n_TOF04: 186 Os, 187 Os, 188 Os capture cross sections

1. n_TOF04: 186Os, 187Os, 188Os capture cross sections Measured on August 2003 for a total of 2.3e18 protons Neutron flux monitor : 6Li/silicon detector -ray detection: C6D6 scintillators (PHWT) MACS at 25 keV fundamental for the determination of the Re/Os cosmochronometer Marita Mosconi1, Kaori Fujii5, Alberto Mengoni2, Michael Heil1,6, Ralf Plag1, Franz Käppeler1, Gaelle Aerts3, Rita Terlizzi4, Cesar Domingo Pardo1 and the n_TOF collaboration 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 6 GSI, Darmstadt,Germany n_TOF04: Os cross sections

2. Samples measured • 186Os (2 g, 79 %) with and without filters (5.9e17 p) • 187Os (2 g, 70 %) with and without filters (3.6e17 p) • 188Os (2 g, 95 %) with and without filters (6.0e17 p) • Al can background subtraction (7e16 p) • 197Au (1.2g) with and without filters  flux normalization (3.3e17 p) • natPb (2 g)  in-beam gamma background (1.4e17 p) • natPb (2 g) with filters  gamma background attenuation (1.6e17 p) • natC (0.5 g)  in-beam neutron background (1.1e16 p) n_TOF04: Os cross sections

3. Count rate n_scattering in-beam g Background subtraction by decoupling the three components of the background • Analysis split in two: • resonance analysis with R-matrix code SAMMY (Kaori Fujii) • unresolved resonance region n_TOF04: Os cross sections

4. Derivation of the cross sections in the unresolved region • double counting rejection • -response corrected by weighted functions (CEA/INFN) • neutron flux determined by Au count rate 4.9 eV resonance for the resonance analysis, Ratynski experimental MACS for the continuum* • background subtraction (next slide) • correction for multiple scattering and self shielding (SESH) • subtraction of the (n,n’g) events at high energy • isotopic correction * background subtraction and SESH correction also to determine Au count rate n_TOF04: Os cross sections

5. background evaluation by sample changer technique • ambient backgroundsubtracted to the count rate of all samples (Pb, C included) • neutron scattering backgroundby C count rate (fitted) rescaled according to the elastic scattering cross section of the sample we look for the background • shape of the in-beam g-rays scattering background by Pb count rate minus its neutron scattering background • level of the -background determined by GEANT3.21 simulations Crosschecks: • Comparison with filters (next slides) • Comparison with SAMMY n_TOF04: Os cross sections

6. n_TOF04: Os cross sections

7. n_TOF04: Os cross sections

8. n_TOF04: Os cross sections

9. Comparison with previous results cross sections by coupling continuum and resonance results n_TOF in agreement with theoretical predictions deviations from previous sets of data n_TOF04: Os cross sections

10. Systematic errors 186Os 0.2% 187Os 0.1% 188Os 0.4% Error due to PHWT application: comparison of the count rate obtained by different weighting functions Error from background determination:(previous slides on background) 186Os 2% 187Os 1% 188Os 3% Error on flux determination: 3 % difference between Ratysnki and the first Au saturated resonance 186Os 1.4% 187Os 3.0% 188Os 1.4% Error due to isotopic correction: uncertainties on unmeasured isotopes n_TOF04: Os cross sections

11. MACSs 186Os Browne Winters n_TOF Bao MACS at 30 keV n_TOF04: Os cross sections

12. MACSs 187Os Browne Winters n_TOF Bao Bokhovko MACS at 30 keV n_TOF04: Os cross sections

13. MACSs 188Os Browne Winters n_TOF Bao MACS at 30 keV n_TOF04: Os cross sections

14. cross sections down to thermal energy cross section count rate resonances from contaminants (187Os, 189Os) 188Os sth=4.7 b contribution from resonances 55.5 % 186Os contribution from resonances 4.9 % 187Os contribution from resonances 2.5 % n_TOF04: Os cross sections

15. The End n_TOF04: Os cross sections

16. 187Os(n,n’) measurement at FZK Neutron monitor 6Li-glass PMT Monoenergetic neutron beam at 30 keV (FWHM≤10 keV)from 7Li(p,n)7Be reaction at threshold ToF technique Samples measured: • 187Os • 188Os (pure elastic component) • Empty can (background ) Fit of the 187Os scattering distribution assuming two separate components n_TOF04: Os cross sections

17. Comparisons of the yield of the 2 samples DET4 Shape of the elastic scattering component DET4 DET4 • TOF translated into energy • Fit of 188Os scattering count rate • Inelastic scattering defined by shifting in energy the elastic 4. Normalization of the 2 curves free parameters of the fit n_TOF04: Os cross sections

18. GEANT simulation of detector response 1000 experimental data experimental data elastic spectrum elastic spectrum NUMBER OF EVENTS inelastic spectrum 800 inelastic spectrum fit as addition of the two fit as addition of the two 600 DET4 400 90o right 200 geometry and presence of moderators strongly affecting the TOF spectra DET1 90o left n_TOF04: Os cross sections

19. background subtraction experimental data elastic spectrum inelastic spectrum background by contaminant fit as addition of the three DET2 30o left huge signature from the moderator material around the sample This will define the background from contaminant for the three detectors n_TOF04: Os cross sections

20. Which challenges? Range of (n,g) cross sections down to lower energies reliable MACS at low kT HF calculation (n,g) • 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 Graaff Data needed: 186,7Os(n,g) cross sections 187Os(n,n’) cross section 186Os(n,g) cross section determination Improved accuracy for 187Os(n,n’) cross section n_TOF04: Os cross sections