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

The 78Se(𝑛,𝛾)79Se cross section has a high impact on the abundances of 78Se produced during the slow neutron capture process (s-process) in massive stars. A measurement of the 78Se 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 sections (MACS) 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 78Se abundances produced in the s-process in massive stars and AGB stars. Massive stars are now predicted to produce sub-solar 78Se/76Se ratios, which is expected since 76Se is an s-only isotope, while solar 78Se abundances have also contributions from other nucleosynthesis processes.