Neutron Activation of 74 Ge and 76 Ge

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1 Neutron Activation of 74 Ge and 76 Ge people involved: P. Grabmayr J. Jochum Georg Meierhofer P. Kudejova L. Canella J. Jolie IKP, Universität zu Köln Eurograd, Hallstatt

2 Outline Motivation: Neutrinoless double beta decay experiments Neutrinoless double beta decay GERDA experiment Background in GERDA Neutron capture on 74 Ge and 76 Ge How to reject background Measurements with cold neutrons Prompt γ-ray spectrum in 75 Ge and 77 Ge Cross section of the 74 Ge(n,γ) and 76 Ge(n,γ) reactions Summary Talk by Kai Freund

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

Background in GERDA Radiopurity of: Germanium detector (cosmogenic 68 Ge) Germanium detector (cosmogenic 60 Co) Germanium detector (bulk) Germanium detector (surface) Cabling Copper holder

4 Background in GERDA Radiopurity of: Germanium detector (cosmogenic 68 Ge) Germanium detector (cosmogenic 60 Co) Germanium detector (bulk) Germanium detector (surface) Cabling Copper holder Electronics Cryogenic liquid Infrastructure Sources: Natural activity of rock Muons and neutrons Muons produce neutrons close to the experiment, the neutrons can propagate undetected through the muon veto to the Ge-diodes and be captured by a 74 Ge or 76 Ge nucleus.

5 Neutron capture by 76 Ge LNGS: 1 muon/(m 2 h) MC-simulations: ~1 n-capture/(kg y) Limit from previous experiments: max. 6 0νββ-counts/y in phase I

6 Neutron capture by 76 Ge / /2-0 7/2+ E (kev) J π 77 Ge IT t1/2 = 52.9 s t1/2 = 11.3 h MC-simulations: ~1 n-capture/(kg y) β - Limit from previous experiments: max. 6 0νββ-counts/y in phase I 215 3/2-0 3/2- E (kev) J π 77 As

7 Neutron capture by 76 Ge /2+ prompt γ-cascade after neutron capture E max = 5911 kev 159 1/2-0 7/2+ E (kev) J π 77 Ge IT β /2- unstable, decay to 77 Se 0 3/2- E (kev) J π 77 As

8 Neutron capture by 76 Ge /2+ prompt γ-cascade after neutron capture E max = 5911 kev delayed β-spectrum E max = 2862 kev continouos mimics 0νββ signal 159 1/2-0 7/2+ E (kev) J π 77 Ge IT β /2- unstable, decay to 77 Se 0 3/2- E (kev) J π 77 As

9 Neutron capture by 76 Ge /2+ prompt γ-cascade after neutron capture E max = 5911 kev delayed β-spectrum E max = 2862 kev continouos mimics 0νββ signal 159 1/2-0 7/2+ E (kev) J π 77 Ge IT β - delayed γ-rays E max = 2353 kev 215 3/2- unstable, decay to 77 Se 0 3/2- E (kev) J π 77 As

10 Neutron Capture by 76 Ge /2+ γ-rays can be rejected by pulse shape analysis and/or segmentation of detectors (multi-site events). β-particles deposit their energiy in single-site events like 0νββ-decay. If β-particles occure together with γ-rays -> multi-site event -> rejection /2-0 7/2+ E (kev) J π 77 Ge IT β - after neutron capture 215 3/2- unstable, decay to 77 Se 0 3/2- E (kev) J π 77 As

11 Neutron Capture by 76 Ge /2+ γ-rays can be rejected by pulse shape analysis and/or segmentation of detectors (multi-site events). β-particles deposit their energiy in single-site events like 0νββ-decay. If β-particles occure together with γ-rays -> multi-site event -> rejection /2-0 7/2+ E (kev) J π 77 Ge IT β - after neutron capture This does not work for decays directly to the ground state. 50% of all nuclei undergo this decay! Only coincidences with prompt transitions can be used /2- unstable, decay to 77 Se 0 3/2- E (kev) J π 77 As

12 Neutron Capture by 74 Ge γ-rays can be rejected by pulse shape analysis and/or segmentation of detectors (multi-site events). after neutron capture β-particles deposit their energiy in single-site events like 0νββ-decay. If β-particles occure together with γ-rays -> multi-site event -> rejection. This does not work for decays directly to the ground state. 50% of all nuclei undergo this decay! Only coincidences with prompt transitions can be used.

