signal
Charge Signal
–
–
–
–
–
600
1000 1400
Time (ns)
200
600
1000 1400
Time (ns)
X
0νββ Backgrounds in Ge Crystals
68Ge(EC, T½ = 270 d) → 68Ga(90% 1.9 MeV β+, T½ = 68 min):
Data from MJD R&D detector - MALBEK
28 Feb. 2014
Simulations for the Demonstrator
5
0νββ Backgrounds in Ge Crystals
68Ge(EC, T½ = 270 d) → 68Ga(90% 1.9 MeV β+, T½ = 68 min):
Data from MJD R&D detector - MALBEK
electron equiv. energy (keVee)
28 Feb. 2014
5
Using Ge to Search for light WIMPS
Fit for a 8 GeV WIMP ( 90% CL exclusion)
After cuts
CoGeNT (circa 2011)
45
Results from a Search for Light-Mass Dark
Matter with a P-type Point Contact Germanium
Detector", C. E. Aalseth et al. [CoGeNT
Collaboration], Phys. Rev. Lett. 106, 131301
(2011)
(point contact) detector.
•49.7 kg-days (150 days)
•Analysis - ML-based
exclusion calculation
30
25
20
15
10
5
0
0.5
•Background uncertainty
handled as nuisance pars:
Rolke, et al., NIM A 551 (2005) 493
28 Feb. 2014
35
Counts/keV/kg/d
•.44 kg modified BEGe
40
1
1.5
2
2.5
3
3.5
Energy (keV)
analysis from M.G. Marino, PhD Thesis
Univ. of Washington (2010)
6
Using Ge to Search for light WIMPS
Fit for a 8 GeV WIMP ( 90% CL exclusion)
After cuts
CoGeNT (circa 2011)
45
Results from a Search for Light-Mass Dark
Matter with a P-type Point Contact Germanium
Detector", C. E. Aalseth et al. [CoGeNT
Collaboration], Phys. Rev. Lett. 106, 131301
(2011)
(point contact) detector.
•49.7 kg-days (150 days)
•Analysis - ML-based
exclusion calculation
30
Do simulations based on
direct assays agree with
the observed spectum?
25
20
15
10
5
0
0.5
•Background uncertainty
handled as nuisance pars:
Rolke, et al., NIM A 551 (2005) 493
28 Feb. 2014
35
Counts/keV/kg/d
•.44 kg modified BEGe
Do we understand all
possible backgrounds
processes?
40
1
1.5
2
2.5
3
3.5
Energy (keV)
analysis from M.G. Marino, PhD Thesis
Univ. of Washington (2010)
6
MALBEK Measurement ≠ Simulations
3
Counts / 5.0 keV / 55.2 days
10
102
spectrum measured in shielding
at Kimballton
model prediction
10
1
10-1
10-2
0
28 Feb. 2014
A. Schubert,
Univ. Washington,
Thesis 2012
500
1000
1500
2000
2500
3000
Energy [keV]
7
Background model for MALBEK
MAGE / GEANT4 simulations to determine efficiencies for
contamination to deposit energy in our detectors.
ORCA/Struck data
Fit Result
232-Th Lower Chain (224-Ra -- 208-Pb) in GermaniumNat
50k CPU hours
232-Th Lower Chain (224-Ra -- 208-Pb) in StainlessSteel304
8k+ runs, 40+ contaminants, 56 components, 21 materials
232-Th Lower Chain (224-Ra -- 208-Pb) in Teflon
Based on assays and estimates of backgrounds
232-Th Upper Chain (232-Th -- 224-Ra) in Zeolite
232-Th Upper Chain (232-Th -- 224-Ra) in StainlessSteel304
238-U Lower Chain I (226-Ra -- 210-Pb) in MoxtekFET
238-U Lower Chain I (226-Ra -- 210-Pb) in StainlessSteel304
238-U Lower Chain II (210-Pb -- 206-Pb) in Brass
3
10
238-U Lower Chain II (210-Pb -- 206-Pb) in LeadAin
238-U Lower Chain II (210-Pb -- 206-Pb) in LeadMod
238-U Lower Chain II (210-Pb -- 206-Pb) in RnExposureOutsideCryostat
Counts / 5.0 keV / 55.2 days
238-U Upper Chain (238-U -- 226-Ra) in Brass
238-U Upper Chain (238-U -- 226-Ra) in CopperOFHC
102
238-U Upper Chain (238-U -- 226-Ra) in GermaniumNat
3-H in GermaniumNat
40-K in Brass
56-Co in CopperOFHC
10
60-Co in GermaniumNat
60-Co in NickelSilver
65-Zn in GermaniumNat
68-Ge in GermaniumNat
76-Ge 2vBB in GermaniumNat
1
Cosmogenic muons in KURFExperimentalHall
A. Schubert,
Univ. Washington, Thesis 2012
10-1
10-20
28 Feb. 2014
500
1000
1500
2000
2500
3000
Energy [keV]
8
Excess of
210
210Pb
Pb
210Bi
210Po
Counts / 0.5 keV / day
T1/2 = 22 years
γ 46.5 keV
T1/2 = 5 days
β 1162 keV end point
206Pb
T1/2 = 138 days
α 5.3 MeV
10
1
