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

TL;DR

This study presents the first direct measurement of the $^{59}$Cu(p,$ extalpha$)$^{56}$Ni reaction, providing crucial data to refine models of nucleosynthesis in supernovae and X-ray bursts, and suggesting the $ u p$-process can operate at higher temperatures.

Contribution

First direct measurement of the $^{59}$Cu(p,$ extalpha$)$^{56}$Ni reaction rate, challenging theoretical predictions and impacting nucleosynthesis models in astrophysical environments.

Findings

Reaction proceeds mainly to the ground state of $^{56}$Ni.

Experimental rate is lower than theoretical predictions.

Results imply the $ u p$-process can occur at higher temperatures.

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,$\alpha$)$^{56}$Ni and $^{59}$Cu(p,$\gamma$)$^{60}$Zn, could potentially halt the production of heavier elements. Here, we report the first direct measurement of $^{59}$Cu(p,$\alpha$)$^{56}$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 $\nu p$-process could operate at higher temperatures than previously inferred and therefore remains a viable site for synthesizing the heavier elements.