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

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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

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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%

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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

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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

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252Cf

AmBe

Moderation through shielding

Source spectra scaled – AmBe by 10-3, 252Cf by 10-2

Neutron spectra

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AmBe

Diverges at ~13 phe

QF a constant value?

Capture peak

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252Cf

QF energy dependent

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252Cf

252Cf

QF energy dependent

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Minimizing overallChi2/ndf (2-35 phe):AmBe 1.56 252Cf 1.69

QF energy dependent

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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

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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

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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

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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

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Special thanks to:

The ZEPLIN-III Collaboration

The Boulby Team

SKY Experiment

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