  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 gamma-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
(gamma,n)/(gamma,alpha) branching at 148Gd. A measurement of
144Sm(alpha,gamma)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 alpha+nucleus potential, no global
optical potential has yet been found which can consistently describe the
results of this and similar alpha-induced reactions at the low energies
encountered in astrophysical environments. The untypically large deviation in
144Sm(alpha,gamma) 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,alpha) reactions in this mass
range. Low-energy (alpha,gamma) and (alpha,n) data on other nuclei can also be
consistently explained in this way. Since Coulomb excitation does not affect
alpha-emission, the 148Gd(gamma,alpha) rate is much higher than previously
assumed. This leads to small 146Sm/144Sm stellar production ratios, in even
more pronounced conflict with the meteorite data.
