Results of a PSA with a BEGe Detector for the GERDA 0νββ. νββ-decay Experiment

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1 Results of a PSA with a BEGe Detector for the GERDA 0νββ νββ-decay Experiment Marik Barnabé Heider Dušan Budjáš Oleg Chkvorets Stefan Schönert Nikita Xanbekov* MPI für r Kernphysik Heidelberg now at: Laurentian University Sudbury * also: ITEP Moscow MAX-PLANCK-INSTITUTNSTITUT FÜR KERNPHYSIK

technology, substantial expertise Disfavoured by 0νββ Disfavoured by Cosmology Phase I: 18 kg existing detectors

2 GERDA (GERmanium Detector Array) Search for 0νββ decay with HPGe detectors: 86% enrichment in 76 Ge (source = detector) HPGe is extremely pure (low background) very good energy resolution (~3 kev at Q ββ ) existing technology, substantial expertise Disfavoured by 0νββ Disfavoured by Cosmology Phase I: 18 kg existing detectors background: <10-2 cts/(kev kg y) (10x improvement) Phase II: 20 kg new detectors background: <10-3 cts/(kev kg y) (100x improvement) 2

Du Duššan an Budj Budjáš áš MPIK Heidelberg GERDA

Underground Lab, Italy (3800 m w.e.) Clean room

PMTs Design focused on background reduction: bare

detector anticoincidence, passive shielding and

3 Du Duššan an Budj Budjáš áš MPIK Heidelberg GERDA design Lock Under construction at Gran Sasso Underground Lab, Italy (3800 m w.e.) Clean room LAr cryostat Ge detector array Water tank with PMTs Design focused on background reduction: bare HPGe detectors submersed in ultra-pure LAr detector anticoincidence, passive shielding and µ-veto Phase II: further active background suppression necessary 3

0νββ decay backgrounds Most important backgrounds: external: 228

and 68 Ge cosmogenically produced in detectors γ-ray emitters

and multi-site events (MSE) required for background suppression.

4 0νββ decay backgrounds Most important backgrounds: external: 228 Th and 226 Ra in materials around the detectors intrinsic: 60 Co and 68 Ge cosmogenically produced in detectors γ-ray emitters external γ ββ (SSE) (MSE) Distinguishing single-site events (SSE) and multi-site events (MSE) required for background suppression. detector segmentation signal time-structure analysis γ γ 60 Co (MSE) 4

5 Modified-electrode electrode detectors for 0νββ search Majorana collaboration: detectors with point-like contact (custom made) have excellent MSE / SSE discrimination performance avoid external background from multiple contacts also: excelent energy resolution and low-energy treshold Standard commercial detector with similar features: BEGe (Broad Energy Ge detector) from Canberra research program within GERDA collaboration P.S. Barbeau, J.I. Collar and O. Tench JCAP 0709:009,2007 (Crystal mass: 475 g) p-type germanium mass: 878 g n + contact (+HV) p + contact (read-out) 5

6 Pulse shape discrimination typical electron event raw preamplifier output: typical gamma-ray event charge pulse SSE E MSE E Time [10 ns] after differentiation: Time [10 ns] current pulse A A Time [10 ns] Time [10 ns] A / E discrimination parameter 6

7 PSD parameter distribution from SSE 228 Th SCS Single-Compton scattering event (SCS) coincidence with a second detector Double-escape event DEP from 208 Tl ( kev) good representation of ββ events 7

8 PSD results 8

9 PSD results 9

10 PSD results 10

11 PSD results summary Tl DEP 228 Th Q ββ 226 Ra (37 kbq) survival probability Th 226-Ra 226 Ra Q ββ 60 Co Q ββ 60-Co 40-K (from background) 208 Tl SEP 60 Co summmation peak 228 Th (3.5 kbq) 60 Co (2.4 kbq) 18.5 cm 70 cm Energy [kev] 11

12 PSD results summary acceptance of 0νββ decay events is (89.2 ± 0.9)% (determined by DEP measurements, validated by coincident single-compton scattering) survival probability of external background events: 228 Th (40.2 ± 1.6)% 226 Ra (20.6 ± 3.4)% survival probability of intrinsic background events: 60 Co (0.93 ± 0.08)% 68 Ge is expected to be also strongly suppresed (creates background at Q ββ via the summation of a β + energy loss and an interaction of the annihilation γ-rays => strong MSE signature) 12

13 Conclusions the PSD properties of BEGe detector allow strong suppresion of main external and intrinsic backgrouds in 0νββ decay experiments results comparable to highly segmented detectors potentially lower background coming from signal contacts compared to segmented detectors BEGe is in standard commercial production as a consequence, the GERDA collaboration has included BEGe in the R&D for Phase II 13

14 Backup slides 14

15 Energy resolution 241 Am 60 Co FWHM 1.59 kev FWHM 0.49 kev

16 áš Surface variation of charge collection collimated 241 Am scan peak position FWHM d collimated 59.5 kev γ-ray beam peak position [kev] distance from the top of crystal, d [mm] FWHM [kev] Gain variation: 0.055% Majorana PPC detector: Gain variation: 0.15% P.S. Barbeau, J.I. Collar and O. Tench JCAP 0709:009,

17 Charge collection stability 0.015% Relative peak position corrected for pulser fluctuation 0.010% 0.005% 0.000% % % % 1333 kev 1174 kev LN refilling weekends 1 st set 2 nd set Setup rearrangement % Date 17

18 Charge collection stability Counts per 3 hr kev 1333 kev Counts per 3 hr (corrected) Days since

19 e h + +HV read-out

Pulse shape analysis DEP 1592.5 kev FEP 1620.7 kev SEP 2103.5 kev FEP 2614.

20 Pulse shape analysis DEP kev FEP kev SEP kev FEP kev DEP SSE line accepted 10 2 rejected MSE region

21 Coincident Compton scattering Pb/Cu shield 23.6 cm solid angle: ~ cm - 80 cm 228 Th source (no collimator) 21

22 2.6 MeV ~43 scattering (E 1.5 MeV) Energy Dario [channels] 1.46 MeV E total = 2.6 MeV 511 kev DEP 2.6 MeV SEP Energy BEGe [channels] 22

23 Coincident backgrounds 23

24 Compton scattering events 70 scattering 40 scattering ~69 ± µs coincidence window counts / 0.5 kev (unnormalised) ~43 ± 7 E total = kev ± 6 kev α = ar cos E equivalent scattering angle α [ ] 24

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