First measurements of the total and partial stellar cross section to the s-process branching-point ⁷⁹Se

Although ⁷⁹Se represents an important branching in the weak s process, the stellar neutron capture cross sections to this isotope have not yet been measured experimentally. In this case, experimental data is essential for evaluating the important branching in the s-process reaction path at ⁷⁹Se. The total cross section of ⁷⁸Se at a stellar energy of kT = 25 keV has been investigated with a combination of the activation technique and accelerator mass spectrometry (AMS), since offline decay counting is prohibitive due to the long terrestrial half life of ⁷⁹Se (2.80±0.36 ×10⁵ y [1]) as well as the absence of suitable γ -ray transitions. The preliminary result for the total Maxwellian averaged cross section is 〈 σ 〉_{30 keV}= 60.1±9.6 mbarn, significantly lower than the previous recommended value. In a second measurement, also the partial cross section to the 3.92 minisomer was determined via g -spectroscopy and yielded 〈σ 〉 _{30 keV}(part.)= 42.0±2.0 mbarn.