Interplay of direct and compound-nucleus mechanisms in neutron capture by light nuclides

The authors discuss the direct-capture theory pertaining to primary electric-dipole (E1) transitions following slow-neutron capture. For approximately 20 light nuclides that we have studied, estimates of direct-capture cross sections using optical-model potentials with physically realistic parameters are in reasonable agreement with the data. Minor disagreements that exist are consistent with extrapolations to light nuclides of generally accepted formulations of compound-nucleus capture. In dealing with nuclei soft to vibrations, we have considered the possible effects of coupling of the collective motion with the optical potential in the framework of R-matrix theory. In such cases, we find that the inclusion of inelastic channels results in systematic changes in the calculated cross sections