Constraints on the ²²Ne(α,n)²⁵Mg s-process neutron source from analysis of ^natMg+n total and ²⁵Mg(n,γ) cross sections

The ²²Ne(α,n)²⁵Mg reaction is thought to be the neutron source during the s process in massive and intermediate mass stars as well as a secondary neutron source during the s process in low-mass stars. Therefore, an accurate determination of this rate is important for a better understanding of the origin of nuclides heavier than iron as well as for improving s-process models. Also, the s process produces seed nuclides for a later p process in massive stars, so an accurate value for this rate is important for a better understanding of the p process. Because the lowest observed resonance in direct ²²Ne(α,n)²⁵Mg measurements is considerably above the most important energy range for s-process temperatures, the uncertainty in this rate is dominated by the poorly known properties of states in ²⁶Mg between this resonance and threshold. Neutron measurements can observe these states with much better sensitivity and determine their parameters (except Γ_α) much more accurately than direct ²²Ne(α,n)²⁵Mg measurements. I have analyzed previously reported ^natMg+n total and ²⁵Mg(n,γ) cross sections to obtain a much improved set of resonance parameters for states in ²⁶Mg between threshold and the lowest observed ²²Ne(α,n)²⁵Mg resonance, and an improved estimate of the uncertainty in the ²²Ne(α,n)²⁵Mg reaction rate. For example, definitely two, and very likely at least four, of the states in this region have natural parity and hence can contribute to the ²²Ne(α,n)²⁵Mg reaction, but two others definitely have non-natural parity and so can be eliminated from consideration. As a result, a recent evaluation in which it was assumed that only one of these states has natural parity has underestimated the reaction rate uncertainty by at least a factor of 10, whereas evaluations that assumed all these states could contribute probably have overestimated the uncertainty.