${}_{}{}^{22}{}_{}{}^{}Ne(\alpha ,\mathit{n}){}_{}{}^{25}{}_{}{}^{}Mg$: The Key Neutron Source in Massive Stars

The excitation function of the reaction ${}^{22}\mathrm{Ne}(\alpha ,n{)}^{25}\mathrm{Mg}$, the key neutron source in the astrophysical $s$ process in massive stars, has been determined from threshold at $E_{\alpha }=570\mathrm{}$ up to 1450 keV with an experimental sensitivity of ${10}^{-11\mathrm{}}\mathrm{b}$. For all resonances in this energy range new resonance parameters have been measured. For a possible resonance at about 635 keV a new upper limit $\omega \gamma <60\mathrm{}\mathrm{neV}$ for the strength was obtained. Based on the new data, improved reaction rates were calculated as a function of temperature. The new uncertainty limits are considerably smaller than in previous determinations, ruling out the large enhancement factors, up to 500, assumed in some stellar model calculations.