Neutron Activation of 76 Ge Georg Meierhofer people involved: P. Grabmayr Kepler Center for Astro and Particle Physics J. Jochum University Tübingen P. Kudejova L. Canella J. Jolie IKP, Universität zu Köln
Outline � Introduction � Neutron capture and decay processes � Background by neutron capture on 76 Ge � Measurements with cold neutrons � Cross section of the 74 Ge(n, � ) and 76 Ge(n, � ) reactions � Prompt � -ray spectrum in 75 Ge and 77 Ge � Summary Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Background in G ERDA 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 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Background in G ERDA 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 Fast neutrons produced by cosmic muons can propagate through the water Sources: tank and LAr to the Ge-diodes. There they can be captured by a 74 Ge or 76 Ge Natural activity of rock nucleus. Muons and neutrons Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Neutron Capture by 76 Ge 6072 1/2+ after neutron capture 159 1/2- IT 0 7/2+ E (keV) J π 77 Ge β - Half-life times 215 3/2- 77m Ge: t 1/2 = 52.9 s unstable, decay 0 3/2- to 77 Se 77 Ge: t 1/2 = 11.3 h 77 As E (keV) J π Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Neutron Capture by 76 Ge 6072 1/2+ after neutron capture prompt � -cascade E max = 5911 keV 159 1/2- IT 0 7/2+ E (keV) J π 77 Ge β - Half-life times 215 3/2- 77m Ge: t 1/2 = 52.9 s 0 3/2- 77 Ge: t 1/2 = 11.3 h 77 As E (keV) J π Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Neutron Capture by 76 Ge 6072 1/2+ after neutron capture prompt � -cascade E max = 5911 keV 159 1/2- IT 0 7/2+ E (keV) J π 77 Ge β - delayed � -spectrum E max = 2862 keV continouos, mimics 0 ��� signal Half-life times 215 3/2- 77m Ge: t 1/2 = 52.9 s 0 3/2- 77 Ge: t 1/2 = 11.3 h 77 As E (keV) J π Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Neutron Capture by 76 Ge 6072 1/2+ after neutron capture prompt � -cascade E max = 5911 keV 159 1/2- IT 0 7/2+ E (keV) J π 77 Ge β - delayed � -spectrum E max = 2862 keV continouos, mimics 0 ��� signal delayed � -rays E max = 2353 keV Half-life times 215 3/2- 77m Ge: t 1/2 = 52.9 s 0 3/2- 77 Ge: t 1/2 = 11.3 h 77 As E (keV) J π Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Neutron Capture by 76 Ge 6072 1/2+ after neutron capture prompt � -cascade E max = 5911 keV 159 1/2- IT 0 7/2+ E (keV) J π 77 Ge β - delayed � -spectrum E max = 2862 keV continouos, mimics 0 ��� signal delayed � -rays 34% of all activated nuclei decay from the E max = 2353 keV Half-life times isomeric state to the 215 3/2- ground state of 77 As 77m Ge: t 1/2 = 52.9 s without emitting � - 0 3/2- 77 Ge: t 1/2 = 11.3 h 77 As rays. E (keV) J π Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Neutron Capture by 76 Ge In GSTR-06-012 Luciano discussed this problem: Production rate: 0.5 – 1 nuclei/kg/y (LAr) Counts in ROI due to � -particles 8 x 10 -5 counts/keV/decay (can be reduced by factor of 3 by anti-coincidence). 77 Ge: 77m Ge: 2.1 x 10 -4 counts/keV/decay (small reduction due to direct transition to ground state). Rejection strategy for � -particles from 77m Ge: t 1/2 ( 77m Ge)=52.9s � dead time 4min ( � dec = 0.96) 1. Trigger on muon veto (rate: 2.5 per min.). not feasible 2. Trigger on muon veto & prompt gamma-rays (after neutron capture) in HPGe (9 events/day). � = � mv x � Ge x � dec favoured � = 0.95 x 0.56 x 0.96 = 0.51 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Prompt transitions in 77 Ge 3 5 9 2 3 8 4 1 4 2 9 0 9 4 0 8 1 0 5 5 4 4 4 5 6072 keV not in decay IAEA scheme Nuclear Data Services 2064 keV E [keV] E [keV] 1880 keV 196 862 1251 431 808 1903 1248 keV 851 3895 1021 keV 4514 5420 619 keV Only 15% of the 9 159 keV 5 emitted energy known 1 0 keV Nuclear Data Sheets 81 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