13 Prompt transitions in 77 Ge kev not in decay scheme IAEA Nuclear Data Services 2064 kev 1880 kev E [kev] E [kev] 1248 kev 1021 kev kev 159 Nuclear Data Sheets kev 0 kev Only 15% of energy known

14 FRM II Beam 2.9 x 10 9 n th /(cm 2 s 1 ) <λ n > = 6.7 Å <E n > = 1.83 mev Detectors 2 HPGe with Comptonsuppresion Li/Cd/Pb shielding

15 Neutron source FRM II Power: 20 MW Moderator for neutrons: liquid D 2 (25 K) neutron guides to experiments Pictures: Neutronenquelle Heinz-Meier-Leibniz

16 Prompt γ-spectrum in 77 Ge 1e+06 "A0126.txt" u Counts Channel

17 Prompt γ-spectrum in 77 Ge 1e+06 Comparing spectra with different isotopical composition allows to determine unambiguously the transitions in 77 Ge. "A0126.txt" u 1 Counts E [kev] (preliminary) Channel

18 Coincidence CS HPGedetector CS m ~ 300 mg of enriched 76 GeO 2 Irradiation time 8 d t between detectors Lead shielding Lead shielding FWHM=20ns Neutron beam prompt gammas 76 Ge Target Au Foil prompt gammas t Lead shielding Lead shielding Time difference is used to distinguish between random and true coincidences. CS HPGe-detector

19 Decay scheme in 77 Ge (preliminary) new transitions transitions known from β-decay of 77 Ga

20 Decay scheme in 77 Ge (preliminary) new transitions transitions known from β-decay of 77 Ga

Cross-section 76 Ge target was activated together with a gold foil and after irradiation the γ-rays after β-decay were measured by HPGe detectors.

21 Cross-section 76 Ge target was activated together with a gold foil and after irradiation the γ-rays after β-decay were measured by HPGe detectors. The total cross-section was calculated relative to 198 Au using the emission probabilities. Possible because of 1 / v - l a w f o r c o l d n e u t r o n s. decay spectrum of 77 Ge Compton suppression lead neutron beam HPGedetector decay gammas 76 Ge target Au foil Compton suppression lead

22 Cross-section 76 Ge target was activated together with a gold foil and after irradiation the γ-rays after β-decay were measured by HPGe detectors. The total cross-section was calculated relative to 198 Au using the emission probabilities. Possible because of 1 / v - l a w f o r c o l d n e u t r o n s / /2-0 7/2+ E (kev) J π 77 Ge IT σ Ge A ( λ) = A σ 0, Ge Ge Au = ( I ) ( I ) ( Au,γ ) nau ( r) Φ( r) ( Ge,γ ) nge ( r) Φ( r) ( AGe I( Au,γ ) nau ) σ 0, Au ( A I n ) Au ( Ge,γ ) Ge σ ( λ) Au decay spectrum of 77 Ge β /2-0 3/2- E (kev) J π 77 As

23 Cross-section results Evaluating the data one could see that the emission probabilities given in literature are not consistent. Some transitions lead to lower crosssections than those used here. Check needed. 76 Ge σ( 77 Ge) direct σ( 77 Ge) σ( 77m Ge) 46.9 ± 4.7 mb 68.8 ± 3.4 mb 115 ± 16 mb Meierhofer et al. EPJA, 40, (2009)

24 Cross-section results Evaluating the data one could see that the emission probabilities given in literature are not consistent. Some transitions lead to lower crosssections than those used here. Check needed. 76 Ge 74 Ge (preliminary) σ( 77 Ge) direct 46.9 ± 4.7 mb σ( 75 Ge) direct 369 ± 52 mb σ( 77 Ge) 68.8 ± 3.4 mb σ( 75 Ge) 500 ± 53 mb σ( 77m Ge) 115 ± 16 mb σ( 75m Ge) ± 6.8 mb

