Detailed Study of the $^{59}$Cu(p,$α)^{56}$Ni Reaction and Constraints on Its Astrophysical Reaction Rate

TL;DR

This work directly measures the $^{59}$Cu$(p,\alpha)^{56}$Ni excitation function at $E_{\mathrm{cm}}=2.43$–5.88 MeV with the MUSIC active-target detector at FRIB, constraining the astrophysical rate relevant for X-ray bursts and the $\nu p$-process. Hauser–Feshbach TALYS calculations, with aDEM-3 $\alpha$-OMP and ld=4, msL=5, plus a 0.86 scaling, best reproduce the data and enable extrapolation to lower energies; the ground-state contribution to the stellar rate is quantified via $X(T)$, showing $X\approx0.75$ at $T_9\approx2.6$ down to $X\approx0.10$ by $T_9\gtrsim10$. The resulting rate is consistently lower than REACLIB across $0.1\le T_9\le3$, and remains below the competing $^{59}$Cu$(p,\gamma)^{60}$Zn rate, implying a comparatively weak Ni--Cu cycle in the astrophysical sites considered. This work reduces model uncertainties for proton-rich nucleosynthesis and provides tighter experimental constraints on a key reaction in the Ni–Cu cycle.

Abstract

The $^{59}$Cu$(p,α)^{56}$Ni reaction plays an important role in explosive astrophysical scenarios such as Type I X-ray bursts and the $νp$-process in neutrino-driven winds following a core-collapse supernova. In both cases, this reaction has been proposed to significantly affect the synthesis of heavier nuclei by regulating the flow of nucleosynthesis through the Ni--Cu cycle. In this work, we present a direct measurement of the $^{59}\mathrm{Cu}(p,α)^{56}\mathrm{Ni}$ excitation function from 2.43--5.88 MeV in the center-of-mass frame. The experiment was performed in inverse kinematics using the high-efficiency MUSIC active-target detector at FRIB. This measurement allowed tight constraints to be placed on the astrophysical reaction rate. The derived stellar rate is systematically lower than the REACLIB rate and remains below the competing $(p,γ)$ rate for $T_9 \lesssim 3$.

Figures (12)

• Figure 1: Beam profile on Strip 12 (right) of the MUSIC anode. The 56Ni beam contaminant is cleanly separated from the primary 59Cu component.
• Figure 2: (left) Energy loss as a function of strip number "traces" of $(p,\alpha)$ events occurring in strip 5 (purple line), the unreacted $^{59}$Cu beam (black line), and the $(p,p)$ and $(p,p')$ events (green line). (right) $\Delta E$--$\Delta E$ spectrum of events occurring in strip 5.
• Figure 3: Measured $^{59}\mathrm{Cu}(p,\alpha)^{56}\mathrm{Ni}$ cross sections from the present work, together with previous data from Randhawa et al.Jaspreet59Cu and Bhathi et al.Bhathi25, compared to NON-SMOKER predictions scaled by 0.49 and TALYS predictions scaled by 0.86 using the Demetriou and Goriely dispersive $\alpha$-nucleus optical-model potential DEMETRIOU2002253 (black line).
• Figure 4: Measured $^{59}\mathrm{Cu}(p,\alpha)^{56}\mathrm{Ni}$ S-factors from the present work (red diamonds), compared with TALYS calculations using different $\alpha$-OMP, non-Smoker and the new pOMP derived for this reaction by Avrigeanu Avrigeanu2022. Note that the scaling factor of 0.86 applied to the Demetriou and Goriely dispersive $\alpha$-OMP (see the best-fit result in Fig. \ref{['fig:placeholder_xsec']}) is not included in this comparison of predictions from the different $\alpha$-OMPs.
• Figure 5: Astrophysical S-factors for the $^{59}$Cu$(p,x)$ reactions calculated with the DEM-3 potential as a function of $E_{\mathrm{c.m}}$. The total S-factor (solid black line) and the $(p,\gamma)$ contribution (dotted red line) decrease with increasing energy, as typically expected. In contrast, the $(p,\alpha)$ S-factor (dashed blue line) increases with energy and is strongly suppressed at low energies.