Abstract
Recent models of the rapid proton (rp) capture process indicate that a competition between the 59Cu(p, gamma)60Zn and 59Cu(p, alpha)56Ni reactions may result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between 56Ni and 60Zn. Here, we report the identification of 15 proton-unbound levels in 60Zn, populated via 59Cu(d, n) transfer, which govern the rate of the 59Cu(p, gamma)60Zn reaction in XRBs. Precise excitation energies for levels in 60Zn were obtained from observed gamma decays, and spectroscopic factors were determined from angle-integrated cross sections. Incorporating these results into stellar-model calculations, we find that with experimentally constrained uncertainties a NiCu cycle in XRBs is indeed possible, though we limit its branching strength to less than 38%. While modest, such a branching has significant impact on the light curve, motivating further studies of the relevant rates. Our calculations also indicate that a significant NiCu cycle leads to an increase in the amount of odd-A nuclei in the burst ashes, which may affect Urca cooling processes in neutron star crusts.