Decay properties of ${}^{22}\mathrm{Ne}+\hat{I}\pm$ resonances and their impact on $s$-process nucleosynthesis

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 ${}^{22}\mathrm{Ne}(\hat{I}\pm ,n){}^{25}\mathrm{Mg}$ reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing ${}^{22}\mathrm{Ne}(\hat{I}\pm ,{\hat{I}}^3){}^{26}\mathrm{Mg}$ reaction, is not well constrained in the important temperature regime from $\sim 0.2$--$0.4$~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 ${}^{22}\mathrm{Ne}({}^6\mathrm{Li},d){}^{26}\mathrm{Mg}$ reaction in inverse kinematics, detecting the outgoing deuterons and ${}^{25,26}\mathrm{Mg}$ recoils in coincidence. We have established a new $n/{\hat{I}}^3$ decay branching ratio of $1.14(26)$ for the key $E_x=11.32$ MeV resonance in ${}^{26}\mathrm{Mg}$, which results in a new $(\hat{I}\pm ,n)$ strength for this resonance of $42(11)\hat{I}¼$eV when combined with the well-established $(\hat{I}\pm ,{\hat{I}}^3)$ strength of this resonance. We have also determined new upper limits on the $\hat{I}\pm$ 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 ${}^{22}\mathrm{Ne}(\hat{I}\pm ,n){}^{25}\mathrm{Mg}$ and ${}^{22}\mathrm{Ne}(\hat{I}\pm ,{\hat{I}}^3){}^{26}\mathrm{Mg}$ rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from $\sim 0.2$--$0.4$~GK.

17 pages, 4 figures, accepted for publication in Phys. Lett. B