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Re/Os cosmochronometer: measurements of relevant Os cross sections

Re/Os cosmochronometer: measurements of relevant Os cross sections. Marita Mosconi 1 , Alberto Mengoni 2 , Michael Heil 1 , Ralf Plag 1 , Franz Käppeler 1 , Kaori Fuji 5 , Gaelle Aerts 3 , Rita Terlizzi 4 and the n_TOF collaboration.

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Re/Os cosmochronometer: measurements of relevant Os cross sections

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  1. Re/Os cosmochronometer: measurements of relevant Os cross sections Marita Mosconi1, Alberto Mengoni2, Michael Heil1, Ralf Plag1, Franz Käppeler1, Kaori Fuji5, Gaelle Aerts3, Rita Terlizzi4 and the n_TOF collaboration • 186Os, 187Os and 188Os (n,) cross sections at the n_TOF facility operating at CERN • Inelastic scattering cross section of 187Os at 3.7 MV FZK Van de Graaf 1 Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany 2 CERN, Geneva, Switzerland; IAEA, Vienna, Austria 3 CEA/Saclay, Gif-sur-Yvette, France 4 INFN Bari, Italy 5 INFN Trieste,Italy Marita Mosconi,FZK

  2. The Re/Os cosmochronometer s-process local approximation: ANA= A-1NA-1 At stellar temperatures ,187Os low-lying excited states are populated: theoretical calculations are needed to evaluate the stellar capture rate Marita Mosconi,FZK

  3. 187Os stellar rate 100.0 keV 7/2- 5/2- 75.0 keV 3/2- 74.3 keV 3/2- 9.75 keV 1/2- 187Os Theoretical calculations can use as input experimental results: the measurement of the inelastic scattering cross section for the first excited level allows to define the competition by the scattering channels under stellar conditions Thermal population at 30 keV: P(gs) = 33% P(1st) = 47% P(all others) = 20% Marita Mosconi,FZK

  4. Clock determination • temperature dependence of t1/2(187Re) (Bosch et al., Phys.Rev.Lett 77 (1996)) • 186Os, 187Os(n,) cross sections from 0.1 keV to 500 keV • evaluation of stellar cross sections including the effect of excited states (inelastic scattering cross section on 187Os ) Nuclear quantities • astration • galactic history of the 187Re enrichment Astrophysics Marita Mosconi,FZK

  5. Measurements of the (n,g) cross section at n_TOF Marita Mosconi,FZK

  6. Experimental setup at n_TOF (2003) Spallation neutron source: 20GeV protons on a lead target providing a wide range of neutron energies -ray detection: C6D6 scintillators Sample changer C6D6 C6D6 Neutron beam Pulse height weighting technique Correction of the -response by weighting function to make the detector efficiency proportional to -ray energy Neutron flux monitor: Silicon detectors viewing a thin 6LiF foil Marita Mosconi,FZK

  7. Samples measured and yield • 186Os (2 g, 79 %) • 187Os (2 g, 70 %) • 188Os (2 g, 95 %) • Al can  environmentalbackground • natPb (2 g)  in-beam gamma background • natC (0.5 g) • neutron scattering background • 197Au (1.2g) • flux normalization (using Ratynski and Macklin cross section measurements Marita Mosconi,FZK

  8. Preliminary results Maxwellian averaged cross sections at 30 keV Marita Mosconi,FZK

  9. Scattering experiment at FZK Van de Graaff Marita Mosconi,FZK

  10. Experimental method • narrow neutron distributions for En≥ 60 keV • Gaussian neutron energy distribution centered at 30 keV θ=60° θ=45° θ=30° θ=25° θ=15° FWHM≤10 Ep-Eth≤0.3 keV θ=0° • identification of the inelastic scattering component by energy separation • measure the cross section as closer as possible to the energies relevant for s-process • Time-of-Flight technique • neutron beam with a narrow energy distribution (FWHM≤10 keV) • use of7Li(p,n)7Be reaction: Marita Mosconi,FZK

  11. Experimental setup and DAQ Main drawback of 7Li(p,n)7Be at threshold: small yield • improved neutron detectionefficiency by use of KG2 (NE912) • backgroundsubtraction by pulse shape analysis Neutron monitor PMT 6Li-glass Beam unstable due to proton source instabilities: computer monitoring of the neutron energy distribution Samples measured • 187Os • 188Os pure elastic component • Empty canning background Marita Mosconi,FZK

  12. Data Two week of data taking with proton current 3-6 μA Data analysis • Runs with similar FWHM extracted • Parameter of the scattering component extracted • Fit of the 187Os scattering distribution thought as made by two separated components Marita Mosconi,FZK

  13. Summary 186,7,8Os(n,g) cross sections: • background subtraction concluded • resonances analysis in progress • isotopic correction to be improved using the results of the previous item 187Os(n,n’) cross sections: • data taken for neutron distribution centered at 30 keV and with FWHM from 7 to 10 keV • analysis in progress Marita Mosconi,FZK

  14. The End Marita Mosconi,FZK

  15. Derivation of the cross sections in the unresolved region 188Os with filters Estimated level of background • -response corrected by weighting functions • background subtraction • neutron flux determined by Au count rate (using Ratynski and Macklin cross section measurements) • isotopic correction • neutron scattering background evaluated using the C measurement (environmental background corrected) rescaled for the ratio of the neutron scattering cross sections of carbon and of the sample analyzed • shapeof the -background determined using Pb measurement after subtraction of it own neutron scattering background and environmental background • level of the -background determined by GEANT3.21 simulations Normalisation 0.992 Au 4.9 eV resonance SAMMY fit Background 0.000 Marita Mosconi,FZK

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