New Precompound Decay Model: Angular Distributions

A new Monte Carlo precompound decay model is used to calculate double-differential spectra using an approach based on conservation of linear momentum between an incident nucleon and the three quasiparticles activated in the collisions of the equilibration process. The angular distribution theory is presented as it is applied to the individual three-exciton cascades of the precompound model. The importance of multiple precompound decay beyond two nucleons per nucleus is illustrated for Zr-90(p,xn) reactions for incident protons up to 256 MeV. These components are shown to be important for extending the useful range of the precompound approach to energies beyond 80 MeV. The contribution to nucleon spectra due to hole conversion processes is illustrated. We compare results of the formulation described with experimental angular distributions for Zr-90(p,xn) reactions at incident energies of 45, 80, and 160 MeV, and for the Zr-90(p,xp) reactions at incident energies of 80, 160, and 200 MeV. Additionally, Pb-208(p,xn) spectra at 7.5 degrees, 30 degrees, 60 degrees, 120 degrees, and 150 degrees are compared with two experimental data sets for 256 MeV incident proton energy, and Zr-90(p,xn) spectra at the same five angles are compared with predicted results for 256 MeV incident protons. At 256 MeV incident proton energy, the higher energy emission spectra are seriously overpredicted at 60 degrees. All other results are reproduced quite well, including back-angle yields. The quasielastic peak measured at 7.5 degrees at 256 MeV incident proton energy is shown to be in quite reasonable agreement with experimental results.