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Study of plastic scintillator quenching factors

Study of plastic scintillator quenching factors. Lea Reichhart, IOP Glasgow, April 2011. www.amcrys-h.com. 1/17. Quenching factor. What is quenching? Difference in light yield output between nuclear recoils and electron recoils. Energy dependent! Theoretical/semi-empirical approaches:

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Study of plastic scintillator quenching factors

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  1. Study of plastic scintillator quenching factors Lea Reichhart, IOP Glasgow, April 2011 www.amcrys-h.com 1/17

  2. 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

  3. Motivation • Important in situations of low energy neutron detection • Extremely limited data below 1 MeV nuclear recoil energy [2] D.L. Smith, R.G. Polk, T.G. Miller, Nucl. Instr. And Meth. 64 (1968) 157-166. [3] G.V., N.R .Kolb, R.E. O’RiellyPywell, Nucl. Instr. And Meth. In Phys. Res. A 368 (1996) 745-749. [1] V.I. Tretyak, Astroparticle Phys. 33 (2010) 40-53. 3/17

  4. Measurements/Method/Simulation • UPS-923 A Polystyrene (C8H8) based plastic scintillator • 100 cm long, 15 cm thick parallelepiped • PMT model 9302KB from ETEL AmBe/252Cf sources Scintillator bar • Low background measurement • 2850 m water-equivalent • Reduction of cosmic ray muon flux by a factor of ~106 4/17

  5. Measurements/Method/Simulation Scintillator module Production of secondary optical photons, photoelectron count at photo-cathode of PMT Incl. thermal neutron scattering model <4eV  increase of neutron capture by 20% TAL = 100 cm Light yield: 7 phe/keV PMT quantum efficiency: 30% 5/17

  6. Calibration 137Cs ADC channel to photoelectron conversion with 137Cs spectrum at high k-a bias gain (1100V) on PMT 60Co Effects from electronics (after-pulsing, ion feedback, pre-amplifiers,..) visible in MAESTRO data -> more dominant at high rates 6/17

  7. Gamma-ray contamination (from neutron sources) Simulations: Const. gamma-ray spectrum 0-10 MeV attenuation factor for 14 cm of lead shielding: (2.6+0.5)*10-5  negligible contributions to background from neutron sources Experimentally: No increase above background from 60Co source Variation of lead shield by +0.5 cm does not have a significant effect on the end result – included in error 7/17

  8. Neutron spectra 252Cf Moderation through shielding AmBe Source spectra scaled – AmBe by 10-3, 252Cf by 10-2 8/17

  9. QF a constant value? AmBe Diverges at ~13 phe Capture peak 9/17

  10. QF energy dependent 252Cf 252Cf 10/17

  11. QF energy dependent 252Cf 11/17

  12. QF energy dependent Minimizing overall Chi2/ndf (2-35 phe): AmBe 1.56 252Cf 1.69 12/17

  13. Birks factor, kB • 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. 13/17

  14. 12C nuclei fraction • < 500 keV: 12C ~30% of overall • At 350 keV: 12C ~10% • towards 0 keV: 12C raises up to almost 50% Sign. lower QF values Example for pseudocumene [1] Significant contribution from carbon nuclei to nuclear recoil energy depositions at energies below 500 keV 14/17

  15. Birks factor, kB • 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 15/17

  16. Conclusions • 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 16/17

  17. Special thanks to: The ZEPLIN-III Collaboration The Boulby Team SKY Experiment 17/17

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