Pulse Shape Analysiswith Segmented Germanium Detector

Xiang Liu, Max-Planck-Institut für Physik

1. Motivation2. Pulse properties3. Analysis procedure4. Some results5. Outlook

Physics of Massive Neutrinos, Blaubeuren, July 1-5, 2007

Motivation: Single-site vs. Multi-site

Blaubeuren, July 1-5, 2007 Page 2

Photon: Compton scattering multiple energy deposits

02: energy deposit locally, within 1mm.

208Tl

2614keV photon(Geant4 simulation)

02: 2 electrons

Range log(R [mm])

NIM A 570 (2007) 479-486

SSE MSE

Motivation: 3-D segmentation along z, & r

Blaubeuren, July 1-5, 2007 Page 3

GERDA Phase-II prototype detector: 18 segments (3 fold along z, 6 along ) No segment along r, technical difficultySolution Pulse Shape Analysis (PSA)

Electrons & holes drift along the applied field at ~1cm/100ns, inducing charge in electrodes. Rise part of pulse contains information about energy deposit locations.

Pulse shape properties

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Single site event (SSE):

Multiple site event (MSE):

MSE tends to have more complicated pulse structures.

Knee indicates that one charge carrier reaches electrode and stops drifting

Pulse shape analysis procedure

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SSE candidate

MSE candidate

PSA procedure:1. Collect SSE and MSE samples,

independent of pulse shapes.2. Study the PS difference,

train PSA package.3. Apply the trained PSA package to

identify signal and background.

Test stand with 18-fold Ge detector

Collecting SSE sample:

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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

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Likelihood method: pulse rise time current FWHM

Neural Network method: sampled pulses as input

Define discriminator

Neural Network PSA results

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Samples DEP 1620keV 2614keV

Fraction identified by NN as signal 89% 54% 44%

MC simulation: Fraction with R90<2mm 89% 55% 30%

Single segment events

DEP1592keV

Bi-212 1620keV

Outlook1: Pulse shape simulation

Blaubeuren, July 1-5, 2007 Page 9

Need simulation to understand: energy deposit range pulse shape

Understand second order effect: charge carrier drifting velocity depends on crystal axis, impurity concentration & non-uniformity, trapping, preamplifier bandwidth, …

Close collaboration with Majorana MC group

NIM doi:10.1016/j.nima.2007.03.035[Nucl-ex/0701004]

Outlook2: New test stand under construction

Blaubeuren, July 1-5, 2007 Page 10

Both Ge detector and source in Vacuum: photon UV laser

3-D scan of the detector.

Outlook3: application example: Mirror charge

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Mirror charge from neighboring segments gives additional information along z & .

enhance MSE identification

identify signal at segment boundaries

identify surface contamination ()

phiz

Mirror charge amplified by factor of 4

Conclusion

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Measurement with 18-fold Ge detector proves the technique and its ability for background identification. NIM doi:10.1016/j.nima.2007.03.035 [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

Backup2: NN PSA results with DEP, continued

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Fraction of SSE (MSE) as correctly identified by NN as SSE (MSE):

Backup1: NN PSA results with DEP

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Fraction of SSE:

Fraction identified by NN as SSE:

Backup2: Pulse shape analysis packages

Blaubeuren, July 1-5, 2007 Page 15

Backup3: Mirror charge

Blaubeuren, July 1-5, 2007 Page 16

Mirror charge asymmetry of neighbor segments:

NIM doi:10.1016/j.nima.2007.03.035 [Nucl-ex/0701004]