Background by Neutron Activation in GERDA

Transcription

1 Background by Neutron Activation in GERDA Georg Meierhofer people involved: Kepler Center for Astro and Particle Physics University Tübingen P. Grabmayr J. Jochum P. Kudejova L. Canella J. Jolie IKP, Universität zu Köln

2 Outline Motivated by Neutrinoless double beta decay experiments GERDA) Neutron capture and decay processes on Ge Background by neutron capture on Ge Measurements with cold FRM II Cross section of the 74Ge(n,γ) and Ge(n,γ) reactions Prompt γ-ray spectrum in 75Ge and Ge Summary

3 Double beta decay Double beta decay (2νββ) can be observed if single beta decay is energetically forbidden, but the transition of two neutrons into two protons (or pp -> nn) is allowed. The nucleus emits two electrons (positrons) and two anti-neutrinos (neutrinos). (2νββ ) 2νββ was observed in 11 isotopes: 48Ca, Ge, 82Se, 96Zr, 100Mo, 116Cd, 128Te, 130Te, 150 Nd, 238U, 130Ba (β+β+) Zn βga 31 EC A= 30 β- EC As 33 Ge 32 β-β- β- EC Br Rb 37 Kr Se 34

4 GERDA: The GERmanium Detector Array + Isotope: Ge (Qββ = 2039 kev) Phase I: ~18 kg of Ge Phase II: ~40 kg of Ge + Location: LNGS, Gran Sasso, Italy + + Design: Bare HPGe detectors (~86% Ge) submerged into LAr. LAr acts as cooling liquid and γ-ray shield. Cerenkov muon veto (water tank with Ø=10 m) no high Z-materials used, 3400 m.w.e. of rock to shield cosmic radiation

5 Background in GERDA Radiopurity of: Germanium detector (cosmogenic 68Ge) Germanium detector (cosmogenic 60Co) Germanium detector (bulk) Germanium detector (surface) Cabling Copper holder Electronics Cryogenic liquid Infrastructure Background level on ROI < 10-2 cts/(kev kg y) (Phase I) < 10-3 cts/(kev kg y) (Phase II) Sources: Natural activity of rock Muons and neutrons

6 Background in GERDA Radiopurity of: Germanium detector (cosmogenic 68Ge) Germanium detector (cosmogenic 60Co) Germanium detector (bulk) Germanium detector (surface) Cabling Copper holder Electronics Cryogenic liquid Infrastructure Sources: Natural activitybyofcosmic rock muons can propagate through the water tank and LAr to the Ge-diodes. neutrons produced Muons and neutrons

7 Neutron Capture by Ge /2+ after neutron capture LNGS: 1 muon/(m h) MC-simulations: ~1 n-capture/(kg y) 2 E (kev) 1/2- IT 7/2+ Jπ Ge β- Limit from previous experiments: max. 6 0νββ-counts in phase I Half-life times m Ge: t1/2 = 52.9 s Ge: t1/2 = 11.3 h 215 3/2- unstable, 0 decay 3/2- to Se E (kev) Jπ As

8 Neutron Capture by Ge /2+ after neutron capture prompt γ-cascade Emax = 5911 kev E (kev) 1/2- IT 7/2+ Jπ Ge β- Half-life times m Ge: t1/2 = 52.9 s Ge: t1/2 = 11.3 h 215 3/2-0 3/2- E (kev) Jπ As

9 Neutron Capture by Ge /2+ after neutron capture prompt γ-cascade Emax = 5911 kev E (kev) delayed β-spectrum 1/2- IT 7/2+ Jπ Emax = 2862 kev continouos, mimics 0νββ signal Half-life times m Ge: t1/2 = 52.9 s Ge: t1/2 = 11.3 h Ge β /2-0 3/2- E (kev) Jπ As

10 Neutron Capture by Ge /2+ after neutron capture prompt γ-cascade Emax = 5911 kev E (kev) delayed β-spectrum 1/2- IT 7/2+ Jπ Emax = 2862 kev continouos, mimics 0νββ signal Ge β- delayed γ-rays Emax = 2353 kev Half-life times m Ge: t1/2 = 52.9 s Ge: t1/2 = 11.3 h 215 3/2-0 3/2- E (kev) Jπ As

11 Neutron Capture by Ge /2+ after neutron capture prompt γ-cascade Emax = 5911 kev delayed β-spectrum E (kev) 1/2- IT 7/2+ Jπ Ge Emax = 2862 kev 34% of all activated continouos, mimics 0νββ signal β- nuclei decay from the isomeric s delayed γ-rays Emax = 2353 kev 215 Background by β-particles from decay of mge: 2.1 x 10-4 counts/kev/decay 0 E (kev) Required background level Phase II: 10 counts/(kev kg y) -3 3/23/2Jπ As

