Solution of the α-potential mystery in the γ process and its impact on the Nd/Sm ratio in meteorites

Rauscher, Thomas (2013) Solution of the α-potential mystery in the γ process and its impact on the Nd/Sm ratio in meteorites. PHYSICAL REVIEW LETTERS, 111 (6). ISSN 0031-9007

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Abstract

The Sm146/Sm144 ratio in the early solar system has been constrained by Nd/Sm isotope ratios in meteoritic material. Predictions of Sm146 and Sm144 production in the γ process in massive stars are at odds with these constraints, and this is partly due to deficiencies in the prediction of the reaction rates involved. The production ratio depends almost exclusively on the (γ,n)/(γ,α) branching at Gd148. A measurement of Sm144(α,γ)Gd148 at low energy had discovered considerable discrepancies between cross-section predictions and the data. Although this reaction cross section mainly depends on the optical α+nucleus potential, no global optical potential has yet been found that can consistently describe the results of this and similar α-induced reactions at the low energies encountered in astrophysical environments. The untypically large deviation in Sm144(α,γ) and the unusual energy dependence can be explained, however, by low-energy Coulomb excitation, which is competing with compound nucleus formation at very low energies. Considering this additional reaction channel, the cross sections can be described with the usual optical potential variations, compatible with findings for (n, α) reactions in this mass range. Low-energy (α, γ) and (α, n) data on other nuclei can also be consistently explained in this way. Since Coulomb excitation does not affect α emission, the Gd148(γ,α) rate is much higher than previously assumed. This leads to very small Sm146/Sm144 stellar production ratios, in even more pronounced conflict with the meteorite data. © 2013 American Physical Society.

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