Possible solution to the α-potential mystery in the γ-process and the Nd/Sm ratio in meteorites

The ¹⁴⁶Sm/¹⁴⁴Sm ratio in the early solar system has been constrained by Nd/Sm isotope ratios in meteoritic material. Predictions of ¹⁴⁶Sm and ¹⁴⁴Sm production in the γ-process in massive stars are at odds with these constraints and this is partly due to deficiences in the prediction of the reaction rates involved. The production ratio depends almost exclusively on the (γ,n)/(γ,α) branching at ¹⁴⁸Gd. A measurement of ¹⁴⁴Sm(α,γ) ¹⁴⁸Gd 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 which can consistently describe the results of this and similar a-induced reactions. The untypically large deviation in ¹⁴⁴Sm(α,γ) can be explained, however, by low-energy Coulomb excitation which is competing with compound nucleus formation at very low energies. 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 ¹⁴⁸Gd(γ,α) rate is much higher than previously assumed. This leads to very small ¹⁴⁶Sm/ ¹⁴⁴Sm stellar production ratios, in even more pronounced conflict with the meteorite data.