Single-nucleon transfer unveils NiCu cycle in astrophysical X-ray bursts

The rise of space-based astronomy has revealed that X-ray bursts (XRBs) represent the most frequent stellar explosions to occur in our galaxy. These bursts are driven by thermonuclear flashes on accreting neutron stars, and lead to the formation of some of the most neutron-deficient nuclei known to exist. However, it has been found that so-called “waiting points” along the rapid proton (rp) capture process path can halt the progress of nucleosynthesis. Moreover, it has been postulated that a competition between the astrophysical ⁵⁹ Cu(p,γ) ⁶⁰ Zn and ⁵⁹ Cu(p,α) ⁵⁶ Ni reactions may even 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 on the spectroscopic measurement of proton-unbound resonant states in ⁶⁰ Zn, which govern the rate of the ⁵⁹ Cu(p,γ) ⁶⁰ Zn reaction in XRBs. Incorporating these results into stellar-model calculations, we provide the first experimental evidence in support of a NiCu cycle and limit its contribution within rp-process flow to a maximum of ∼38%. In assuming maximum NiCu cycling, our models highlight an increase in the production of low-mass isotopes which may influence Urca cooling processes in neutron star crusts.