J.Pancin AT MSU 2015

Nulcear physics with active targets
Quenchers
Dynamical range
High intensity beams
Energy resolution
Electron drift
Calibrations
AT-TPC Workshop may, 16th-18th, 2015
J. Pancin
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
D2
1 neutron transfer (d,p)
 1 proton pickup (d,3He)
 inelastic scattering (d,d')
 For the same energy loss, gain up to 100 in target thickness compared to CD2 solid target


iC4H10
1 neutron pickup (p,d)
 2 neutron pickup (p,t)
 High gain, usefull for high energy recoils (positive Q-value reactions)


H2: same reactions as iC4H10


If no important gain is needed. Gain up to 50 in target thickness compared to iC4H10
CF4: 1 proton pickup (19F,20Ne)

suitable for very negative Q-value reactions (gain 7.3 MeV in Q-value compared to (d,3He)
reaction)
 Drawback: low gain
 Treatment, heavy DWBA (19F is deformed)

3He: 1 proton transfer (3He,d)  Costly, bad gain

4He: Inelastic scattering (,‘) T=0 probe, used for Isoscalar Giant Resonances excitations

…
AT-TPC Workshop 16-18 mai 2015/J. Pancin
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C.E. Demonchy et al., Nucl. Instrum. and Meth. A 583 (2007) 341
Either polyatomic pure gas or mixture with a quencher
 Various pressures (compromise between target thickness/beam
energy/reaction rate and reaction products range)
 No heavy noble gas (>He) for reaction chambers

AT-TPC Workshop 16-18 mai 2015/J. Pancin
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

Gas mixture at NTP or pure quencher at low pressure
Polyatomic gases (several vibration and rotation modes: non radiative
mode)





Absorbe the radiated photons
Limit the breakdown/increase the sparking voltage limit
Low ionization energy with high XS
Can improve the gain: penning effect
Typical quenchers: i-C4H10 , CF4...CO2, CH4
AT-TPC Workshop 16-18 mai 2015/J. Pancin
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2 complementary aspects: achievable gain/sparking limit
Paschen empirical formulae and
townsend approximation:
V
dEi
aPd

ln( pd )  b e ln( d )
e
e
 Usually more complicated systems than
parallel plates
 Gas mixture
 large size: sparking limit at 1 bar very high
 Choice of gas for active target
Too low pressure with He
Pure argon gas…without any source
Light emission in He+CF4 at 300 mbar
55Fe
AT-TPC Workshop 16-18 mai 2015/J. Pancin

Ionization models in gas with Heed program
Electron transport properties in gas with Monte Carlo simulation
(magboltz) based on electron cross sections (velocity, diffusion,
attachment, townsend…),

GARFIELD contains all this modules and permits full simulations of
gaseous detectors from ionization and field mapping to final signals

AT-TPC Workshop 16-18 mai 2015/J. Pancin
http://garfield.web.cern.ch/garfield/, R.
Veenhof et al.,
http://garfieldpp.web.cern.ch/garfieldpp/
Field map calculation using 3D FEM ( Maxwell 3D, COMSOL or
GARFIELD (neBEM))

Experiments with wide range of energy loss: dE ∝ z2/v2 (80Zn(d,p)81Zn for
instance):
 Dynamical range <200 due to
electronics
 Raether limit 106-107 e-/mm
 Gain degradation

Masking completely the beam with a metallic foil over the pads or a gating grid
(E. Pollacco et a/NIMA723(2013) )
Decrease the gain below the beam by pad biasing
 Use a tuneable mask to lower the amount of ionization electrons created by the
beam

AT-TPC Workshop 16-18 mai 2015/J. Pancin
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Example of pad biasing in AT-TPC

10Be
4He
D. Suzuki et al., NIM A 660, 64 (2011)
10Be
Low gain strips: (θ, TKE) for 10Be / low-energy 4He
4
 High gain strips every five anode pads: θ for He

AT-TPC Workshop 16-18 mai 2015/J. Pancin
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
Tuneable mask below the beam, factor 10 on dynamical range
J. Pancin et al., JINST 7, P01006 (2012)
Used for 68Ni(,’) in 2011 in MAYA (M. Vandebrouck et al.,PRL113 (2014))
AT-TPC Workshop 16-18 mai 2015/J. Pancin
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Space charge effect
in amplification gap
(overlapping avalanches)
 Reduce the collection time of Ions:
- Faster ions
- Reduce the gaps: micropattern detectors
with sub mm gaps (MPGD)

RD51 note 2009-004
Ion feed back from the amplification region:
a fraction of the positive ions produced in the
avalanches slowly drift back in the sensitive
volume and modify the electric field
MWPC: 5%, micromegas 1%...
GEM or mumegas stack <0.01% or use of ion gate

