Spherical optical model potential for the Re/Os stellar nucleosynthesis chronometer froms-wave neutrons on186,187,188Os

This paper reports resonance parameters and spherical optical model potentials obtained from the analyses of neutron transmission measurements for ${}_{}{}^{186}{}_{}{}^{}$,188Os over the incident neutron energy range 27–1000 eV and for ${}_{}{}^{187}{}_{}{}^{}\mathrm{Os}$ over the range 27–140 eV. Multilevel R-matrix analyses of 77 s-wave resonances yielded sets of resonance parameters ($E_{\lambda }$,${\Gamma }_{\mathrm{n})}$ and the associated external (background) R functions. The statistical properties of the resonance parameters, mean-level spacings, and strength functions are reported. The external R functions and strength functions were used to determine spherical optical model potential well depths, which in the case of ${}_{}{}^{187}{}_{}{}^{}\mathrm{Os}$ we find to be strongly dependent on the spin of the compound nucleus. These optical model well depths can be used to estimate the capture cross section of the first excited state in ${}_{}{}^{187}{}_{}{}^{}$Os+n. Since this state is highly populated at stellar temperatures, the ratio of excited to ground-state capture cross sections is an important parameter in using the s-process model of stellar nucleosynthesis to determine what fraction of the abundance of ${}_{}{}^{187}{}_{}{}^{}\mathrm{Os}$ should be ascribed to the radiogenic decay of ${}_{}{}^{187}{}_{}{}^{}\mathrm{Re}$. That fraction determines, in part, the estimate of the duration Δ of stellar nucleosynthesis to be derived from ${}_{}{}^{187}{}_{}{}^{}\mathrm{Re}$ beta decay. A determination of Δ for the Re/Os chronometer based on our data leads to an age of the universe of (17±3)×${10}^9$ yr.