25 Summary The knowledge of the prompt spectrum after neutron capture by 76 Ge is important for background analysis in GERDA. The observation of a prompt cascade in GERDA would allow to veto the delayed electrons offline from the β-decay of 77m Ge Measurement of the prompt spectrum using PGAA To predict the background contribution by neutron capture in GERDA quantitatively the capture cross sections have to be known well. Measurement using the PGAA facility

26 Pulse shape analysis PhD thesis D. Budjas, Heidelberg 2009

27 Analysis Detector II Time information Detector I

28 Cross Section σ Ge ( λ) = A A Ge Au ( I ) ( I ) ( Au, γ) nau( r) Φ( r) ( Ge, γ) nge ( r) Φ( r) σ Au ( λ) σ 0, Ge = ( AGe I( Au, γ) nau) σ0, Au ( A I n ) Au ( Ge, γ) Ge

29 What can we lern from 0νββ? If 0νββ is observed: Hd-M best value PLB586(2004)198 m 3 m 3 m 2 m² 12 (inverse) m² 23 m² 12 m 2 m 1 m ν ² in ev m 1 m² 23 (normal) m² 12 = (7.92±0.07) 10-5 ev² (solar ν) m² 23 = (2.6±0.4) 10-3 ev² (atm. ν) Mass hierachy (degenerate, inverted or normal) F.Feruglio, A. Strumia, F. Vissani, NPB 637

30 Sensitivity Background Phase I Phase II Phase I: 10-2 cts/(kev kg y) Phase II: 10-3 cts/(kev kg y) Hd-M best value PLB586(2004)198 phase II Phase III (inverse) phase I KKDC claim (normal) F. Feruglio, A. Strumia, F.Vissani, NPB 637

31 Coincidence Detector II Detector I

Cross Section Target: enriched target (87% 76 Ge) Ø = 12 mm d = ~2 mm

77 Ge g ) ~60/120/180 s ( 77 Ge m ) Measurement: ~15 h ( 77 Ge g )

beam HPGe- Detector Decay gammas 76 Ge Target Au Foil Compton

34 Cross Section Target: enriched target (87% 76 Ge) Ø = 12 mm d = ~2 mm m = 470/500 g Flux determination: Au foil Irradiation: ~1200/1800 s ( 77 Ge g ) ~60/120/180 s ( 77 Ge m ) Measurement: ~15 h ( 77 Ge g ) ~120/180 s ( 77 Ge m ) Compton suppression Lead shielding Neutron beam HPGe- Detector Decay gammas 76 Ge Target Au Foil Compton suppression Lead shielding Sketch not to scale Shielding against neutrons not shown

35 0νββ-Decay segmented detector 0νββ event, energy is deposited in a very small volume due to the short range of electrons

36 Neutron Capture in GERDA ~1 n-capture/(kg y) (MC simulation) Possible background in the region of interest (2039 kev) Source γ-ray Background in ROI Rejection method Prompt Gamma Rays Peak? Compton scattering β-decay of 77 Ge β-decay of 77 Ge m β-decay of 77 As Peak ( kev) Compton scattering (E max = kev) (E max = kev) (E max =682.9 kev) γ segmented detector

37 Neutron Capture in GERDA ~1 n-capture/(kg y) (MC simulation) Possible background in the region of interest (2039 kev) Source γ-ray Background in ROI Rejection method Prompt Gamma Rays Peak? Compton scattering multisite events β-decay of 77 Ge β-decay of 77 Ge m β-decay of 77 As Peak ( kev) Compton scattering (E max = kev) (E max = kev) (E max =682.9 kev) multisite events γ segmented detector

38 Neutron Capture in GERDA ~1 n-capture/(kg y) (MC simulation) Possible background in the region of interest (2039 kev) Source β- Background in ROI Rejection method Prompt Gamma Rays segmented detector β-decay of 77 Ge Continuous (E max = kev) β-decay of 77 Ge m Continuous (E max = kev) detection of prompt gamma rays β-decay of 77 As (E max =682.9 kev)

