Understanding Pulse Shape Discrimination in Germanium Detectors: Diffusion Effects Barbara Understanding Pulse Shape Discrimination in Schweisshelm Germanium Detectors: Diffusion Effects Motivation Diffusion in DPG W¨ urzburg 2018 Simulation Results Conclusion Barbara Schweisshelm Max Planck Institute for Physics, Munich March 22, 2018 DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 1
Understanding Pulse Shape Gerda Experiment Discrimination in Germanium Detectors: Diffusion Effects Barbara Schweisshelm Motivation • Outstanding background Diffusion in reduction Simulation • But: some background Results Conclusion always remains • Signal and background [ Gerda Homepage] discrimination is indispensible 30 Broad Energy Germanium (BEGe) detectors → Pulse Shape • High energy resolution Discrimination (PSD) • Pronounced weighting potential → simplifies PSD DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 2
Understanding Pulse Shape Weighting Potential Discrimination in Germanium Detectors: Diffusion Effects Barbara Schweisshelm Motivation Shockley - Ramo Theroem Diffusion in Q = − q ϕ 0 ( � x ) Simulation Results Q = induced charge at Conclusion electrode q = drifting charge ϕ 0 = weighting potential DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 3
Understanding Pulse Shape Pulse Shape Discrimination in Discrimination in Germanium Detectors: Gerda BEGes Diffusion Effects Barbara • Pronounced weighting Schweisshelm potential at electrode Motivation • Distinguish single site (SSE) Diffusion in Simulation and multi site events (MSE) Results • Rejection of surface events Conclusion [adapted from arXiv: 1307.2610] Amplitude / Energy (A/E) of current signal gives information over event topology → A/E used to classify events DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 4
Understanding Pulse Shape 2D Calibration Spectrum Discrimination in Germanium Detectors: Diffusion Effects A/E spectrum for Th calibration data Barbara Schweisshelm Motivation Diffusion in Simulation Results Conclusion Double escape peak (DEP), single escape peak (SEP) and full energy peak (FEP) of 208 Tl What influence do diffusion effects have on this spectrum? → Simulation DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 5
Understanding Pulse Shape Charge Diffusion in SigGen Discrimination in Germanium Detectors: Simulation Diffusion Effects Barbara • Spherical charge clouds Schweisshelm • Holes and e − separately Motivation Diffusion in • Cloud is traveling through Simulation the detector Results → velocity depends on Conclusion field strength at each point • Charge cloud size affected by diffusion and self-repulsion SigGen code by David Radford, • Track drift of point charge open source: https://radware.phy.ornl.gov/MJ/mjd_siggen/ and cloud dimensions separately → convolution → How does the diffusion affect A/E? DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 6
Understanding Pulse Shape Effect on Simulated Pulses Discrimination in Germanium Detectors: Diffusion Effects Energy deposition at a specified position Barbara → Energy dependence of the current pulse amplitude (A)? Schweisshelm Motivation 98 Diffusion in 97 Simulation 96 Results A/E [a.u.] 95 Conclusion 94 diff on, size = 0mm 93 diff on, size = 1mm diff off, size = 0mm 92 diff off, size = 1mm 91 0 1000 2000 3000 4000 Energy [keV] • Diffusion creates a slope • Initial charge cloud size additionally decreases A/E DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 7
Understanding Pulse Shape Effect on Simulated Pulses Discrimination in Germanium Detectors: Diffusion Effects Energy deposition at a specified position Barbara → Initial charge cloud size dependence of pulse amplitude? Schweisshelm Motivation Diffusion in 97 Simulation Results A/E [a.u.] 96 Conclusion diff on, E = 10keV 95 diff on, E = 2500keV diff off, E = 10keV diff off, E = 2500keV 94 0.00 0.25 0.50 0.75 1.00 initial charge cloud size [mm] • A/E decreases with increasing charge cloud size • Diffusion leads to an additional decrease with energy DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 8
