Measurement of 59Cu(p,α) 56Ni reaction rate to constraint the flow of νp-process

A central question in nuclear astrophysics is where and how elements heavier than iron are synthesized. While the r-process (rapid neutron capture) and s-process (slow neutron capture) account for most heavy-element production, they cannot explain several neutron-deficient p-nuclei, notably 92,94Mo and 96,98Ru. Two promising sites for their synthesis are the ν p-process in neutrinodriven, proton-rich winds of core-collapse supernovae (CCSNe) and the rp-process in type-I X-ray bursts (XRBs). In both environments, nucleosynthesis beyond iron is governed by the competition between 59Cu(p,γ) 60Zn and 59Cu(p,α) 56Ni, known as the Ni–Cu cycle. At high temperatures (T > 3 GK), the (p,α) channel dominates, recycling material back to 56Ni and limiting the synthesis of heavier nuclei. Thus, precise knowledge of the 59Cu(p,α) 56Ni rate is critical for understanding the flow of nucleosynthesis in proton-rich conditions. We report a direct measurement of the 59Cu(p,α) 56Ni reaction at two center-of-mass energies, 4.01 MeV and 4.68 MeV, performed at the IRIS facility at TRIUMF, Canada. A high-purity 59Cu beam, produced via the ISOL technique, was incident on a solid hydrogen target, and the resulting reactions were analyzed using silicon and CsI(Tl) detectors to measure the energies and scattering angles of the reaction products. The total cross sections for the 59Cu(p,α) 56Ni reaction were measured at the two energies. The measured results lie systematically below the NON-SMOKER predictions. Comparisons with TALYS calculations using various α-optical model potentials show similar trends. The 4.01 MeV measurement lies within the νp-process Gamow window, providing the first direct experimental constraint on this reaction in the relevant astrophysical region. The lower measured rate implies a delayed onset of the Ni-Cu cycle, favoring the competing 59Cu(p,γ) 60Zn channel and enabling a stronger flow toward heavier nuclei. These results suggest enhanced efficiency of p-nuclei production in νp-process environments such as core-collapse supernovae and X-ray bursts.