The neutron capture cross sections of 186;187;188 Os and their application to Re/Os cosmochronometer

The aim of this thesis is a fine determination of 186;187;188Os neutron capture cross sections, in order to remove principal nuclear physics uncertainties on the age of the universe determined using the Re/Os cosmochronometer. A general introduction including stellar nucleosynthesis, nuclear cosmochronometry,available neutron facilities and neutron reaction features is given. Dedicated measurements of the 186;187;188Os capture cross sections have been performed at the CERN neutron time-of- ight facility, n TOF, in the neutron energy range from 1 eV up to 1 MeV. The details of the measurement and the data treatment are reported. Using obtained capture yields, resolved resonance region analysis is completed. Resonance parameters have been extracted from a full R-matrix fit. A statistical analysis has been performed and the related averaged resonance parameters are derived. This information is crucial for a complete understanding and modelling in terms of the Hauser-Feshbach statistical model of the capture and inelastic reaction channels, required for the evaluation of the stellar reaction rates of these isotopes. The neutron capture cross sections averaged over the thermal neutron spectrum, Maxwellian averaged cross sections, for the range of temperatures relevant for s-process nucleosynthesis have been derived from the combined information of the experimental data in the resolved and unresolved resonance regions. The im plications of this analysis in the estimation of the s-process component of the 187Os abundance and the related impact on the evaluation of the time-duration of the Galactic nucleosynthesis through the Re/Os cosmochronometer is shown. Moreover, further considerations for a realistic stellar condition, phenomena of 187Re decay and s-process branchings are discussed. In conclusion, results of present analysis are compared with the alternative cosmic age by means of astronomical and cosmological ways.