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Study of plastic scintillator
quenching factorsLea Reichhart, IOP Glasgow, April 2011
www.amcrys-h.com
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Quenching factor
What is quenching?
Difference in light yield output between nuclear recoils and electron recoils.
Energy dependent!
Theoretical/semi-empirical approaches:
Lindhard factor -> energy dissipation into atomic motion or heat
Birks factor kB -> dependence on energy and stopping power dE/dr
2/17
Important in situations of low energy neutron detection
Extremely limited data below 1 MeV nuclear recoil energy
[1] V.I. Tretyak, Astroparticle Phys. 33 (2010) 40-53.
[3] G.V., N.R .Kolb, R.E. O’RiellyPywell, Nucl. Instr. And Meth. In Phys. Res. A 368 (1996) 745-749.
[2] D.L. Smith, R.G. Polk, T.G. Miller, Nucl. Instr. And Meth. 64 (1968) 157-166.
Motivation
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Measurements/Method/Simulation
AmBe/252Cf sources
•Low background measurement•2850 m water-equivalent•Reduction of cosmic ray muon flux by a factor of ~106
Scintillator bar
•UPS-923 A Polystyrene (C8H8) based plastic scintillator•100 cm long, 15 cm thick parallelepiped•PMT model 9302KB from ETEL
4/17
Measurements/Method/Simulation
Production of secondary optical photons, photoelectron count atphoto-cathode of PMT
Incl. thermal neutron scattering model <4eV increase of neutron capture by 20%
Scintillator module
TAL = 100 cmLight yield: 7 phe/keVPMT quantum efficiency: 30%
5/17
137Cs
60Co
Effects from electronics (after-pulsing, ion feedback, pre-amplifiers,..) visible in MAESTRO data-> more dominant at high rates
ADC channel to photoelectron conversion with 137Cs spectrum at high k-a bias gain (1100V) on PMT
Calibration
6/17
Gamma-ray contamination (from neutron sources)
Experimentally:No increase above background from 60Co source
Simulations:Const. gamma-ray spectrum 0-10 MeVattenuation factor for 14 cm of lead shielding: (2.6+0.5)*10-5
negligible contributions to background from neutron sources
Variation of lead shield by +0.5 cm does not have a significant effect on the end result – included in error
7/17
252Cf
AmBe
Moderation through shielding
Source spectra scaled – AmBe by 10-3, 252Cf by 10-2
Neutron spectra
8/17
AmBe
Diverges at ~13 phe
QF a constant value?
Capture peak
9/17
252Cf
QF energy dependent
10/17
252Cf
252Cf
QF energy dependent
11/17
Minimizing overallChi2/ndf (2-35 phe):AmBe 1.56 252Cf 1.69
QF energy dependent
12/17
Quenching factor only depends on the stopping power dE/dr of a specific particle in a specific material (shape of the curve)
Scaled by kB factor -> (should be) independent of particle species
[1] V.I. Tretyak, Astroparticle Phys. 33 (2010) 40-53.
Birks factor, kB
13/17
Example for pseudocumene [1]
• < 500 keV: 12C ~30% of overall• At 350 keV: 12C ~10%• towards 0 keV: 12C raises up to almost 50%
Significant contribution from carbon nuclei to nuclear recoil energy depositions at energies below 500 keV
12C nuclei fraction
Sign. lower QF values
14/17
kB factor from best fit to the data: 0.01045 g MeV-1 cm-2
Good agreement with theory above ~350 keV – below steep drop
Birks factor, kB
15/17
Constant quenching factor is only a good approximation for high recoil energies.
Energy dependent quenching factor measurements down to 100 keV.
kB factor of 0.01405 g MeV-1 cm-2 obtained for best fit to data points above 350 keV.
Measured energy dependent quenching factor falls very rapidly below 350 keV.
Contributions to the overall quenching at low energies not sufficient described by Birks model
Further investigation of low energy electron recoil efficiencies
Conclusions
16/17
Special thanks to:
The ZEPLIN-III Collaboration
The Boulby Team
SKY Experiment
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