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Prompt Gamma Activation Analysis on 76Ge

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1 Prompt Gamma Activation Analysis on 76Ge
Georg Meierhofer Kepler Center for Astro and Particle Physics Universität Tübingen

2 Motivation GERDA is a low background experiment. The expected count rate for 0νββ decay is ~6 cts/y in Phase I if the claim by KK is true. The goal is a background level of the order of 10-2 cts/(keV kg y) in Phase I and cts/(keV kg y) in Phase II. Background by neutron capture on 76Ge Measurement by PGAA Prompt gamma ray spectrum Cross section of 76Ge(n,γ) reaction Summary Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

3 0νββ-Decay segmented detector 0νββ event, energy is deposited in a
very small volume due to the short range of electrons Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

4 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 activity of rock Muons and neutrons Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

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 Sources: Natural activity of rock Muons and neutrons Muons produce neutrons close to the experiment, the neutrons can penetrate undetected through the muon veto to the 76Ge diodes and can be captured there. Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

6 Neutron Capture by 76Ge Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren Burson. Et.al. 1957

7 Background from Neutron Capture
~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 77Ge t1/2=11.3 h Peak ( keV) (Emax= keV) β-Decay of 77Gem t1/2=53 s X (Emax= keV) β-Decay of 77As t1/2=38.8 h (Emax=682.9 keV) γ segmented detector Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

8 Background from Neutron Capture
~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 77Ge t1/2=11.3 h Peak ( keV) (Emax= keV) β-Decay of 77Gem t1/2=53 s X (Emax= keV) β-Decay of 77As t1/2=38.8 h (Emax=682.9 keV) γ segmented detector Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

9 Background from Neutron Capture
~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 X β-Decay of 77Ge t1/2=11.3 h Continuous (Emax= keV) β-Decay of 77Gem t1/2=53 s (Emax= keV) detection of prompt gamma rays β-Decay of 77As t1/2=38.8 h (Emax=682.9 keV) segmented detector Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

10 Background from Neutron Capture
~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 X β-Decay of 77Ge t1/2=11.3 h Peak ( keV) (Emax= keV) Continuous (Emax= keV) β-Decay of 77Gem t1/2=53 s (Emax= keV) (Emax= keV) detection of prompt gamma rays β-Decay of 77As t1/2=38.8 h (Emax=682.9 keV) Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

11 Background from Neutron Capture
~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 X β-Decay of 77Ge t1/2=11.3 h Peak ( keV) (Emax= keV) Continuous (Emax= keV) β-Decay of 77Gem t1/2=53 s (Emax= keV) (Emax= keV) detection of prompt gamma rays β-Decay of 77As t1/2=38.8 h (Emax=682.9 keV) Only 15% of the energy weighted intensity known Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

12 Prompt Gamma Activation Analysis
Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

13 PGAA @ FRM II 7.83 x 109 n/(cm2 s1) <λn> = 6.7 Å
<E> = 1.83 meV Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

14 PGAA @ FRM II m ~ 300 mg of enriched GeO2
Irradiation time > s Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

15 Prompt Gamma Spectrum in 77Ge
Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

16 Analysis of the Spectra
depleted sample (%) enriched sample (%) 70Ge 22.1 0.0 72Ge 30.0 0.03 73Ge 8.4 0.13 74Ge 38.9 12.1 76Ge 0.6 86.9 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

17 Prompt Gamma Ray Energies
Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

18 Coincidence m ~ 300 mg of enriched GeO2 Irradiation time 10 d HPGe- CS
Detector Au Foil 76Ge Target Neutron beam prompt gammas Lead shielding CS HPGe-Detector m ~ 300 mg of enriched GeO2 Irradiation time 10 d Δt between detectors FWHM=20ns Channels Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

19 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

20 Coincidence Detector II Detector I 02.10.2008
Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

21 First look on Coincidence Data
5911 5650 5049 4824 4536 4192 4008 3893 6072 keV 2179 keV 2064 keV 1880 keV 1536 keV 1248 keV 1047 keV 1021 keV 630 keV 619 keV 504 keV 421 keV 159 keV 0 keV 1756 1903 1558 1458 1250 830 1113 1087 861 402 392 887 469 459 Known transition/level New transition/level Known from β-decay to 77Ge 504 421 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

22 Capture Cross Section of 76Ge(n,γ) in the Literature
[mbarn] σ(77Geg) Seren (1947): ±17 Pomerance (1952): 350 ± 70 Brooksbank (1955): 300 ± 60 Metosian (1957): 76 ± 15 Lyon (1957): 43 ± 2 σ(77Gem) Metosian (1957): 87 ± 15 Lyon (1957): 137 ± 15 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

23 Capture Cross Section of 76Ge(n,γ)
Because of cascades with more than 1 step, therefore weighting with energy: Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

24 Capture Cross Section of 76Ge(n,γ)
Because of cascades with more than 1 step, therefore weighting with energy: Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

25 HPGe- Detector Au Foil 76Ge Target Neutron beam Decay gammas Lead shielding Compton suppression Cross Section 76Ge target was activated and after irradiation the γ-radiation of the β-decay was measured by HPGe detectors. 12mm Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

26 Cross Section Decay spectrum of 77Ge Energy region of neutrons
Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

27 Cross Section Preliminary results: Our measurement cross section
NUDAT Our measurement cross section [mbarn] σ(77Geg direct) 46.2 ± 5.5 σ(77Geg) 64.9 ± 3.5 σ(77Gem) using IT using β-decay 98 ± 12 112 ± 14 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

