176Lu/176Hf: A Sensitive Test ofs‐Process Temperature and Neutron Density in AGB Stars

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Abstract

The s-process branching at A ¼ 176 has been analyzed on the basis of significantly improved experimental cross sections.Thisworkreportsonactivationmeasurementsofthepartial(n ;� )crosssectionof 176Lufeedingtheisomeric state in 176 Lu. 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 ¼ 5:1 and 25 keV, respectively. With these results and a recent time-of-flight measurement of the total stellar (n ;� ) 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 176 Lu was carried out for testing the temperature and neutron density conditions duringHeshellflashesinthermallypulsinglow-massasymptoticgiantbranchstars.Itwasfoundthatthelong-standing problem of the mother/daughter ratio of the two s-only isotopes 176 Lu and 176 Hf could be solved, if the temperaturedependent � -decay half-life of 176 Lu was considered with sufficient resolution over the temperature profile of the

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