PGAA @ FRM II ( P rompt G amma-ray A ctivation A nalysis) Beam ~3 x 10 9 n th /(cm 2 s 1 ) < � n > = 6.7 Å (cold) <E n > = 1.83 meV Detectors 2 HPGe with Compton suppresion Li/Cd/Pb shielding Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Thermal n-capture cross section 76 Ge target was activated together with a gold foil and after irradiation the � -rays after � -decay were measured HPGe- Compton by HPGe detectors. The cross-section Compton suppression detector suppression was calculated relative to 198 Au using known emission probabilities. lead lead decay spectrum of 77 Ge GeO 2 target enriched in 76 Ge + Au foil decay neutron beam gammas 76 Ge target 0 Au foil Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Thermal n-capture cross section 6072 1/2+ 76 Ge target was activated together with a gold foil and after irradiation the � -rays after � -decay were measured isomeric state by HPGe detectors. The cross-section � -decay: 81(2)% was calculated relative to 198 Au using 159 1/2- IT: 19( IT known emission probabilities. 0 7/2+ E (keV) J π 77 Ge β - decay spectrum of 77 Ge ( ) ( ) ( ) � ∗ ∗ ∗ A I n r r ( � ) � Ge Au, Au λ σ λ ( ) = ( ) ( ) Ge ( ) ( ) Au � ∗ ∗ ∗ A I n r r ( � ) Au Ge, Ge 367 ( ) ∗ ∗ A I n 211 ( � ) � Ge Au, Au σ = ( ) 0 , Ge 0 , Au ∗ ∗ A I n ( � ) Au Ge, Ge 215 3/2- 0 215 0 3/2- E (keV) J π 77 As Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Results 76 Ge(n, � ) cross section [mbarn] � ( 77 Ge total) � ( 77 Ge direct) � ( 77m Ge) Seren (1947): 85 ±17 Pomerance (1952): 350 ± 70 Brooksbank (1955): 300 ± 60 Metosian (1957): 76 ± 15 Metosian (1957): 87 ± 15 Lyon (1957): 43 ± 2 Lyon (1957): 6 ± 5 Lyon (1957): 137 ± 15 Wigmann (1962): 120 ± 20 Mannhart (1968): 86 ± 9 New value (2009): 68.8 ± 3.4 46.9 ± 4.7 115 ± 16 G. Meierhofer et al., EPJA 40, 61 (2009) relativly large uncertainties due to branching ratio Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Emission probabilities NDS 81 Depending on the transition used, the . cross section varies by 15%. The same effect was observed by J. Marganiec, PRC79, 065802 (2009). Very likely that the emission probabilities in the literature are not correct. Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Neutron Capture by 74 Ge after neutron capture S n = 6505 keV E max ( � delayed) = 1177 keV E max ( � delayed) = 618 keV Half-life times 75m Ge: t 1/2 = 47.7 s stable 75 Ge: t 1/2 = 82.78 h Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Results 74 Ge(n, � ) cross section [mbarn] � ( 75 Ge total) � ( 75 Ge direct) � ( 75m Ge) Seren (1947): 380 ±76 Pomerance (1952): 600 ± 60 Metosian (1957): 180 ± 40 Metosian (1957): 40 ± 8 Lyon (1960): 550 ± 55 Wigmann (1962): 200 ± 20 Mannhart (1968): 143 ± 16 Koester (1987): 400 ± 200 New value (2010): 497 ± 52 365 ± 51 130.5 ± 5.6 G. Meierhofer et al., PRC 81, 027603 (2010) relativly large uncertainties due to emission probabilities Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
Prompt � -spectra (preliminary) Enriched: 76 Ge 74 Ge 73 Ge 77 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 Further spectra: t(measurement)=54 000 s Empty target (C 2 F 4 ) Decay (enriched) Decay (depleted) Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen GERDA Meeting 1.-3. March 2010, LNGS
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