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 ${ }^{ 22}\mathrm{ Ne}(\alpha, n){ }^{ 25}\mathrm{ Mg}$ reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing ${ }^{ 22}\mathrm{ Ne}(\alpha, \gamma){ }^{ 26}\mathrm{ Mg}$ 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 $n / \gamma$ 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 $(\alpha, n)$ 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 ${ }^{ 22}\mathrm{ Ne}(\alpha, n){ }^{ 25}\mathrm{ Mg}$ and ${ }^{ 22}\mathrm{ Ne}(\alpha, \gamma){ }^{ 26}\mathrm{ Mg}$ 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.
Categories: nucl-ex astro-ph.SR nucl-th