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

The Se 78 ( n , γ ) Se 79 cross section has a high impact on the abundances of Se 78 produced during the slow neutron capture process ( s process) in massive stars. A measurement of the Se 78 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 k T = 5 and 100 keV, with uncertainties between 4.6% and 5.8%. The new MACS values result in substantial decreases of 20–30% of Se 78 abundances produced in the s process in massive stars and AGB stars. Massive stars are now predicted to produce subsolar Se 78 / Se 76 ratios, which is expected since Se 76 is an s -only isotope, while solar Se 78 abundances have also contributions from other nucleosynthesis processes.