Lu-176/Hf-176: A sensitive test of s-process temperature and neutron density in AGB stars

The s-process branching at A (176) has been analyzed on the basis of significantly improved experimental cross sections. This work reports on activation measurements of the partial (n, gamma) cross section of Lu-176 feeding the isomeric state in Lu-176. In total, six irradiations were performed at the Karlsruhe 3.7 MV pulsed Van de Graaff accelerator, and the induced activities were measured with HPGe clover detectors. In combination with previous data, partial cross sections of 3185 +/- 156 and 1153 +/- 30 mbarn were deduced at kT = 5: 1 and 25 keV, respectively. With these results and a recent time-of-flight measurement of the total stellar (n, gamma) cross section, the isomeric ratio was found to be constant in the relevant thermal energy range of the main s-process component. Based on these new data, a comprehensive analysis of the branching at Lu-176 was carried out for testing the temperature and neutron density conditions during He shell flashes in thermally pulsing low-mass asymptotic giant branch stars. It was found that the long-standing problem of the mother/daughter ratio of the two s-only isotopes Lu-176 and Hf-176 could be solved, if the temperature-dependent beta-decay half- life of Lu-176 was considered with sufficient resolution over the temperature profile of the convective He shell flashes.