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

P. E. Koehler
Phys. Rev. C 66, 055805 – Published 27 November 2002
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Abstract

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 Γα) 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.

  • Received 19 July 2002

DOI:https://doi.org/10.1103/PhysRevC.66.055805

©2002 American Physical Society

Authors & Affiliations

P. E. Koehler*

  • Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

  • *Electronic address: koehlerpe@ornl.gov

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Vol. 66, Iss. 5 — November 2002

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