Evidence for an imaginary spin-orbit term in the proton optical potential

Evidence is given that there is a positive imaginary spin-orbit potential in the proton optical potential for ⁴⁰Ca at around 30 MeV, dropping with energy and apparently becoming negative at high energies. This is consistent with probable phonon-coupling effects but inconsistent at lower energies with ab initio OM calculations. Similar behaviour is found for ¹⁶O but with generally much smaller values of the imaginary potential. The authors comment on the need for suitable data to enable them to establish the general properties of W_so, the imaginary spin-orbit strength. The determination of an imaginary spin-orbit potential is made possible since the l-dependent potential used permits unprecedented quality of fit for angular distributions for these nuclei.