Charged-particle optical potentials tested by first direct measurement of 59Cu(p,alpha)56Ni reaction

Due consideration of proton optical–model potential (OMP) anomalies at sub-Coulomb energies for medium–weight nuclei is shown to be critical for the analysis of the unprecedented measurement of ${}^{59}$Cu$(p,\alpha {)}^{56}$Ni reaction cross section at an energy of $\sim$6 MeV [Phys. Rev. C {\bf 104}, L042801 (2021)]. The variation in predicted cross sections from standard statistical–model calculations and the cross–section range corresponding to the anomalous proton imaginary–potential depth, for target nuclei off the line of stability, are distinct and well separated. Consequently, the new measurement provides, under unique conditions, tests of proton isoscalar and isovector real–potential components, the anomalous imaginary potential, as well as previous alpha-particle OMP, for nuclei off the line of stability.