Neutron spectroscopy of ${}^{26}$Mg states: constraining the stellar neutron source ${}^{22}$Ne($\alpha ,n$)${}^{25}$Mg

This work reports on accurate, high-resolution measurements of the ${}^{25}$Mg($n,\gamma$)${}^{26}$Mg and ${}^{25}$Mg($n,tot$) cross sections in the neutron energy range from thermal to about 300 keV, leading to a significantly improved ${}^{25}$Mg($n,\gamma$)${}^{26}$Mg parametrization. The relevant resonances for $n{+}^{25}$Mg were characterized from a combined R-matrix analysis of the experimental data. This resulted in an unambiguous spin/parity assignment of the corresponding excited states in ${}^{26}$Mg. With this information experimental upper limits of the reaction rates for ${}^{22}$Ne($\alpha ,n$)${}^{25}$Mg and ${}^{22}$Ne($\alpha ,\gamma$)${}^{26}$Mg were established, potentially leading to a significantly higher ($\alpha, n$)\mathrm{/}($\alpha, \gamma$) ratio than previously evaluated. The impact of these results have been studied for stellar models in the mass range 2 to 25 $M_{\odot }$.