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

TL;DR

This study provides the first experimental measurements of the total and partial stellar neutron capture cross sections for $^{79}$Se, crucial for understanding the s-process branching at this isotope, using activation and AMS techniques.

Contribution

First experimental determination of the total and partial neutron capture cross sections for $^{79}$Se relevant to the s-process, employing activation and AMS methods.

Findings

Total cross section at 25 keV: 60.1±9.6 mbarn, lower than previous estimates.

Partial cross section to the isomer: 42.0±2.0 mbarn.

Results improve understanding of the s-process branching at $^{79}$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 $\times10^5$ y) as well as the absence of suitable $\gamma$-ray transitions. The preliminary result for the total Maxwellian averaged cross section is $<\sigma>_{30 keV}$= 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 $\gamma$-spectroscopy and yielded $<\sigma>_{30 keV}$(part.)= 42.0$\pm$2.0 mbarn.