Decay properties of 22Ne + resonances and their impacton s-process nucleosynthesis

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 22Ne(�; n)25Mg reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing 22Ne(�;)26Mg reaction, is not well constrained in the important temperature regime from �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 22Ne(6Li; d)26Mg reaction in inverse kinematics, detecting the outgoing deuterons and 25;26Mg recoils in coincidence. We have established a new n= decay branching ratio of 1:14(26) for the key Ex = 11:32 MeV resonance in 26Mg, which results in a new (�; n) strength for this resonance of 42(11) �eV when combined with the well-established (�; ) strength of this resonance. We have also determined new upper limits on the � partial widths of neutron-unbound resonances at Ex = 11:112; 11:163, 11:169, and 11:171 MeV. Monte-Carlo calculations of the stellar 22Ne(�; n)25Mg and 22Ne(�; )26Mg rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from �0:2–0:4 GK.