39 Neutron Capture in GERDA ~1 n-capture/(kg y) (MC simulation) Possible background in the region of interest (2039 kev) Source γ-ray Background in ROI Rejection method β- Background in ROI Rejection method Prompt Gamma Rays Peak? Compton scattering multisite events β-decay of 77 Ge β-decay of 77 Ge m Peak ( kev) Compton scattering (E max = kev) (E max = kev) multisite events Continuous (E max = kev) Continuous (E max = kev) detection of prompt gamma rays β-decay of 77 As (E max =682.9 kev) (E max =682.9 kev)

40 Neutron Capture in GERDA ~1 n-capture/(kg y) (MC simulation) Possible background in the region of interest (2039 kev) Source Prompt Gamma Rays β-decay of 77 Ge β-decay of 77 Ge m γ-ray Background in ROI Peak? Compton scattering Peak ( kev) Compton scattering (E max = kev) (E max = kev) Rejection method multisite events multisite events β- Background in ROI Continuous (E max = kev) Continuous Only 15% of the energy weighted intensity known (E max = kev) Rejection method detection of prompt gamma rays β-decay of 77 As (E max =682.9 kev) (E max =682.9 kev)

41 Cross-section of 76(n,γ) in the literature cross section [mbarn] σ( 77 Ge g ) σ( 77 Ge m ) Seren (1947): 85 ±17 Pomerance (1952): 350 ± 70 Brooksbank (1955): 300 ± 60 Metosian (1957): 76 ± 15 Lyon (1957): 43 ± 2 Metosian (1957): 87 ± 15 Lyon (1957): 137 ± 15

42 Cross Section Preliminary results: NUDAT Our measurement cross section [mbarn] σ( 77 Ge g direct) σ( 77 Ge g ) σ( 77 Ge m ) using IT using β-decay 46.2 ± ± ± ± 14

43 Cross Section Preliminary results: NUDAT. Our measurement cross section [mbarn] σ( 77 Ge g direct) σ( 77 Ge g ) σ( 77 Ge m ) using IT using β-decay 46.2 ± ± ± ± 14

44 Cross Section Preliminary Results: Our measurement cross section [mbarn] σ( 77 Ge g direct) 46.0 ± 5.0 Literature cross section Lyon (1957): 6 ± 5 [mbarn] σ( 77 Ge g ) σ( 77 Ge m ) using IT using β-decay 64.3 ± ± ± 14 Seren (1947): 85 ±17 Pomerance (1952): 350 ± 70 Brooksbank (1955): 300 ± 60 Metosian (1957): 76 ± 15 Lyon (1957): 43 ± 2 Metosian (1957): 87 ± 15 Lyon (1957): 137 ± 15

45 Prompt Gamma Ray Spectrum CS HPGe- Detector CS Rate [Ereign./keV/s] Single spectra m ~ 300 mg Irradiation time > s Coincidence spectra m ~ 300 Depleted mg Enriched Abgereicherte Probe Irradiation Background Leere Probe Angereicherte time Probe 10 d Diff. Differenz (Ang. - Leer - 0,01) Coincident events ,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 73 Ge 1471,75 Lead shielding Neutron beam Lead shielding prompt gammas 76 Ge Target Au Foil prompt gammas Lead shielding Lead shielding 0, Energie [kev] CS HPGe-Detector Sketch not to scale Shielding against neutrons not shown

Prompt Gamma Ray Spectrum CS HPGe- Detector CS Lead shielding Lead shielding prompt gammas Neutron beam 76 Ge Target Au Foil prompt gammas Coincidence spectra m

46 Prompt Gamma Ray Spectrum CS HPGe- Detector CS Lead shielding Lead shielding prompt gammas Neutron beam 76 Ge Target Au Foil prompt gammas Coincidence spectra m ~ 300 mg Irradiation time 10 d Coincident events Lead shielding CS HPGe-Detector Lead shielding Sketch not to scale Shielding against neutrons not shown

Neutron Activation of 76Ge

Neutron Activation of 76Ge Georg Meierhofer people involved: P. Grabmayr J. Jochum Kepler Center for Astro and Particle Physics University Tübingen P. Kudejova L. Canella J. Jolie IKP, Universität zu Köln