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

Abstract

Although ${}^{79}$Se 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 ${}^{79}$Se. The total cross section of ${}^{78}$Se 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 ${}^{79}$Se (2.80$\pm$0.36 $\times 10^5$ y) as well as the absence of suitable ${\hat{I}}^3$-ray transitions. The preliminary result for the total Maxwellian averaged cross section is $<\ddot{I}ƒ{>}_{30keV}$= 60.1$\pm$9.6 mbarn, significantly lower than the previous recommended value. In a second measurement, also the partial cross section to the 3.92 min-isomer was determined via ${\hat{I}}^3$-spectroscopy and yielded $<\ddot{I}ƒ{>}_{30keV}$(part.)= 42.0$\pm$2.0 mbarn.

Proceedings "Nuclei in the Cosmos" NIC- IX, June 25-30 2006, CERN, Geneva/ Switzerland, published with Proccedings of Science, PoS(NIC-IX)089