12 Prompt transitions in Ge 6072 kev not in decay scheme 2064 kev 1880 kev 1248 kev 1021 kev E [kev] IAEA Nuclear Data Services E [kev] kev kev 0 kev Only 15% of the emitted energy known Nuclear Data Sheets 81

13 FRM II (Prompt Gamma-ray Activation Analysis) Beam ~3 x 109 nth/(cm2 s1) <λn> = 6.7 Å (cold) <En> = 1.83 mev Detectors 2 HPGe with Compton suppresion Li/Cd/Pb shielding

14 Thermal n-capture cross section β-decay were measured by HPGe detectors. The cross-section was calculated relative to 198Au using k HPGedetector Compton suppression Compton suppressio lead lead decay spectrum of Ge GeO2 target enriched inge + Au foil neutron beam 0 decay gammas Ge target Au foil

15 Thermal n-capture cross section /2+ β-decay were measured by HPGe detectors. The cross-section was calculated relative to 198Au using k 159 1/2-0 E (kev) 7/2+ Jπ isomeric state β-decay: 81(2)% IT: 19(2)% IT Ge βdecay spectrum of Ge σ Ge λ = AGe I Au,γ n Au r Φ r A Au I Ge,γ nge r Φ r Au σ 0,Ge = 0 AGe I Au,γ n Au A Au I Ge,γ nge 0, Au /2-0 E (kev) 3/2Jπ 215 As

16 Results Ge(n,γ) cross section [mbarn] σ(ge total) Seren (1947): 85 ±17 Pomerance (1952): 350 ± 70 Brooksbank (1955): 300 ± 60 Metosian (1957): ± 15 Lyon (1957): 43 ± 2 New value (2009): 68.8 ± 3.4 σ(ge direct) Lyon (1957): 6 ± ± 4.7 σ(mge) Metosian Lyon Wigmann Mannhart (1957): 87 ± 15 (1957): 137 ± 15 (1962): 120 ± 20 (1968): 86 ± ± 16 G. Meierhofer et al., EPJA 40, 61 (2009) relativly large uncertainties due to branching ratio

17 Results 74Ge(n,γ) cross section [mbarn] σ(75ge total) σ(75ge direct) σ(75mge) Seren (1947): 380 ± Pomerance (1952): 600 ± 60 Metosian (1957): 180 ± 40 Lyon Metosian (1957): 40 ± 8 (1960): 550 ± 55 Wigmann (1962): 200 ± 20 Mannhart (1968): 143 ± 16 Koester (1987): 400 ± 200 New value (2010): 497 ± ± ± 5.6 G. Meierhofer et al., PRC 81, 0203 (2010) relativly large uncertainties due to emission probabilities

18 Prompt γ-spectra (preliminary) Enriched: Ge 74 Ge 73 Ge (Ge (decay 75 Ge (decay) Depleted: 74 Ge 73 Ge 72 Ge 70 Ge 75 Ge (decay) Background: F, H, N, Na, C,Cd, Al, Pb t(measurement)= s Further spectra: Empty target (C2F4) Decay (enriched) Decay (depleted)

19 CS Coincidence m ~ 300 mg of enriched GeO2 Irradiation time 8 d HPGedetector Lead shielding CS Lead shielding Δt between detectors prompt gammas Neutron beam Ge Target FWHM=20ns prompt gammas Lead shielding Lead shielding t CS HPGe-detector Time difference is used to distinguish between random and true coincidences.

20 Example 5049 kev Ge Background detector A detector B

21 Decay scheme in Ge (preliminary) In total 122 transitions assigned to Ge, 75 of them placed in the decay scheme. Some transitions known from other reactions: new transitions -β-decay of Ga - Ge(13C,12C)Ge transitions known from β-dec Now 60% of the emitted energy known

22 Summary Neutron capture on Ge will produce background in GERDA (prompt cascade and delayed decay of Ge). The prompt cascade has to be well known to veto the delayed decay of Ge. The cross sections of the Ge(n,γ) and 74Ge(n,γ) reactions were measured by the activation method. Values of higher reliability obtained The prompt gamma-ray spectrum in Ge and 75Ge were measured and the level schemes reconstructed. about 60% of emitted energy found Data will be used for further MC-simulations

