Abstract
The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the Ne-22(alpha, n)Mg-25 reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing Ne-22(alpha, gamma)Mg-26 reaction, is not well constrained in the important temperature regime from similar to 0.2-0.4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the Ne-22(Li-6, d)Mg-26 reaction in inverse kinematics, detecting the outgoing deuterons and Mg-25,Mg-26 recoils in coincidence. We have established a new n/gamma decay branching ratio of 1.14(26) for the key E-x = 11.32 MeV resonance in Mg-26, which results in a new (alpha, n) strength for this resonance of 42(11) mu eV when combined with the well-established (alpha, gamma) strength of this resonance. We have also determined new upper limits on the alpha partial widths of neutron-unbound resonances at E-x = 11.112, 11.163, 11.169, and 11.171 MeV. Monte-Carlo calculations of the stellar Ne-22(alpha, n)Mg-25 and Ne-22(alpha, gamma)Mg-26 rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from similar to 0.2-0.4 GK. (C) 2020 The Authors. Published by Elsevier B.V.