A. Schubert,
Univ. Washington,
Thesis 2012
-1
10
0
28 Feb. 2014
20
40
60
80
100
120
140
160
180
200
Energy [keV]
9
Possible sources of
210
Pb contamination
ancient lead shims
(< 1.3 x 10-2 Bq/kg)
ultra-low-background
tin solder
brass pins and
connectors
(< 4% lead)
inner shielding:
ancient lead bricks
(< 1.3 x 10-2 Bq/kg)
28 Feb. 2014
A. Schubert,
Univ. Washington,
Thesis 2012
10
A. Schubert,
Univ. Washington,
Thesis 2012
Brass connectors
102
10
Predicted spectral shapes
Inner lead shield
102
10
1
1
10-1
10-1
100
200
300
400
500
600
700
800
Energy [keV]
Solder
102
0
0
10
10-1
10-1
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200
300
400
500
600
700
800
Energy [keV]
300
400
500
600
700
800
Energy [keV]
600
700
800
Energy [keV]
10
1
100
200
Lead shims
102
1
0
100
0
100
200
300
400
500
11
Background model fit of spectrum
Results of fit in which normalizations
of ~90 PDFs were allowed to float
ORCA/Struck data
Fit Result
232-Th Lower Chain (224-Ra -- 208-Pb) in Zeolite
Counts / 10 keV
238-U Lower Chain I (226-Ra -- 210-Pb) in GermaniumNat
238-U Lower Chain I (226-Ra -- 210-Pb) in Zeolite
238-U Lower Chain II (210-Pb -- 206-Pb) in GermaniumNat
238-U Lower Chain II (210-Pb -- 206-Pb) in LeadPatch
lead shims
3
10
100 Bq/kg
instead of 10-2 Bq/kg
238-U Lower Chain II (210-Pb -- 206-Pb) in MoxtekFET
238-U Lower Chain II (210-Pb -- 206-Pb) in Zeolite
238-U Upper Chain (238-U -- 226-Ra) in MoxtekFET
3-H in GermaniumNat
40-K in Resistor
2
10
46-Sc in CopperOFHC
60-Co in Brass
10
χ2 / DOF = 132.8 / 115
P-value = 0.12
1
0
28 Feb. 2014
500
1000
1500
2000
2500
Energy [keV]
A. Schubert,
Univ. Washington,
Thesis 2012
X
MALBEK Spectra Before/After
with lead shims
without lead shims
P. Finnerty, UNC, Thesis 2013
28 Feb. 2014
12
Background model for MALBEK
MAGE / GEANT4 simulations to determine efficiencies for
contamination to deposit energy in our detectors
ORCA/Struck data
50k CPU hours
232-Th Lower Chain (224-Ra -- 208-Pb) in StainlessSteel304
8k+ runs, 40+ contaminants, 56 components, 21 materials
232-Th Upper Chain (232-Th -- 224-Ra) in StainlessSteel304
Fit Result
232-Th Lower Chain (224-Ra -- 208-Pb) in GermaniumNat
232-Th Lower Chain (224-Ra -- 208-Pb) in Teflon
232-Th Upper Chain (232-Th -- 224-Ra) in Zeolite
Counts / 10 keV
238-U Lower Chain I (226-Ra -- 210-Pb) in StainlessSteel304
238-U Lower Chain II (210-Pb -- 206-Pb) in Brass
238-U Lower Chain II (210-Pb -- 206-Pb) in LeadAin
238-U Lower Chain II (210-Pb -- 206-Pb) in LeadMod
χ2 / DOF = 97.2 / 114
P-value = 0.87
103
238-U Lower Chain II (210-Pb -- 206-Pb) in RnExposureOutsideCryostat
238-U Upper Chain (238-U -- 226-Ra) in Brass
238-U Upper Chain (238-U -- 226-Ra) in CopperOFHC
3-H in GermaniumNat
40-K in Brass
56-Co in CopperOFHC
102
60-Co in GermaniumNat
60-Co in NickelSilver
Cosmogenic muons in KURFExperimentalHall
10
1
0
28 Feb. 2014
500
1000
1500
2000
2500
Energy [keV]
A. Schubert,
Univ. Washington,
Thesis 2012
13
Serendipity -
210
Pb shim “source”
with lead shims
68
65
28 Feb. 2014
Zn
without lead shims (221 day data set)
Ge
68
210
Pb
65
Ge
Zn
210
Pb
14
Serendipity -
210
Pb shim “source”
with lead shims
68
65
without lead shims (221 day data set)
Ge
68
Zn
210
65
Zn
68
Pb
65
Ge
Zn
Ge
210
65
Zn
68
Pb
Ge
fast
slow
28 Feb. 2014
14
slow surface events
1000
Voltage (a.u.)
800
600
400
200
~ 20 keV
0
active volume
7000
8000
9000
10000
11000
12000
13000
Time (ns)
n+ dead layer
transition region – partial
charge collection
“Characteristics of signals originating near the lithium-diffused N+ contact of high purity germanium p-type
point contact detectors”, E. Aguayo et al. (MAJORANA Collaboration), Nucl. Instr. and Methods A 701 176
(2013).
