The puzzle of the synthesis of the rare nuclide 138La

The calculations of the p-process in the O/Ne layers of Type II supernovae are quite successful in reproducing the solar system content of p-nuclides. They predict, however, a significant underproduction of the rare odd-odd nuclide La-138. A model for the explosion of a 25 M-. star with solar metallicity is used to suggest that nu (e)-captures on Ba-138 may well be its most efficient production mechanism. The responsibility of an inadequate prediction of the La-138 and La-139 photodisintegration rates in the too low production of La-138 is also examined quantitatively. A detailed discussion of the theoretical uncertainties in these rates suggest that the required rate changes are probably too high to be fully plausible. Their measurement would be most welcome. They would help disentangling the relative contributions of thermonuclear and neutrino processes to the La-138 production.