First direct measurement of ${}^{59}$Cu(p,$\hat{I}\pm$)${}^{56}$Ni: A step towards constraining the Ni-Cu cycle in the Cosmos

Abstract

Reactions on the proton-rich nuclides drive the nucleosynthesis in Core-Collapse Supernovae (CCSNe) and in X-ray bursts (XRBs). CCSNe eject the nucleosynthesis products to the interstellar medium and hence are a potential inventory of p-nuclei, whereas in XRBs nucleosynthesis powers the light curves. In both astrophysical sites the Ni-Cu cycle, which features a competition between ${}^{59}$Cu(p,$\hat{I}\pm$)${}^{56}$Ni and ${}^{59}$Cu(p,${\hat{I}}^3$)${}^{60}$Zn, could potentially halt the production of heavier elements. Here, we report the first direct measurement of ${}^{59}$Cu(p,$\hat{I}\pm$)${}^{56}$Ni using a re-accelerated ${}^{59}$Cu beam and cryogenic solid hydrogen target. Our results show that the reaction proceeds predominantly to the ground state of ${}^{56}$Ni and the experimental rate has been found to be lower than Hauser-Feshbach-based statistical predictions. New results hint that the $\hat{I}½p$-process could operate at higher temperatures than previously inferred and therefore remains a viable site for synthesizing the heavier elements.