AT-TPC Workshop 16-18 mai 2015/J. Pancin
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High intensity/ionising beams (2)
 Active
targets/Nuclear physics: more ionizing particles
the ionization in the drift gap is sufficient to deform the drift electric field
dE
   dx
WVi d
eI
Charge in C/m3 in the drift gap
 Approved
experiment at GANIL: Angular distribution of fission fragment in transfer-induced
fission using MAYA
use a 106 Hz 238U beam @ 6A MeV in isobutane
 Energy deposit ~ 1 PeV/s
 Primary ions electric field: ~ 80 V/cm compared to drift field ~ 150V/cm
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High intensity/ionising beams (3)
 Improved incoming beam intensity / heavy-Z beams
C.Rodriguez-Tajes et al,NIMA768(2014)
 Use a “field cage”
AT-TPC Workshop 16-18 mai 2015/J. Pancin
TACTIC: another mask type
K.A.Chipps et al. Proc. of Sci. 261 (2010)
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Several statistical processes:

Energy loss: landau distribution

and Ionization:

F.wi
E
F: Fano factor (00.5 to 0.4 )
Gain fluctuation: for high gain; variance of a polya dist. given by factor b
(0.5)
Electronical noise
 E   N   M   el 
          
 E   N   M  M 
2
2
2
(Q/Q)2=W(F+b)/E
Ionization chamber : no b…
AT-TPC Workshop 16-18 mai 2015/J. Pancin
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RESOLUTION %

R  2.35
TOTAL
AVALANCHE
NOISE
IONIZATION
GAIN
40%
• Best RMS for a gain
between 3.103 & 6.103
• Degradation increase in inverse
proportion to the quencher
35%
30%
Argon/Isobutane mixture
Iso : 1%
Iso : 2%
Iso : 3%
Iso : 4%
Iso : 5%
[email protected]
RMS
25%
20%
15%
10%
5%
100
1000
10000
100000
1000000
Gain
AT-TPC Workshop 16-18 mai 2015/J. Pancin
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J. Pancin et al, Nucl. Instrum. and Meth. A 735(2014)
 Angular resolution measurements done in
2013 using a pad plane with 22 mm2 pixels
 Trace reconstruction based on the pad
individual charge (T. Roger et al, NIMA638(2011))
 The energy resolution on the pads has a
strong influence.
AT-TPC Workshop 16-18 mai 2015/J. Pancin
E~30 keV
iC4H10 at 50 mbar
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3 α source in Ar+CF4(2%) at 1.1 bar
AT-TPC Workshop 16-18 mai 2015/J. Pancin
 Important variation of the cross section versus energy
 Strong influence of the mixture (noble gas+polyatomic molecule)
 Ramsauer effect
CO2
σT
At low electric fields: symetrical diffusion
At moderate to high fields: longitudinal
diffusion reduced
Transverse diffusion:
 In
E Field
L
Drift
Longitudinal diffusion:
DC the dispersive factor ≈ longitudinal diffusion
In TPC, the dispersive factor ≈ transverse diffusion (center of gravity of charge
induced on pad rows)

 In Active Targets:
- shaping time on the pads suppress longitudinal diffusion effect
- Transversal diffusion madatory when MPGD are used
High velocities to reduce time offset
between nuclei and electron signals
But low velocities to guarantee good
vertical angular resolution
From 10 to 2 cm diffusion
length: 25% degradation of
angular resolution was
observed
AT-TPC Workshop 16-18 mai 2015/J. Pancin
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
Gain PPAC/mumegas G = exp(d), where the Townsend coefficient a
increases with E field
Strong dependance of micromegas response on the gap thickness
 Calibration mandatory for good energy resolution

AT-TPC Workshop 16-18 mai 2015/J. Pancin
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•
•

10% gap variation can lead to more than 50% gain variation…
AT-TPC Workshop 16-18 mai 2015/J. Pancin
Calibration method 1:
1. Pulser calibration for gap variation estimation
2. Gain correction using garfield/magboltz
3. Real data correction
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AT-TPC Workshop 16-18 mai 2015/J. Pancin
Calibration method 2:
- Use of a 2D calibration table with a 55Fe source in Ar mixture
- Gap calculation using Garfield
- Gain calculation for other gas using Garfield
- Real data correction
55Fe
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AT-TPC Workshop 16-18 mai 2015/J. Pancin
H.J. Hilke et al, NIMA252(1986)
Fiel cage homogeneity to guarantee a perfect
reconstruction of the traces:
 Rectangular field cage in ACTAR TPC
 3D Simulations done with SIMION
 Check with alpha source difficult (straggling, long…)
Why not using a laser?
Impurities ionization (like aromatic
hydrocarbons): low ionization potential and
can be added in the gas if needed.
Test performed with the laser of GISELE at
GANIL (Oct 2014)
Laser pulses of about 150 mW
Angular resolution (H) close to 0.1°
Only 1 problem…safety…
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AT-TPC Workshop 16-18 mai 2015/J. Pancin
Comparison between measurements and
ACTARsim:
 Better results from simulations
 Straggling under-estimated in
SRIM/LISE
 Improved resolution without
straggling
 Can it be taken in analysis method?
 Would improve the resolutions?
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
Main nuclei imposed by the physics
Good choice of quencher can help: gain, drift speed,
high counting rate…
GARFIELD/MAGBOLTZ is a good tool to estimate the
gas performances
Some (easy) solutions exist for high dynamics and high
counting rate purposes
Calibrations mandatory
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Any questions?
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AT-TPC Workshop 16-18 mai 2015/J. Pancin
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