The depolarization parameter D in proton${\mathrm{-}}^{13}$C elastic scattering at 72 MeV has been measured with an accuracy of 0.003–0.004 per data point. D was found to deviate from unity by ∼0.02 around 65°, 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 $V_{\mathrm{ss}=0.7\mathrm{}\pm 0.1}$ MeV.

• Received 13 November 1989

DOI:https://doi.org/10.1103/PhysRevLett.64.368