Understanding Pulse Shape E dependence of initial single site Discrimination in Germanium Detectors: cloud size Diffusion Effects Barbara Schweisshelm Motivation Diffusion in Simulation Results Conclusion • σ of hits given by Geant4 Th calibration simulation • Higher E → bigger charge cloud size → lower A/E • Study effect of different clustering sizes DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 9
Understanding Pulse Shape Electronics Model Discrimination in Germanium Detectors: Diffusion Effects Barbara 1250 120 Simulation Simulation Schweisshelm Electronics Electronics 100 1000 Motivation Charge [a.u.] Current [a.u.] 80 750 Diffusion in 60 Simulation 500 40 Results 250 20 Conclusion 0 200 400 600 800 200 400 600 800 time [ns] time [ns] • Optimization of parameters using averaged DEP events from data • Electronics smoothens the pulses DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 10
Understanding Pulse Shape Realistic Noise Discrimination in Germanium Detectors: Diffusion Effects Barbara 1250 120 Schweisshelm Simulation Simulation Noise Noise 100 1000 Motivation Charge [a.u.] Current [a.u.] 80 750 Diffusion in 60 Simulation 500 40 Results 250 20 Conclusion 0 200 400 600 800 200 400 600 800 time [ns] time [ns] • Sharp A/E distribution from simulated pulses • Extract noise from baseline events • Add noise for a more realistic distribution DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 11
Understanding Pulse Shape Energy Dependence of A/E Discrimination in Germanium Detectors: Diffusion Effects Barbara Schweisshelm Motivation Diffusion in Simulation Results Conclusion • A/E distribution with electronics and realistic noise • A determined after 50 ns moving window average filter • Slope of SSE band determined by fit DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 12
Understanding Pulse Shape Results for one BEGe Discrimination in Germanium Detectors: Simulated slopes [10 − 6 keV − 1 ] Diffusion Effects without electronics Barbara Schweisshelm clustering size diff on diff off 0 mm -8.03 ± 0.07 -8.18 ± 0.07 Motivation 0.1 mm -7.67 ± 0.07 -8.15 ± 0.08 Diffusion in Simulation 0.3 mm -7.09 ± 0.07 -7.58 ± 0.07 Results 0.5 mm -7.68 ± 0.06 -5.59 ± 0.21 Conclusion with electronics 0 mm -3.07 ± 0.07 -3.24 ± 0.06 0.1 mm -3.24 ± 0.23 -2.83 ± 0.06 0.3 mm -3.21 ± 0.07 -2.69 ± 0.06 0.5 mm -3.14 ± 0.06 -2.11 ± 0.06 • Uncertainty given by fit • Slope is already induced by event size (not diffusion) • Electronics model reduces slope DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 13
Understanding Pulse Shape Comparison of all Detectors Discrimination in Germanium Detectors: Diffusion Slopes of SSE band for all detectors in data and simulation Effects Barbara Schweisshelm -2.5 Motivation Diffusion in Simulation -5.0 6 1/keV] Results Conclusion slope [10 -7.5 -10.0 run 62 run 75 simulation simulation with electronics 0 1 2 3 4 5 7 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 30 31 32 33 34 35 Detector Channel • Simulated slope and data are same order of magnitude • Simulation shows similar trends as data DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 14
Understanding Pulse Shape Conclusion Discrimination in Germanium Detectors: Diffusion Effects Barbara • BEGe detectors → powerful event by event background Schweisshelm identification Motivation • Energy dependence of A/E seems to be mainly caused by Diffusion in Simulation event size Results • Diffusion goes in the same direction but the effect is not Conclusion significant if the event size is taken into account • Electronics model has a larger influence than diffusion • Electronics model can reproduce the trends within the strings • Pulse shape simulations help Gerda to understand A/E better DPG Fr¨ uhjahrstagung W¨ urzburg - March 22, 2018 15
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