28 Cross Section Preliminary results: Our measurement cross section
NUDAT . Our measurement cross section [mbarn] σ(77Geg direct) 46.2 ± 5.5 σ(77Geg) 64.9 ± 3.5 σ(77Gem) using IT using β-decay 98 ± 12 112 ± 14 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

29 Cross Section Preliminary results: Our measurement cross section
NUDAT Ng et al. Our measurement cross section [mbarn] σ(77Geg direct) 46.2 ± 5.5 σ(77Geg) 64.9 ± 3.5 σ(77Gem) using IT using β-decay 98 ± 12 112 ± 14 Ng: relative data, normalised to NUDAT (A. Ng, Phys. Rev. 176, (1968), 1329 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

30 Summary If the spectrum and the decay scheme of the prompt gamma ray cascade after neutron capture on 76Ge is known, a veto on the delayed beta decay of 77Ge can be triggert. Measurement of the prompt spectrum using PGAA To predict the background contribution by neutron capture in GERDA the capture cross section has to be known well. Measurement using the PGAA facility First results Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

31 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

32 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

33 Cross Section Target: Flux determination: Irradiation: Measurement:
HPGe- Detector Au Foil 76Ge Target Neutron beam Decay gammas Lead shielding Compton suppression Target: enriched target (87% 76Ge) Ø = 12 mm d = ~2 mm m = 470/500 g Flux determination: Au foil Irradiation: ~1200/1800 s (77Geg) ~60/120/180 s (77Gem) Measurement: ~15 h (77Geg) ~120/180 s (77Gem) Sketch not to scale Shielding against neutrons not shown Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

34 Outlook When the analysis is finished the obtained prompt gamma ray energies, their intensities and the cross sections of the 76Ge(n,γ)-reaction will be used in the MC simulations for GERDA The coincidence data will be important to trigger the veto for the suppression of the β-decay of 77Gem The data additionally will give the prompt gamma ray energies of 75Ge after n-capture Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

35 What can we lern from 0νββ?
If 0νββ is observed: Neutrino is a Majorana particle Neutrino mass Mass hierachy (degenerate, inverted or normal) m3 m3 m²12 m2 m²23 m²23 m2 m²12 m² in eV m1 m1 m²12 = (7.92±0.07)·10-5 eV² (solar ) m²23 = (2.6±0.4)·10-3 eV² (atm. ) Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

36 What can we lern from 0νββ?
Hd-M ‚best value‘ PLB586(2004)198 If 0νββ is observed: Mass hierachy (degenerate, inverted or normal) m3 m3 m²12 m2 (inverse) m²23 m²23 (normal) m2 m²12 m² in eV m1 m1 m²12 = (7.92±0.07)·10-5 eV² (solar ) m²23 = (2.6±0.4)·10-3 eV² (atm. ) F.Feruglio, A. Strumia, F. Vissani, NPB 637 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

37 GERDA: The GERmanium Detector Array
Location: LNGS, Gran Sasso, Italy Used isotope: 76Ge Source  Detector HP-Ge detector technologies well established Segmented detectors Enrichment from 7.83% to ~87% Ge chemically very clean ~100% detection efficiency Good energy resolution: FWHM ~3.3 keV at 2039 keV Rather low Qββ value (2039 keV) Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

38 Setup Bare HPGe diodes submerged directly into liquid Ar.
Shielding: LAr, H2O, no high Z-materials, 3400 m.w.e. of rock Phase I: ~18 kg of 76Ge Phase II: ~40 kg of 76Ge Phase II will use segmented detectors for background suppression. Exposure (Phase I+II): 100 kg y Phase III: 1 ton experiment GERDA & MAJORANA Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

39 Sensitivity Background Phase I: 10-2 cts/(keV kg y)
Phase II: 10-3 cts/(keV kg y) phase II Phase I Hd-M ‚best value‘ PLB586(2004)198 Phase II Phase III phase I KKDC claim (inverse) (normal) F. Feruglio, A. Strumia, F.Vissani, NPB 637 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

40 Cross Section Preliminary Results: Our measurement Literature
[mbarn] σ(77Geg direct) 46.0 ± 5.0 Lyon (1957): 6 ± 5 σ(77Geg) 64.3 ± 4.4 Seren (1947): 85 ±17 Pomerance (1952): 350 ± 70 Brooksbank (1955): 300 ± 60 Metosian (1957): 76 ± 15 Lyon (1957): 43 ± 2 σ(77Gem) using IT using β-decay 98 ± 12 112 ± 14 Metosian (1957): 87 ± 15 Lyon (1957): 137 ± 15 Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

41 Prompt Gamma Ray Spectrum
HPGe- Detector Au Foil 76Ge Target Neutron beam prompt gammas Lead shielding CS HPGe-Detector Single spectra m ~ 300 mg Irradiation time > s Coincidence spectra Irradiation time 10 d Coincident events Depleted Enriched Background Diff. Sketch not to scale Shielding against neutrons not shown Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

42 Prompt Gamma Ray Spectrum
HPGe- Detector Au Foil 76Ge Target Neutron beam prompt gammas Lead shielding CS HPGe-Detector Coincidence spectra m ~ 300 mg Irradiation time 10 d Coincident events Sketch not to scale Shielding against neutrons not shown Georg Meierhofer, Kepler Center for Astro and Particle Physics, University Tübingen Eurograd Blaubeueren

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