β-delayed proton emission of 69Kr and the 68Se rp-process waiting point

Abstract

The slow β-decay of the 68Se waiting point in the astrophysical rp-process can in principle be bypassed by a sequential two proton capture. The rate of this reaction sequence depends exponentially on the 69Br proton separation energy. We studied β-delayed proton emission of 69Kr and extracted a proton separation energy of −641(42) keV. In addition, we determined a 69Kr β-decay half-life of T1/2=28(1) ms and an excitation energy of 3153(55) keV of the 69Kr isobaric analog state in 69Br. X-ray burst model calculations show that regardless of the values of other uncertain masses in the region, the new Sp(Br69) allows for a reaction flow via Se68(2p,γ) of at most 20%. Uncertainties are sufficiently reduced to conclude that Se68(2p,γ) has at best a very small effect on burst light curve and composition, and that 68Se is a strong rp-process waiting point. Our results also exclude the possibility of a suggested longer lived, so far unobserved, 69Br ground state.