Os186,187,188(n,γ)cross sections and galactic nucleosynthesis

Abstract

The $^{186,187,188}\mathrm{Os}(n,\ensuremath{\gamma})$ cross sections were measured over the incident neutron energy range 2.6-800 keV. Optimized statistical model fits to the average cross sections were made employing estimates of the $^{186}\mathrm{Os}$, $^{187}\mathrm{Os}$, and $^{188}\mathrm{Os}$ $p$-wave strength functions 0.29 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, 0.45 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, and 0.33 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, respectively, $d$-wave strength functions 1.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, 4.0 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, and 1.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, respectively, and gamma ray strength functions ($\frac{{\overline{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}}{{D}_{0}}$) 26.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, 176 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, and 20.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$, respectively. A lower bound for the $^{187}\mathrm{Os}$ neutron inelastic cross section is estimated as 0.25(20) b at 30 keV. The Maxwellian-averaged capture cross sections are presented as a function of temperature. The ratio of 30 keV Maxwellian-averaged cross sections $\frac{〈{\ensuremath{\sigma}}_{\ensuremath{\gamma}}(186)〉}{〈{\ensuremath{\sigma}}_{\ensuremath{\gamma}}(187)〉}=0.504(17)$ is reported and the lack of agreement with earlier measurements of this ratio is discussed. The use of this cross section ratio in estimating, via the $^{187}\mathrm{Re}$-$^{187}\mathrm{Os}$ beta decay, the duration of galactic nucleosynthesis is discussed. The cross section ratio from this work yields an estimate of 10.4(25) \ifmmode\times\else\texttimes\fi{} ${10}^{9}$ yr for the duration of galactic nucleosynthesis, a result higher than but still consistent with the estimate 7(2) \ifmmode\times\else\texttimes\fi{} ${10}^{9}$ yr derived from U/Th decay.NUCLEAR REACTIONS Neutron capture, cross section, inelastic cross section, strength functions, nucleosynthesis, Maxwellian-averaged cross section.