Development and applications of multi-step Hauser-Feshbach/Pre-equilibrium model theory

A recently developed model that combines compound and precompound reactions with conservation of angular momentum is discussed. This model allows a consistent description of intermediate excitations from which tertiary reaction cross sections can be calculated for transitions to the continuum as well as to the discrete residual levels with known spins and parities. Predicted neutron, proton, and alpha-particle production cross sections and emission spectra from 14-MeV neutron-induced reactions are compared favorably with angle-integrated experimental data for 12 nuclides. The model is further developed to include angular distributions of outgoing particles. The random phase approximation used for the compound stage is partially removed for the precompound stages, allowing off-diagonal terms of the collision matrix to produce both odd and even terms in the Legendre polynomial expansion for the angular distribution. Calculated double-differential cross sections for the 14.6-MeV 23Na(n,n'x) reaction are compared with experimental data. (author)