Spin Distribution in Preequilibrium Reactions for 48Ti + n

Abstract

Cross section measurements were made of prompt gamma-ray production as a function of incident neutron energy on a 48Ti sample. Partial gamma-ray cross sections for transitions in (45-48)Ti, (44-48)Sc, and (42-45)Ca have been determined. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the LANSCE/WNR facility. The prompt-reaction gamma rays were detected with the large-scale Compton-suppressed germanium array for neutron induced excitations (GEANIE).

Neutron energies were determined by the time-of-flight technique. The gamma-ray excitation functions were converted to partial gamma-ray cross sections taking into account the dead-time correction, target thickness, detector efficiency and neutron flux (monitored with an in-line fission chamber). The data are presented for neutron energies E[subscript n] between 1 to 200 MeV. These results are compared with model calculations which include compound nuclear and preequilibrium emission. The model calculations are performed using the STAPRE reaction code for E[subscript n] up to 20 MeV and the GNASH reaction code for E[subscript n] up to 120 MeV. Using the GNASH reaction code the effect of the spin distribution in preequilibrium reactions has been investigated. The preequilibrium reaction spin distribution was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). The multistep direct (MSD) part of the FKK theory was calculated for a one-step process. The contribution from higher steps is estimated to be small. The spin distribution of the multistep compound (MSC) part of FKK theory is assumed to be the same as in the compound nucleus. The FKK preequilibrium spin distribution was incorporated into the GNASH calculations and the gamma-ray production cross sections were calculated and compared with experimental data.

The difference in the partial gamma-ray cross sections using spin distributions with and without preequilibrium effects is found to be significant. Specifically, the probability of gamma transitions from a high spin state is strongly suppressed because of the preequilibrium spin distribution. Preequilibrium reactions are found to be important for neutron energies above 10 MeV.