Pulse Shape Analysis with Segmented Germanium Detector Xiang Liu, Max-Planck-Institut für Physik 1.Motivation 2.Pulse properties 3.Analysis procedure 4.Some results 5.Outlook Physics of Massive Neutrinos, Blaubeuren, July 1-5, 2007

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

Motivation: 3-D segmentation along z,  & r Blaubeuren, July 1-5, 2007Page 3 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.

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

Pulse shape analysis procedure Blaubeuren, July 1-5, 2007Page 5 SSE candidate MSE candidate PSA procedure: 1.Collect SSE and MSE-dominant samples, independent of pulse shapes. 2.Study the PS difference, define discriminator. 3.Use discriminator to identify signal and background. Test stand with 18-fold Ge detector

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

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

Neural Network PSA results Blaubeuren, July 1-5, 2007Page 8 SamplesDEP RoI Fraction identified by NN as signal89%54%44%81% MC simulation: Fraction with R 90 <2mm89%55%30%78% Single segment events DEP 1592keV Bi-212  1620keV

Outlook1: Pulse shape simulation Blaubeuren, July 1-5, 2007Page 9 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/ ]

Outlook2: New test stand under construction Blaubeuren, July 1-5, 2007Page 10 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

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

Conclusion Blaubeuren, July 1-5, 2007Page 12 Measurement with 18-fold Ge detector proves the technique and its ability for background identification. NIM A577 (2007) 574[nucl-ex/ ], nucl-ex/ First analysis proves that properly-trained pulse shape analysis package identifies most multi-site background events. arXiv: 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

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

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

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

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