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.  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 3/17

4. Measurements/Method/Simulati on AmBe/ 252 Cf sources Low background measurement 2850 m water-equivalent Reduction of cosmic ray muon flux by a factor of ~10 6 Scintillator bar UPS-923 A Polystyrene (C 8 H 8 ) based plastic scintillator 100 cm long, 15 cm thick parallelepiped PMT model 9302KB from ETEL 4/17

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

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

7. Gamma-ray contamination (from neutron sources) Experimentally: No increase above background from 60 Co source 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 Variation of lead shield by +0.5 cm does not have a significant effect on the end result – included in error 7/17

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

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

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

11. QF energy dependent 11/17

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

13.  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

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

15.  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

16.  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

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