23 Energy weighted intensities v= Iγ * Eγ Itotal * Etotal I=100% E=1 I=100% E=0.5 I=100% E=0.5 v=1 v=1

24 Neutron Capture by Ge In GSTR Luciano discussed this problem: Production rate: nuclei/kg/y (LAr) Counts in ROI due to β-particles Ge: 8 x 10-5 counts/kev/decay (can be reduced by factor of 3 by anti-coincidence). m Ge: 2.1 x 10-4 counts/kev/decay (small reduction due to direct transition to ground state). 1. Rejection strategy for β-particles from mge: t1/2(mge)=52.9s dead time 4min (εdec = 0.96) Trigger on muon veto (rate: 2.5 per min.). 2. not feasible Trigger on muon veto & prompt gamma-rays (after neutron capture) in HPGe (9 events/day). favoured 5. ε = εmv x εge x εdec 6. ε = 0.95 x 0.56 x 0.96 = 0.51

25 Neutron Capture by 74Ge after neutron capture Sn = 6505 kev Emax(β delayed) = 11 kev Emax(γ delayed) = 618 kev Half-life times 75m Ge: t1/2 = 47.7 s 75 Ge: t1/2 = h stable

26 Emission probabilities Depending on the transition used, the cross section varies by 15%. NDS 81. The same effect was observed by J. Marganiec, PRC79, (2009). Very likely that the emission probabilities in the literature are not correct.

27 kev Depleted GeO2 70 Ge: % 72 Ge: 30.04% 73 Ge: 8.40 % 74 Ge: % Ge: 0.59 % / single spectrum // SE Enriched GeO2 70 Ge: 0.0 % 72 Ge: 0.03 % 73 Ge: 0.13 % 74 Ge: 12.1 % Ge: 86.9 %

28 kev detector A detector B detector A detector B Coincident with transition 38 kev 74/ single spectrum 74// SE Background: 2029: < 10-3 cts/(kg kev y) 2035: < 10-3 cts/(kg kev y)

29 Decay scheme in 75Ge (preliminary) new transitions transitions known from β-dec

30 FRM II (Prompt Gamma-ray Activation Analysis) Beam ~3 x 109 nth/(cm2 s1) <λn> = 6.7 Å (cold) <En> = 1.83 mev a.u. Detectors 2 HPGe with Compton suppresion Li/Cd/Pb shielding Wave length [λ]

31 Prompt Gamma-ray Activation Analysis metorite HPGedetector CS Lead CS Lead PGAA used for non-destructive determination of elemental/isotopic Neutrons composition. Less sensitive than NAA, but in principle all (also non-radioactive) isotopes (except 4He) can be detected. Lead CS prompt gammas prompt gammas Lead HPGe-detector

32 PGAA/PGAI/NT Other techniques: Prompt Gamma-ray Activation Imaging, Neutron Tomography fibula 6 th(century (replica Sensitivity depends on the cross sections of the isotopes (up to 0.1 ppm). Sensitivity high: B, Cd, Sm, Gd medium: H, Cu, Ag, Au, Na, K, low: C, N, O, Mg, Si, Sn, Pb Mn, Fe, Al, Ti

33 Abundances in depleted GeO2 For detection of Ge PGAA is not competitive because

34 Thermal n-capture cross section β-decay were measured by HPGe detectors. The cross-section was calculated relative to 198Au using k decay spectrum of Ge 0

35 Neutron Capture by Ge In GSTR Luciano discussed this problem: Production rate: nuclei/kg/y (LAr) Counts in ROI due to β-particles Ge: 8 x 10-5 counts/kev/decay (can be reduced by factor of 3 by anti-coincidence). m Ge: 2.1 x 10-4 counts/kev/decay (small reduction due to direct transition to ground state). Rejection strategy for β-particles from mge: t1/2(mge)=52.9s dead time 4min (εdec = 0.96) Trigger on muon veto (rate: 2.5 per min.). 2. not feasible Trigger on muon veto & prompt gamma-rays (after neutron capture) in HPGe (9 events/day). favoured 5. ε = εmv x εge x εdec 6. ε = 0.95 x 0.56 x 0.96 = Trigger on energy deposition of >4 MeV (above natural radioactivity) in HPGe. 9. lower efficiency than strategy 2.

36 Prompt γ-spectrum in Ge aring spectra with different isotopical composition allows to determine unambiguously the transitions in Counts E [kev] (preliminary) Channel

37 Analysis Detector II Detector I Time information

38 Cross Section σ Ge λ = AGe I Au,γ n Au r Φ r A Au I Ge,γ nge r Φ r Au AGe I Au,γ n Au σ 0,Ge = A Au I Ge,γ nge 0, Au

39 Coincidence Detector II Detector I