Evidence for spin-spin effects in p

The depolarization parameter {ital D} in proton-{sup 13}C elastic scattering at 72 MeV has been measured with an accuracy of 0.003--0.004 per data point. {ital D} was found to deviate from unity by {similar to}0.02 around 65{degree}, which is unambiguous evidence for a nucleon-nucleus spin-spin interaction. The data are well described by a microscopic calculation as well as by a phenomenological optical-model potential, which includes a real spherical spin-spin potential of strength {ital V}{sub ss}=0.7{plus minus}0.1 MeV.