Charged-particle optical potentials tested by first direct measurement of the ${}^{59}\mathrm{Cu}(p,\alpha ){}^{56}\mathrm{Ni}$ 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 ${}^{59}\mathrm{Cu}(p,\alpha ){}^{56}\mathrm{Ni}$ reaction cross section at an energy of $\approx 6$ MeV [Randhawa et al., Phys. Rev. C 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.

Figure 1

Comparison of ${}^{59}\mathrm{Cu}(p,\alpha ){}^{56}\mathrm{Ni}$ reaction cross sections measured [1], evaluated (TENDL) [6], calculated by TALYS-1.95 and default options [5] (dotted curve), and similarly to Ref. [3] but for proton OMPs of (i)–(ii) Koning and Delaroche [4] (KD) without (dash-dot-dotted) and with (short dash-dotted) DI$+$PE account, (iii) Saini et al. [14] either original parameters (dash-dotted) or (iv) related $\left(N\text{−}Z\right)/A$ dependence (dashed), (v) the imaginary potential $W\left(E\right)$ of Ref. [15] (solid), as well as (vi) cross-section range with the lower and upper limits given by $W$ values of 1 and 2 MeV, respectively [16]. Effect of replacing $\alpha$-particle OMP [2] by [17], for proton OMP [15], is also shown (short-dotted).