Single-Nucleon Transfer on Unstable <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mmultiscripts><mrow><mi>Cu</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>59</mn></mrow></mmultiscripts></mrow></math> Probes the NiCu Cycle in Astrophysical X-Ray Bursts

C. O’Shea; G. Lotay; A. Gade; D. T. Doherty; H. Schatz; J. S. Randhawa; B. A. Brown; D. Weisshaar; J. Pereira; S. A. Gillespie; D. W. Bardayan; S. Byrne; L. Canete; W. N. Catford; G. Cerizza; A. Chester; J. Chung-Jung; C. Cousins; S. Coil; H. L. Crawford; S. Giraud; E. Good; G. Grauvogel; I. Harca; J. Heery; A. Hill; R. Jain; C. Maher; F. Montes; C. Müller-Gatermann; S. Noji; N. D. Pathirana; T. Parry; C. Paxman; A. M. Rogers; A. Sebastian; I. Sultana; J. A. Swartz; A. Tsantiri; S. Uthayakumaar; R. G. T. Zegers

doi:10.1103/gbbj-hpqk

Single-Nucleon Transfer on Unstable ${}^{59}{Cu}$ Probes the NiCu Cycle in Astrophysical X-Ray Bursts

C. O’Shea ^1,*, G. Lotay ¹, A. Gade ^2,3, D. T. Doherty ¹, H. Schatz ^2,3, J. S. Randhawa ^4,†, B. A. Brown ^2,3, D. Weisshaar ², J. Pereira² et al.

S. A. Gillespie ², D. W. Bardayan ⁵, S. Byrne ⁶, L. Canete ^1,‡, W. N. Catford ¹, G. Cerizza ², A. Chester ², J. Chung-Jung^2,3, C. Cousins¹, S. Coil ^5,§,∥, H. L. Crawford ⁷, S. Giraud^2,¶, E. Good ^2,**, G. Grauvogel ^2,3, I. Harca ², J. Heery ¹, A. Hill^2,3, R. Jain ^2,††, C. Maher^2,3, F. Montes ², C. Müller-Gatermann ⁸, S. Noji², N. D. Pathirana ^2,3, T. Parry ^1,‡‡, C. Paxman ^1,‡, A. M. Rogers ⁶, A. Sebastian ², I. Sultana ⁹, J. A. Swartz ², A. Tsantiri ^2,3,§§, S. Uthayakumaar ², and R. G. T. Zegers^2,3

¹School of Mathematics and Physics, University of Surrey, Guildford, United Kingdom
²Facility for Rare Isotope Beams, Michigan State University, East Lansing, Michigan, USA
³Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan, USA
⁴Physics Division, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
⁵Department of Physics and Astronomy, University of Notre Dame, Notre Dame, Indiana, USA
⁶Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, Massachusetts, USA
⁷Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA
⁸Physics Division, Argonne National Laboratory, Lemont, Illinois, USA
⁹Department of Physics, Central Michigan University, Mount Pleasant, Michigan, USA

^*Contact author: c.o’shea@surrey.ac.uk
^†Present address: Department of Physics and Astronomy, Mississippi State University, Starkville, Mississippi, USA.
^‡Present address: GANIL, Caen, France.
^§Present address: Facility for Rare Isotope Beams, Michigan State University, East Lansing, Michigan, USA.
^∥Also at Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan, USA.
^¶Present address: LMCE, CEA-DAM Île-de-France, Université Paris-Saclay, Bruyères-le-Châtel, France.
^**Present address: Pacific Northwest National Laboratory, Richland, Washington, USA.
^††Present address: Lawrence Livermore National Laboratory, Livermore, California, USA.
^‡‡Present address: Facility for Rare Isotope Beams, Michigan State University, East Lansing, Michigan, USA.
^§§Present address: Department of Physics, University of Regina, Regina, Saskatchewan, Canada.

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Phys. Rev. Lett. 137, 022701 – Published 6 July, 2026

DOI: https://doi.org/10.1103/gbbj-hpqk

Abstract

Recent models of the rapid proton ( $r p$ ) capture process indicate that a competition between the ${}^{59}{Cu} (p, γ) {}^{60}{Zn}$ and ${}^{59}{Cu} (p, α) {}^{56}{Ni}$ reactions may result in the formation of a nickel-copper (NiCu) cycle that traps the flux of material between ${}^{56}{Ni}$ and ${}^{60}{Zn}$ . Here, we report the identification of 15 proton-unbound levels in ${}^{60}{Zn}$ , populated via ${}^{59}{Cu} (d, n)$ transfer, which govern the rate of the ${}^{59}{Cu} (p, γ) {}^{60}{Zn}$ reaction in XRBs. Precise excitation energies for levels in ${}^{60}{Zn}$ were obtained from observed $γ$ decays, and spectroscopic factors were determined from angle-integrated cross sections. Incorporating these results into stellar-model calculations, we find that with experimentally constrained uncertainties a NiCu cycle in XRBs is indeed possible, though we limit its branching strength to less than 38%. While modest, such a branching has significant impact on the light curve, motivating further studies of the relevant rates. Our calculations also indicate that a significant NiCu cycle leads to an increase in the amount of odd- $A$ nuclei in the burst ashes, which may affect Urca cooling processes in neutron star crusts.

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Single-Nucleon Transfer on Unstable ${}^{59}{Cu}$ Probes the NiCu Cycle in Astrophysical X-Ray Bursts

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Single-Nucleon Transfer on Unstable Cu59 Probes the NiCu Cycle in Astrophysical X-Ray Bursts

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Single-Nucleon Transfer on Unstable ${}^{59}{Cu}$ Probes the NiCu Cycle in Astrophysical X-Ray Bursts