Waiting points in the astrophysicalrpprocess: How unbound areBr69andRb73?

Abstract

One of the key uncertainties in our understanding of the astrophysical $\mathit{rp}$ process is the extent to which even-even $N=Z$ nuclei such as $^{68}\mathrm{Se}$ and $^{72}\mathrm{Kr}$ act as waiting points in the process because of the next nucleus in the proton-capture chain being proton unbound. For these specific cases, much of the argument is based on the nonobservation of $^{69}\mathrm{Br}$ and $^{73}\mathrm{Rb}$ following projectile fragmentation. An examination of the nuclear structure involved, and the high population of low-lying isomers in the mirror nuclei, $^{69}\mathrm{Se}$ and $^{73}\mathrm{Kr}$ in fragmentation, suggests that this nonobservation may be down to a shadowing mechanism where the majority of the population goes to the isomer that proton decays before the ground state can be reached. Strong conclusions on the extent to which the $^{69}\mathrm{Br}$ and $^{73}\mathrm{Rb}$ are unbound may therefore be unsafe. The possible astrophysical implications of this are discussed along with the relevance of this shadowing mechanism to the search for exotic nuclei in fragmentation.