Pulse-shape shape analysis with a Broad-energy. Ge-detector. Marik Schönert. MPI für f r Kernphysik Heidelberg

Transcription

1 Pulse-shape shape analysis with a Broad-energy Ge-detector Marik Barnabé é Heider Dušan Budjáš Oleg Chkvorets Stefan Schönert MPI für f r Kernphysik Heidelberg

2 Outline 1. Motivation and goals 2. BEGe detector and set-up description 3. Detector performance 4. Pulse-shape shape analysis first results 5. Comparison BEGe and 18-segment coax 6. Outlook 2

3 Novel p-type p point contact HP-Ge detector for DBD search J. Collar/Majorana collab. recognized potential of p-type point contact (ppc) HP-Ge for PSA DEP 1620 FE line Performance in terms of MSE suppression / SSE acceptance comparable or even better than segmented detectors Number of electr. contacts same as standard p-type coax. detect (1 HV + 1 signal cable per detector), less than for segmented detectors As signal cables & contacts are potential source of backgrounds, achievable background level of ppc-detector potentially superior to high-segmented detectors Discussions Stefan Schönert / Canberra Olen standard BEGe detectors could have similar pulse shape performance as the ppc P.S. Barbeau, J.I. Collar and O. Tench JCAP 0709:009,2007 (Crystal mass: 475 g) BEGe ordered by MPIK through DFG/TR27 special funds end of December 07, detector delivered end of March 08 3

4 Broad-energy Ge-detector covers energy range 3 kev - 3 MeV enhanced efficiency for low-energy gammas low capacitance ( low noise) Specifications: depletion voltage 122 kev 1.33 MeV mass 4000 V 0.63 kev 1.8 kev 870 g φ 81 mm 32 mm p-type germanium n + contact p + contact 4

5 The set-up FADC Shaping amplifier HV unit Timing filter amplifier 5

6 Data acquisition layout RC-feedback preamplifier 2002CSL with cooled FET noise: (with 0 pf input C) FWHM = 570 ev risetime: (with 30 pf input C) < 20 ns Analog spectroscopy amplifier with an ADC system Struck SIS 3301 flash-adc with 14-bit resolution, 100 MHz sampling rate, digital shaping Analog timing-filter amplifier Canberra model 2111 Preamp Timing output Energy output Amplifier (no shaping) Timing-filter amplifier FADC (two channels read-out) 6

7 Detector performance 60 Co spectrum recorded by an ADC with analog spectroscopy amplifier, shaping time = 12 µs bias voltage: 3800 V Pulser resolution: 7

8 Background spectrum Count rate in 228 Ac line 1588 kev (background for 208 Tl DEP): 0.77 ± 0.18 counts / hour ( 0.5 % of recorded DEP) Counts / day / 0.16 kev 1459 kev 40 K Compton background near DEP: ~ 4% of 228 Th spectrum Compton 1765 kev 214 Bi 2204 kev 214 Bi 2614 kev 208 Tl 8

9 Preamplifier performance pulser connected directly to a digital osciloscope (resolution: 50 ps/point) pulser connected to preamp read out by osciloscope rise time: ~ 20 ns (resolution: 200 ps/point) pulser connected to preamp read out by flash-adc (resolution: 10 ns/point) 9

10 Performance with FADC Co-60 spectrum, shaping time = 10 µs (digital shaping) Pulser resolution: 10

11 typical SSE candidate Pulse-shape shape analysis raw preamplifier output: typical MSE candidate charge pulse Time [10 ns] after analog differentiation with TFA: Time [10 ns] current pulse pulse maximum Time [10 ns] Time [10 ns] TFA parameters: 10 ns integration, 10 ns differentiation 11

12 Pulse-shape shape analysis Single site event: signal generated by one cluster of charge carriers charge signal Smaller energy less steep pulse (if risetime is constant) Multiple site event = superposition of more smaller pulses maximum differentiated charge pulse (current) 12

13 Current-maximum distribution Current-maximum / energy Counts γ 2.61 MeV DEP γ 1.62 MeV SEP Energy [kev] 13

14 Current-maximum discrimination cut-profile determination from SSE dominated regions: Compton near DEP SSE dominated Current-maximum / energy DEP cut at SSE dominated 3 st. dev. fit with gaussian distribution function Current-maximum / energy γ-line 1.62 MeV MSE dominated SEP Current-maximum / energy MSE dominated Current-maximum / energy 14

15 Current-maximum discrimination Cut function interpolated from cut-values at several points Current-maximum / energy Compton continuum DEP γ 1.62 MeV SEP γ 2.61 MeV Energy [channels] 15

16 PSA discrimination results 228 Th spectrum 228 Th spectrum after PSA cut 2.61 MeV counts 15 cm DEP 1.62 MeV SEP energy [kev] 16

17 PSA discrimination results Zoom (linear scale) 228 Th spectrum 228 Th spectrum after PSA cut counts Fractions remaining after cut: DEP 91.01% ± 0.62% 1.62 MeV 13.20% ± 0.45% SEP 9.10% ± 0.29% 2.61 MeV 13.19% ± 0.06% ROI Q ββ 49.06% ± 0.40% energy [kev] 17