28 Feb. 2014
15
a qualitative slow pulse diffusion model
Counts
D.C. Radford and P. Finnerty
104
103
102
0
10
20
30
40
50
60
Energy (keV)
Big first step towards understanding
physical mechanism responsible for
slow-signals in PPC detectors.
P. Finnerty, UNC, Thesis 2013
28 Feb. 2014
16
• Low-energy 80 keV γ that interacts
near the surface of the Ge detector.
• Measure drift time in Ge detector
using the 356 γ as start time.
Rise Time t10-90 (ns)
A test using γ-γ coincidence
2000
1800
103
1600
1400
1200
102
1000
800
600
10
400
200
0
0
1
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Drift Time t10_NaI-10_Ge (ns)
Drift Time t10_NaI-10_Ge (ns)
1400
1200
102
1000
800
600
10
400
200
0
0
28 Feb. 2014
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20
30
40
50
60
70
80
90
100
Energy (keV)
1
17
Lessons learned from MALBEK
• Events
interacting in the transition region near the surface can
result in slow rise time, energy degraded signals in P-type point
contact (PPC) Ge detectors.
-
Have identified a physical process that can contribute background
signals into the WIMP energy region.
•A
-
diffusion model qualitatively describes the observed behavior.
Find agreement (within about a factor of 2) for the rise times of the
degraded signals, using only measured Li thickness & hole mobilities.
• Searching
for a WIMP signal with PPC detectors requires:
a sub-keV energy threshold and good energy resolution
• low backgrounds
• efficient ID & rejection of slow pulses (contamination vs. acceptance)
• calibration & characterization of the detector (in energy & rise time)
•
• Detailed
predictive simulations based on measured assays have
proven powerful tools for understanding detector response.
• Work continues to further understand potential systematic effects
in PPC detectors.
28 Feb. 2014
18
Risetime Cuts for MALBEK
Flat cut- sacrifice some signal, while optimizing rejection
of slow backgrounds.
See G. Giovanetti, TAUP 2013
68
Ge
Prelimi
nary
this cut results in a reduced slow pulse
contribution in the signal region
55
Fe
65
Zn
68
Ge
fast
slow
slow event cut that is constant in wpar
28 Feb. 2014
19
MALBEK Spectrum - Best Fit
Example best fit MALBEK spectrum with 10 GeV WIMP mass
DataExclusion WIMP Mass: 10 GeV (Initial fit)
Prelimi
nary
12
Total fit
L lines (65Zn, 68Ge )
Flat background
WIMP signal
Counts/keV/kg/d
10
8
6
4
2
0
28 Feb. 2014
1
1.5
2
2.5
3
3.5
4
4.5
Energy (keV)
20
MALBEK Spectrum - 90% CL WIMP Mass
Example MALBEK spectrum with fit to a 10 GeV WIMP at 90% CL
DataExclusion WIMP Mass: 10 GeV (Final fit, 0.9 CL, σ : 1.2717e-41 cm )
2
Prelimi
nary
12
Total fit
L lines (65Zn, 68Ge )
Flat background
WIMP signal
Counts/keV/kg/d
10
8
6
4
2
0
28 Feb. 2014
1
1.5
2
2.5
3
3.5
4
4.5
Energy (keV)
21
Sensitivity to light WIMPS
• With
WIMP-nucleon σSI (cm2)
221 days livetime, the
MALBEK R&D detector
(0.45 kg) sees no evidence
of a WIMP signal.
- See G. Giovanetti,
TAUP 2013
10-39
CoGeNT 2013
CoGeNT 2014 ML
MALBEK constant cut
LUX 85 d
MALBEK 99% cut
10-40
MALBE
K Prelim
inary
10-41
10-42
6
28 Feb. 2014
8
10
12
WIMP mass (GeV)
14
16
18
22
Sensitivity to light WIMPS
-38
221 days livetime, the
MALBEK R&D detector
(0.45 kg) sees no evidence
of a WIMP signal.
- See G. Giovanetti,
TAUP 2013
• The
MAJORANA
DEMONSTRATOR (40 kg)
may have sensitivity to
WIMP nuclear recoils and
other non standard
model physics if
- meets background
goals
- achieves low energy
threshold specs.
28 Feb. 2014
DAMA (no ch.)
CDMSlite
CDMS II Si
SuperCDMS SNOLAB
DarkSide-50
MJD 100 kg-yr, 300 eV
-39
10
CoGeNT 2014 ML
MALBEK constant cut
SuperCDMS Soudan
LUX 85 d
MJD 100 kg-yr, 500 eV
MALBE
K Prelim
inary
-40
10
WIMP-nucleon σSI (cm2)
• With
10
10-41
10-42
-43
10
10-44
-45
10
-46
10
10
WIMP mass (GeV)
MJD simulation assumes 0.001 counts/keV/kg/day
continuum and 15 days of 3H activation time
( M.G. Marino, PhD Diss., Univ. of Washington (2010))
22

A search for low-mass WIMPS with MALBEK

Transcription

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