Measurement of the Se$^{78}$ (n , γ) Se$^{79}$ cross section up to 600 keV at the n_TOF facility at CERN

The $^{78}$Se(n, γ ) $^{79}$Se cross section has a high impact on the abundances of $^{78}$Se produced during the slow
neutron capture process (s process) in massive stars. A measurement of the $^{78}$Se radiative neutron capture cross
section has been performed at the Neutron Time-of-Flight facility at CERN using a set of liquid scintillation
detectors that have been optimized for a low sensitivity to neutrons. We present resonance capture kernels up
to 70 keV and cross section from 70 to 600 keV. Maxwellian-averaged cross section (MACS) values were
calculated for stellar temperatures between kT = 5 and 100 keV, with uncertainties between 4.6% and 5.8%.
The new MACS values result in substantial decreases of 20–30% of $^{78}$Se abundances produced in the s process
in massive stars and AGB stars. Massive stars are now predicted to produce subsolar$^{78}$Se / $^{76}$Se ratios, which
is expected since $^{76}$Se is an s-only isotope, while solar $^{78}$Se abundances have also contributions from other
nucleosynthesis processes.