S-process branchings at151Sm,154Eu,and163Dy

Abstract

The $(n,\ensuremath{\gamma})$ cross sections of the stable europium isotopes ${}^{151}\mathrm{Eu}$ and ${}^{153}\mathrm{Eu}$ have been measured by irradiating oxide samples in a quasistellar neutron spectrum. From the induced activities, the stellar cross sections of ${}^{151}\mathrm{Eu}$ and ${}^{153}\mathrm{Eu}$ at a thermal energy of $kT=30 \mathrm{keV}$ were found to be $〈\ensuremath{\sigma}v〉{/v}_{T}=3821\ifmmode\pm\else\textpm\fi{}152 \mathrm{mb}$ and $2733\ifmmode\pm\else\textpm\fi{}110 \mathrm{mb},$ respectively. These results allowed us to resolve severe discrepancies among previous data. Similar activation measurements were also performed on ${}^{152,154}\mathrm{Sm}$ and ${}^{164,170}\mathrm{Er}.$ Among these results, the stellar cross section of the s-only isotope ${}^{164}\mathrm{Er}$ $〈\ensuremath{\sigma}v〉{/v}_{T}=1084\ifmmode\pm\else\textpm\fi{}51 \mathrm{mb}$ at $kT=30 \mathrm{keV}$ is particularly important. Statistical model calculations were performed with emphasis on the effect of excited states as well as on the unstable isotopes ${}^{151}\mathrm{Sm},$ ${}^{152}\mathrm{Eu},$ and ${}^{154}\mathrm{Eu}.$ The combined set of cross sections was used for an updated analysis of the branchings at $A=151,$ 152, 154, and 163. The temperature and density estimates derived via the classical approach are discussed and compared to stellar models for helium shell burning in low mass stars.