S-process branchings at Sm-151, Eu-154, and Dy-163

The (n, gamma) cross sections of the stable europium isotopes Eu-151 and Eu-153 have been measured by irradiating oxide samples in a quasistellar neutron spectrum. From the induced activities, the stellar cross sections of Eu-151 and Eu-153 at a thermal energy of kT=30 keV were found to be [sigmav]/v(T)=3821+/-152 mb and 2733 +/- 110 mb, respectively. These results allowed us to resolve severe discrepancies among previous data. Similar activation measurements were also performed on Sm-152,Sm-154 and Er-164,Er-170. Among these results, the stellar cross section of the s-only isotope Er-164 [sigmav]/v(T)=1084+/-51 mb at kT=30 keV is particularly important. Statistical model calculations were performed with emphasis on the effect of excited states as well as on the unstable isotopes Sm-151, Eu-152, and Eu-154. 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.