The effects of unthermalized isomeric states and of a time-varying neutron flux on s-process branching ratios

A study has been made of branched s-process nucleosynthesis in the convective helium shells that appear in intermediate-mass red giants during thermal pulses. It is shown that, even though the average neutron flux may be several orders of magnitude larger than fluxes previously thought to be appropriate for s-process nucleosynthesis, resulting branching ratios still reflect solar abundances for most nuclides. This stems from the fact that, in the course of a single pulse, the neutron flux is not constant with time, in particular being a monotonically decreasing function of time after temperature maximum. Of the branches which do not at first sight lead to solar abundances, it appears that at least the worst of them can be explained if one accepts Ward's (1977) suggestion that the isomeric levels of several nuclei do not thermalize on a time scale short compared to any other time scale of interest.