Neshad Deva Pathirana

Recent models of the rapid proton (rp) capture process indicate that a competition between the ^{59}Cu(p,γ)^{60}Zn and ^{59}Cu(p,α)^{56}Ni reactions may result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between ^{56}Ni and ^{60}Zn. Here, we report the identification of 15 proton-unbound levels in ^{60}Zn, populated via ^{59}Cu(d,n) transfer, which govern the rate of the ^{59}Cu(p,γ)^{60}Zn reaction in XRBs. Precise excitation energies for levels in ^{60}Zn were obtained from observed γ 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.

Short-Lived Radionuclides (SLRs) with million-year lifetimes serve as chronometers of Early Solar System (ESS) formation and can reveal active nucleosynthesis in our Galaxy. Among the nuclides whose decay signatures are preserved in meteorites, the extinct radionuclide ⁹² Nb is one of the few confirmed proton-rich SLRs, and the ESS ⁹² Nb/ ⁹² Mo abundance ratio serves as a sensitive probe of proton-rich nucleosynthesis. However, the interpretation of this ratio is complicated by uncertainties in both nuclear reaction rates and the astrophysical environments. Because ⁹² Nb is shielded by the stable isobars ⁹² Zr and ⁹² Mo, it cannot be produced through β-decay chains. Instead, it must be synthesized through direct reactions in explosive supernova environments, where the photoneutron reaction on ⁹² Nb is a key regulator and currently dominates the uncertainty in the nuclear reaction networks producing ⁹² Nb. Direct measurements of this reaction are presently not feasible. Therefore, recommended values have relied on theoretical calculations. Here, we present the first experimental constraint on this reaction. This newly constrained reaction rate significantly increases the predicted ⁹² Nb abundance in relevant supernova environments, thereby shifting model predictions in the direction needed to ease the long-standing discrepancy in the ESS ⁹² Nb/ ⁹² Mo ratio by removing one of the dominant nuclear uncertainties in the ⁹² Nb production network.