Spin Dependence of thes-Wave Neutron Interaction withLi7

The angular distributions of the neutrons scattered elastically by ${\mathrm{Li}}^7$ have been measured at four energies in the vicinity of the $p$-wave resonance at 256 kev. Pronounced departures from symmetry are observed, the scattering being predominantly forward below resonance and backward above resonance. These departures are analyzed for information on the nature of the $s$-wave "background" interaction. By taking into account the known values of the coherent and incoherent scattering cross sections at zero energy, it is possible to conclude that the parallel-spin ($J=2^{-}$) $s$-wave interaction is the stronger. Several other examples are cited from the literature on light nuclei which also indicate a stronger parallel-spin $s$-wave interaction, the magnitudes of the splittings being of the order of 1 Mev.