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

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 $T_9 \lesssim 3$.

💡 Research Summary

The paper presents a comprehensive experimental and theoretical investigation of the 59Cu(p,α)56Ni reaction, a key process that influences nucleosynthesis pathways in proton‑rich explosive astrophysical environments such as Type I X‑ray bursts (XRBs) and the νp‑process occurring in neutrino‑driven winds after core‑collapse supernovae. In both scenarios, the competition between the (p,γ) and (p,α) channels on 59Cu determines whether the reaction flow proceeds beyond the Ni–Cu cycle into heavier nuclei. At temperatures below roughly 3 GK, the (p,γ) channel dominates, allowing the flow to escape the cycle; at higher temperatures the (p,α) channel can close the cycle, suppressing the production of elements heavier than iron.

To obtain reliable reaction rates, the authors performed a direct measurement of the excitation function from 2.43 to 5.88 MeV in the center‑of‑mass frame using inverse kinematics at the Facility for Rare Isotope Beams (FRIB). A 59Cu beam, produced by fragmentation of a 64Zn primary beam and re‑accelerated to 8.418 MeV/u, was injected into the Multi‑Sampling Ionization Chamber (MUSIC) active target filled with 440 torr of methane. MUSIC’s 18 segmented anode strips record the energy loss (ΔE) of each ion as it traverses the gas, providing near‑100 % detection efficiency and allowing clean separation of reaction products based on their Z and energy‑loss signatures. The (p,α) events were identified by characteristic ΔE traces of the heavy recoil 56Ni, which differ markedly from the unreacted 59Cu beam and from elastic/inelastic scattering channels. A ΔE–ΔE two‑dimensional representation further confirmed the identification on a strip‑by‑strip basis.

Cross sections were extracted for eight effective center‑of‑mass energies corresponding to strips 2–9. The authors applied a correction formula to account for the finite thickness of each strip and for the variation of the cross section across the strip. Energy‑loss uncertainties were evaluated by comparing ATIMA and Ziegler stopping‑power tables, leading to a conservative 5 % systematic uncertainty on the beam energy loss and consequently on the effective energies (0.6–2 % depending on the strip). Statistical uncertainties were derived from Poisson statistics; for the low‑count strips (8 and 9) the Feldman–Cousins unified approach was used to construct 95 % confidence intervals, ensuring proper coverage even with very few events. Systematic uncertainties, dominated by event‑selection criteria and space‑charge effects at the high instantaneous beam rate, averaged 17 % but rose to ~33 % for the noisier strips.

The measured excitation function is compared with previous direct data (Randhawa et al., Bhathi et al.) and with statistical‑model predictions. Earlier experiments relied on detecting α particles over a limited angular range (≈18°–40°) and required angular‑distribution assumptions to obtain total cross sections, introducing sizable model‑dependent uncertainties. In contrast, the MUSIC active target provides fully angle‑integrated cross sections, eliminating that source of error. The new data agree within uncertainties with the previous measurements when the TALYS‑based angular distribution is used, but they differ when a simple Legendre polynomial fit is applied, highlighting the sensitivity to the assumed angular shape.

The authors performed Hauser–Feshbach calculations with the TALYS 2.0 code, employing the Demetriou–Goriely dispersive α‑optical potential (DEM‑3). By scaling the TALYS predictions by a factor of 0.86 they achieve the best fit to the experimental points; a similar scaling of NON‑SMOKER predictions (0.49) also reproduces the trend but less accurately. These calculations were also used to evaluate the stellar enhancement factor, i.e., the contribution of thermally populated excited states of 59Cu in a stellar plasma, allowing conversion of the laboratory cross sections to stellar rates.

The resulting stellar reaction rate is systematically lower than the rate currently adopted in the REACLIB library. Importantly, the new rate remains below the competing 59Cu(p,γ)60Zn rate for temperatures up to T₉ ≈ 3 (≈3 GK). Consequently, in astrophysical models of X‑ray bursts and νp‑process nucleosynthesis, the Ni–Cu cycle will not be closed as efficiently as previously thought at temperatures below this threshold. This implies that the flow can proceed to heavier nuclei more readily, potentially enhancing the synthesis of elements with mass numbers A > 64 in the νp‑process and affecting the shape of X‑ray burst light curves.

In summary, the work delivers the most precise and lowest‑energy direct measurement of the 59Cu(p,α)56Ni reaction to date, substantially reduces the nuclear‑physics uncertainties associated with this key reaction, and provides an updated stellar reaction rate that should be incorporated into future astrophysical network calculations. The combination of an active‑target technique with rigorous statistical treatment sets a new standard for measuring low‑cross‑section reactions on short‑lived isotopes relevant to explosive nucleosynthesis.