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Pulse Shape Analysis with Segmented Germanium Detector

Pulse Shape Analysis with Segmented Germanium Detector. Xiang Liu, Max-Planck-Institut f ü r Physik. Motivation Pulse properties Analysis procedure Some results Outlook. Physics of Massive Neutrinos, Blaubeuren, July 1-5, 2007. Motivation: Single-site vs. Multi-site. Range log(R [mm]).

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Pulse Shape Analysis with Segmented Germanium Detector

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  1. Pulse Shape Analysis with Segmented Germanium Detector Xiang Liu, Max-Planck-Institut für Physik • Motivation • Pulse properties • Analysis procedure • Some results • Outlook Physics of Massive Neutrinos, Blaubeuren, July 1-5, 2007

  2. Motivation: Single-site vs. Multi-site Range log(R [mm]) SSE MSE 208Tl 2614keV photon (Geant4 simulation) NIM A 570 (2007) 479-486 Photon: Multiple Compton scattering  mean free path (@2MeV) ~ cm 02: energy deposit locally, within 1mm. 02: 2 electrons Blaubeuren, July 1-5, 2007 Page 2

  3. Motivation: 3-D segmentation along z,  & r GERDA Phase-II prototype detector: 18 segments (3 fold along z, 6 along ) No segment along r, technically impossible  Pulse Shape Analysis (PSA) Electrons & holes drift along the applied field at ~1cm/100ns, inducing charge in electrodes.  Rising part of the pulse contains information about energy deposit locations. Blaubeuren, July 1-5, 2007 Page 3

  4. Pulse shape properties Single site event (SSE): Knee indicates that one type of charge carriers reach electrode. Multiple site event (MSE): MSE tends to have more complicated pulse structures. Blaubeuren, July 1-5, 2007 Page 4

  5. Pulse shape analysis procedure SSE candidate MSE candidate Test stand with 18-fold Ge detector PSA procedure: Collect SSE and MSE-dominant samples, independent of pulse shapes. Study the PS difference, define discriminator. Use discriminator to identify signal and background. Blaubeuren, July 1-5, 2007 Page 5

  6. Collecting SSE sample: • Double escape events (DEP): • one electron and one positron • with sum energy 1592keV 2614keV • Single Compton scattering events: • single electron • with energy 2039keV Blaubeuren, July 1-5, 2007 Page 6

  7. Pulse shape analysis packages • Likelihood method: • pulse rise time 10-90% • pulse rise time 10-30% • current FWHM • current asymmetry Library method: use collected SSE sample as reference library Neural Network method: sampled pulses as input (most efficient) Define discriminator Blaubeuren, July 1-5, 2007 Page 7

  8. Neural Network PSA results DEP 1592keV Bi-212  1620keV Single segment events Blaubeuren, July 1-5, 2007 Page 8

  9. Outlook1: Pulse shape simulation • Need simulation to understand: • energy deposit range  pulse shape • Understand second order effects: • charge carrier drifting velocity • depends on crystal axis, • impurity concentration • & non-uniformity, • trapping, • preamplifier bandwidth, • … • Close collaboration with • Majorana MC group NIM A577 (2007) 574 [Nucl-ex/0701004] Blaubeuren, July 1-5, 2007 Page 9

  10. Outlook2: New test stand under construction • Both Ge detector and source in Vacuum: • photon •  • UV laser •  • 3-D scan of the detector • Detector properties • Mirror charge (next page) • Identify surface  • Identify multi-segment signals Blaubeuren, July 1-5, 2007 Page 10

  11. Outlook3: application example: Mirror charge • Mirror charge from neighboring segments gives additional information along z & . • enhance MSE identification • identify signal at segment boundaries z phi Mirror charge amplified by factor of 5 Blaubeuren, July 1-5, 2007 Page 11

  12. Conclusion Measurement with 18-fold Ge detector proves the technique and its ability for background identification. NIM A577 (2007) 574[nucl-ex/0701004], nucl-ex/0701005 First analysis proves that properly-trained pulse shape analysis package identifies most multi-site background events. arXiv:0704.3016 A new test stand is under construction at MPI Munich, where segmented Ge detector will be studied in detail with , , e sources and UV laser. With detailed simulation and mirror charge, PSA is expected to further identify photon and surface backgrounds improve signal efficiency Blaubeuren, July 1-5, 2007 Page 12

  13. Backup2: NN PSA results with DEP, continued Fraction of SSE (MSE) as correctly identified by NN as SSE (MSE): Blaubeuren, July 1-5, 2007 Page 13

  14. Backup1: NN PSA results with DEP Fraction of SSE: Fraction identified by NN as SSE: Blaubeuren, July 1-5, 2007 Page 14

  15. Backup2: Pulse shape analysis packages Blaubeuren, July 1-5, 2007 Page 15

  16. Backup3: Mirror charge Mirror charge asymmetry of neighbor segments: NIM doi:10.1016/j.nima.2007.03.035 [Nucl-ex/0701004] Blaubeuren, July 1-5, 2007 Page 16

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