Direct Measurement of the $^{59}$Cu$(p,α)^{56}$Ni Excitation Function to Constrain the Ni--Cu Cycle Strength and Its Impact on Explosive Nucleosynthesis

Abstract

A new direct measurement of the $^{59}$Cu$(p,α){}^{56}$Ni excitation function from 2.43 to 5.88~MeV in the center-of-mass frame was performed in inverse kinematics using the high-efficiency MUSIC active-target detector at FRIB. This reaction plays a critical role in constraining the strength of the Ni--Cu cycle in explosive astrophysical environments such as Type~I X-ray bursts and the $ν$p-process in neutrino-driven winds following core-collapse supernovae. The newly derived stellar reaction rate is systematically lower than the REACLIB evaluation, resulting in less than 0.1\% recycling through the Ni--Cu cycle in X-ray bursts and an enhanced efficiency of the $ν$p-process up to temperatures of $T_9 \approx 3.7$.

Figures (3)

• Figure 1: Measured $^{59}\mathrm{Cu}(p,\alpha)^{56}\mathrm{Ni}$ cross sections from the present work, Randhawa et al.Jaspreet59Cu, and Bhathi et al.Bhathi25, compared with the NON-SMOKER predictions scaled by 0.49 (gray dotted line) and TALYS calculations scaled by 0.86 using the Demetriou and Goriely dispersive $\alpha$-OMP DEMETRIOU2002253 (black line).
• Figure 2: (Top) Recommended $^{59}\mathrm{Cu}(p,\alpha)^{56}\mathrm{Ni}$ stellar reaction rate from the present work (purple line) with propagated uncertainties (shaded band) compared to the REACLIB rate (green line). (Bottom) Ratio of the recommended stellar rate from this work to the REACLIB rate.
• Figure 3: Branching ratio of the $^{59}\mathrm{Cu}(p,\alpha)^{56}\mathrm{Ni}$ to $^{59}\mathrm{Cu}(p,\gamma)^{60}\mathrm{Zn}$ stellar reaction rates as a function of temperature. The purple curve and shaded band show the recommended ratio and its uncertainty based on the experimentally constrained $(p,\alpha)$ rate. The green curve and band illustrate the REACLIB rate varied by $\pm 100$