Effect of pre‐equilibrium spin distribution on neutron‐induced reaction cross sections

Cross section measurements were made of prompt gamma‐ray production as a function of neutron energy using the germanium array for neutron induced excitations (GEANIE) at LANSCE. Measuring the prompt reaction gamma rays as a function of incident neutron energy provides more precise understanding of the spins populated by the pre‐equilibrium reaction. The effect of the spin distribution in pre‐equilibrium reactions has been investigated using the GNASH reaction code. Widely used classical theories such as the exciton model usually assume that the spin distribution of the pre‐equilibrium reaction is the same as the spin distribution of the compound nucleus reaction mechanism. In the present approach, the pre‐equilibrium reaction spin distribution was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). This pre‐equilibrium spin distribution was incorporated into the GNASH code and the gamma‐ray production cross sections were calculated and compared with experimental data. Spin distributions peak at lower spin when calculated with the FKK formulation than with the Compound Nuclear theory. The measured partial gamma‐ray cross sections reflect this spin difference. Realistic treatment of the spin distribution improves the accuracy of calculations of gamma‐ray production cross sections.