Strong collective excitations in low energy neutron scattering from transitional nuclei

Elastic and inelastic scattering cross sections for 2.47 MeV neutrons bombarding ${}_{}{}^{148,150,152}{}_{}{}^{}\mathrm{Sm}$ nuclei were measured. The effects of the stable nuclear deformation on the scattering are unexpectedly pronounced. The inelastic scattering cross sections to the $2^{+}$, $4^{+}$, and $6^{+}$ rotational levels of ${}_{}{}^{152}{}_{}{}^{}\mathrm{Sm}$ are anomalously large, and there is evidence for excitation of the $8^{+}$ level. The corresponding effect on elastic scattering shows up as large cross-section reductions for ${}_{}{}^{152}{}_{}{}^{}\mathrm{Sm}$ compared with ${}_{}{}^{148}{}_{}{}^{}\mathrm{Sm}$ at all angles beyond 60°. The same sort of coupled elastic-inelastic effects are evinced by the ${}_{}{}^{150}{}_{}{}^{}\mathrm{Sm}$ cross sections, but are less marked. We show that these deformation effects are not explained using the usual coupled-channel optical model. The distribution of inelastic scattering strength to many levels of several shape transitional nuclei shows marked enhancements above statistical model expectations for the collective levels at the expense of cross sections to higher excited levels. The total nonelastic cross sections for all of these nuclei are about equal, indicating that deformation effects principally redistribute the scattering strength to emphasize collective levels strongly coupled to the ground state.