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

Abstract

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 Wso, 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.