Inelastic neutron scattering cross section of 187Os at 30 keVM. Mosconi1, M. Heil1, 2, F. Käppeler1, A. Mengoni3 and R. Plag1
1 Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
2 GSI, Darmstadt, Germany
3 International Atomic Energy Agency, P.O. Box 100, Wagramer Strasse 5, 1400 Vienna, Austria
Published online: 21 May 2008
The Re/Os cosmochronometer requires a precise determination of the neutron capture cross section of 187Os at the stellar conditions of s-process nucleosynthesis, i.e., the effective stellar values for thermal energies between 8 and 25 keV. This implies a correction to the cross section measured in the laboratory, because at these temperatures most of the 187Os nuclei are in the first exited state at 9.75 keV. In order to support the necessary Hauser-Feshbach calculations of the capture cross section for the excited states, it is important to consider the competition by the inelastic channels, including the so-called super-elastic scattering, where the neutron gains in energy. In this context, the inelastic scattering cross section of 187Os has been measured using a pulsed neutron beam with an energy of 30 ± 9 keV (FWHM) and a repetition rate of 1 MHz. Neutrons were produced directly at the threshold of the 7Li(p,n)7Be reaction in order to take advantage of the sharp collimation from reaction kinematics. To compensate for the low neutron yield close to threshold, scattered neutrons were recorded with three efficient 6Li-glass scintillators 11 cm in diameter, located at 90 and 120 deg with respect to the neutron beam. Data have been taken with isotopically enriched 187Os and 188Os, the latter for measuring the elastic component separately. The analysis of the data was performed by a complete Monte Carlo simulation of the experiment. The results for the inelastic cross section are compared to previous data and are discussed with respect to the stellar (n,γ) rates, which are instrumental for an evaluation of the Re/Os cosmochronometer.
© CEA 2008