S-process studies - The effects of a pulsed neutron flux

Recent investigations of the astrophysical site of the s-process strongly suggest the helium-burning shell of the helium-shell-flashing stars of intermediate mass as the most likely site for the s-process event which produced the solar-system abundances. Previous analyses of branching in the s-process have been in terms of a constant temperature and density environment. The occurrence of periodic thermal instabilities seems to make this assumption inappropriate. The current effort is to reformulate the mathematics of the branched s-process to allow for the influence of thermal pulses. The effect of pulses on the unique isobars between branches is generally small; the largest effect of pulses is at those branch points where the branching ratio for beta-decay during a pulse is small, but the beta-decay lifetime during the interpulse period is shorter than the time between pulses. It is shown that the decay of the unstable branch nuclei between neutron irradiations allows the mean s-process neutron flux responsible for the solar-system abundances in such a pulsed environment to be considerably larger than that for a single continuous-exposure event.