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,γ ) 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,γ ) 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( α,n ) 25 Mg and 22 Ne( α,γ ) 26 Mg were established, potentially leading to a significantly higher (α,n)/(α,γ) ratio than previously evaluated. The impact of these results has been studied for stellar models in the mass range 2 to 25 M⊙ .