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

Reactions on 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 ${}^{59}\mathrm{Cu}(p,\alpha ){}^{56}\mathrm{Ni}$ and ${}^{59}\mathrm{Cu}(p,\gamma ){}^{60}\mathrm{Zn}$, could potentially halt the production of heavier elements. Here, we report the first direct measurement of ${}^{59}\mathrm{Cu}(p,\alpha ){}^{56}\mathrm{Ni}$ using a reaccelerated ${}^{59}\mathrm{Cu}$ beam and a cryogenic solid hydrogen target. Our results show that the reaction proceeds predominantly to the ground state of ${}^{56}\mathrm{Ni}$, and the experimental rate has been found to be lower than Hauser-Feshbach based statistical model predictions. New results hints that the $\nu p$ process could operate at higher temperatures than previously inferred and therefore remains a viable site for synthesizing the heavier elements.

Figure 1

The upper panel shows the schematic of IRIS set-up which includes an ionization chamber followed by a solid ${\mathrm{H}}_2$ target and two $\mathrm{\Delta }E\text{−}E$ telescopes for particle identification. The bottom left panel shows the energy loss spectrum of beam particles in the ionization chamber and the bottom right panel shows the particle identification using the Si(YY1)-CsI(Tl) $\mathrm{\Delta }E\text{−}E$ telescope.

Figure 2

The upper panel shows the excitation energy spectrum with (blue) and without (red) ${\mathrm{H}}_2$ target. The lower panel shows the blue histogram fitted with a Gaussian $+$ polynomial function (solid pink line) and the red histogram, i.e., the background fitted with polynomial only (green dotted line).

Figure 3

The upper panel shows the simulated detector efficiency as a function of ring number (which provides the scattering angle). The dark blue in the inset shows the simulated hit pattern. The lower panel shows the calculated angular distributions using different $\alpha$-OMPs in talys. Vertical dotted lines shows the detector coverage.

Figure 4

The cross section obtained in the current work compared to various statistical model calculations at $E_{c.m.}=6.0\mathrm{MeV}$. Error bars on the calculated cross sections include the total change in the cross sections due to change in the energy inside the target.