18 PSA discrimination results 60 Co spectrum 60 Co spectrum after PSA cut 1.17 MeV 1.33 MeV counts Fraction remaining after cut in ROI Q ββ : 1.57% ± 0.03% summation peak energy [kev] 18

19 PSA discrimination results 60 Co spectrum 60 Co spectrum after PSA cut Low energy tail caused by pile-ups Fraction of events in summation peak after cut: 0.49% ± 0.01% counts energy [kev] 19

20 PSA discrimination results Zoom ROI (linear scale) MeV Ra spectrum 226 Ra spectrum after PSA cut MeV MeV 2.12 MeV 2.45 MeV counts Fractions remaining after cut: 1.76 MeV 13.29% ± 0.13% 1.85 MeV 12.88% ± 0.38% 2.12 MeV 14.75% ± 0.42% 2.20 MeV 15.28% ± 0.21% 2.45 MeV 14.55% ± 0.21% ROI Q ββ 30.96% ± 0.55% energy [kev] 20

21 PSA discrimination results results summary: suppression factors Suppression factor = counts before cut counts after cut suppression Ra 60 Co 228 Th Suppression in γ-peaks 208 Tl SEP 206 ± Co sum-peak Suppression in ROI 2039 kev ± 56 kev: 228 Th 2.04 ± Ra 3.23 ± Co 63.6 ± Tl DEP energy [kev] 21

22 BEGe vs. 18-fold segmented coax comparison of discrimination power for 228 Th spectrum counts counts BEGe point-contact Fractions remaining after PSA cut: DEP 91.01% ± 0.62% 1.62 MeV 13.20% ± 0.45% 2.61 MeV 13.19% ± 0.06% ROI Q ββ 49.06% ± 0.40% 18-fold segmented coax Fractions remaining after combined single-segment and PSA cut: DEP 81.93% ± 2.22% 1.62 MeV 18.98% ± 0.39% 2.61 MeV 14.57% ± 0.31% ROI Q ββ 48.10% ± 1.12% (PSA data without Compton background subtraction) I. Abt, A. Caldwell, K. Kroeninger, J. Liu, X. Liu, and B. Majorovits: Nucl. Instr. Methods A 583 (2007), Eur. J. Phys. C 52 (2007) 19-27, GERDA meeting Nov

23 BEGe vs. 18-fold segmented coax comparison of discrimination power for 60 Co spectrum counts counts Zoom ROI BEGe point-contact Fractions remaining after PSA cut: 1.17 MeV 11.96% ± 0.05% 1.33 MeV 11.45% ± 0.05% 2.51 MeV 0.49% ± 0.01% ROI Q ββ 1.57% ± 0.03% Zoom ROI 18-fold segmented coax Fractions remaining after single-segment cut: 1.17 MeV 39.06% ± 0.15% 1.33 MeV 38.02% ± 0.14% 2.51 MeV 2.89% ± 0.48% ROI Q ββ 7.04% ± 1.04% Note: no PSA cut applied! I. Abt, A. Caldwell, K. Kroeninger, J. Liu, X. Liu, and B. Majorovits: Nucl. Instr. Methods A 583 (2007) 23

24 Summary / Outlook SSE/MSE discrimination with BEGe comparable to 18-fold segmented detector only first PS analysis performed based on a simple cut parameter, other PS parameters under study room for improvement! risetime DEP number of peaks risetime length current maximum / energy risetime risetime SEP current maximum / energy 24

25 Outlook (continued) Single compton-scattering measurements using coincidence with another HPGe detector (Dario) Collimator-scan of the Ge crystal to investigate pulse-shape dependance on interaction-position PSA efficiency dependance on HV (influence on chargecarrier mobility) P. S. Barbeau, J. I. Collar, O. Tench arxiv:nucl-ex/ v1 25

26 Backup slides 26

27 Coax vs. ppc pulse-shapes shapes P. S. Barbeau, J. I. Collar, O. Tench arxiv:nucl-ex/ v1 27

28 228 Th current-maximum distribution histograms 28

29 228 Th current-maximum distribution histograms Compton near ~1.2 MeV Compton near ~1.9 MeV Compton near ~2.3 MeV 29

30 228 Th current-maximum distribution (unnormalised) 30

31 60 Co current-maximum distribution with cut 31

32 60 Co current-maximum distribution with cut (zoom) 32

33 226 Ra current-maximum distribution with cut 33

34 226 Ra spectrum with cut 34

35 Table of results 226 Ra E [kev] reduction 38.74% ± 0.33% bck. red % ± 1.29% suppresion 2.58 ± % 0.18% 35.80% 1.81% % 0.20% 35.91% 1.23% % 0.13% 25.91% 1.51% % 0.39% 43.92% 1.65% % 0.42% 33.27% 1.51% % 0.21% 28.86% 1.99% % 0.21% 18.25% 2.27% Co % 0.05% 12.97% 0.59% % 0.05% 7.76% 0.80% % 0.01% 0.57% 0.15% Th % 0.25% 41.84% 1.23% % 0.06% 39.43% 1.63% % 0.15% 50.19% 1.14% % 0.62% 58.30% 1.34% % 0.45% 56.42% 0.90% % 13.19% 0.29% 0.06% 46.86% 8.97% 0.61% 3.51%