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

A new direct measurement of the $^{59}$Cu$(p,α){}^{56}$Ni excitation function from 2.43 to 5.88MeV 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 TypeI 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 $T_9 \approx 3.7$.

💡 Research Summary

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The authors present the first extensive direct measurement of the 59Cu(p,α)56Ni reaction cross section over a center‑of‑mass energy range of 2.43–5.88 MeV. The experiment was carried out at the Facility for Rare Isotope Beams (FRIB) using the high‑efficiency MUSIC active‑target detector in inverse kinematics. A 59Cu beam (8.418 MeV/u) produced by fragmentation of a 240 MeV/u 64Zn primary beam was purified to ~94 % and delivered at ~9 × 10³ particles per second. The MUSIC chamber, filled with methane at 440 Torr, provided continuous energy‑loss tracking across 18 anode strips, allowing identification of (p,α) events through ΔE–ΔE analysis of the heavy recoil (56Ni) and the emitted α particle. Eight effective energy bins were extracted, with statistical uncertainties derived from Poisson counting and Feldman–Cousins confidence intervals for the lowest‑statistics points.

The measured cross sections are compared with previous sparse data (Randhawa et al. at ~6 MeV and Bhathi et al. at 4–5 MeV) and with theoretical predictions from the NON‑SMOKER statistical model (scaled by 0.49) and TALYS calculations employing the Demetriou‑Goriely dispersive α‑optical model potential (α‑OMP). The TALYS results, after a uniform scaling factor of 0.86, reproduce the experimental excitation function very well, indicating that the α‑OMP and the nuclear level density (LD) in the compound nucleus 60Zn dominate the reaction rate. Sensitivity tests show that variations in the proton optical model potential have a minor effect because changes in the entrance‑channel transmission affect both T_p⁰ and the total transmission similarly.

To obtain a stellar reaction rate, the authors combined the experimental ground‑state cross sections with the scaled TALYS predictions using the Exp2Rate tool, thereby extending the rate to temperatures beyond the measured window. The contribution of thermally populated excited states in 59Cu was evaluated through the stellar enhancement factor (SEF) and the normalized partition function G₀(T). The ground‑state contribution X(T₉) drops from ~0.75 at T₉≈2.6 to ~0.10 at T₉≈10, meaning the experiment directly constrains roughly three‑quarters of the stellar rate at the lower end of the temperature range.

When compared with the widely used REACLIB rate (based on NON‑SMOKER calculations), the new rate is more than an order of magnitude lower at temperatures relevant for X‑ray bursts (T₉≲1) and remains systematically lower up to T₉≈4.5, where it briefly exceeds the REACLIB value due to a dip in the REACLIB parametrization, before falling below again at higher temperatures. This systematic reduction implies that the (p,α) branch of the Ni–Cu cycle is far weaker than previously assumed.

Astrophysical implications are explored with one‑zone X‑ray burst models and νp‑process network calculations. In X‑ray bursts, the reduced (p,α) rate limits recycling through the Ni–Cu cycle to less than 0.1 % of the total flow, leading to higher energy generation and a shift in the final ash composition toward heavier nuclei. In νp‑process conditions (T₉≈1–3), the suppression of the (p,α) channel delays the breakout temperature, allowing the competing (p,γ) channel to dominate up to T₉≈3.7. Consequently, the efficiency of neutron production via antineutrino capture on protons and subsequent (n,p) reactions is enhanced, facilitating the synthesis of nuclei beyond the iron group.

In summary, this work provides the most comprehensive experimental constraint on the 59Cu(p,α)56Ni reaction to date, delivers a revised stellar rate that is significantly lower than previous evaluations, and demonstrates the substantial impact of this revised rate on nucleosynthesis pathways in both Type I X‑ray bursts and the νp‑process in core‑collapse supernovae. The authors suggest that future efforts should focus on simultaneous measurements of the competing 59Cu(p,γ)60Zn reaction and on refined theoretical treatments of level densities and α‑optical potentials to further reduce uncertainties in the Ni–Cu cycle.