Theoretical study of multistep direct reaction mechanism in preequilibrium nuclear reaction processes

点击翻译按钮获取中文摘要

We have proposed a new semi-quantal model to describe the multistep direct (MSD) reaction processes in preequilibrium nuclear reactions, which is called the semi-classical distorted wave model : it is referred to as the SCDW model. The purpose of this work is to investigate the MSD mechanism in the preequilibrium particle emission of nucleon-induced reactions in the intermediate energy region.First of all, we have developed a way of calculating the MSD cross sections with the Quasi-Random Number Monte Carlo method, and succeeded in the SCDW calculations for the leading three MSD steps. The code was applied to proton-induced reactions on medium-heavy nuclei (i.e., ^<58>Ni and ^<90>Zr) for incident energies of several tens of MeV to 200 MeV.The result showed good agreement with the experimental data without any adjustable parameters, except at very forward and backward angles. We found that the underestimation seen at large angles is due to the momentum distribution of target nucleons given by the local Femi-gas model. To improve this situation, we have developed a new approach in which the Wigner transform of one-body density matrix is incorporated in the SCDW model. The discrepancy was indicated to be removed by using more realistic single-particle wave functions in the harmonic oscillator or Woods-Saxson ponetial, instead of the local Fermi-gas model.In addition, we have found that the SCDW model is applicable to analyzes of (p, nx) reactions by taking into account the Q value correctly. Moreover, the result of SCDW calculations with in-medium NN cross sections showed that the in-medium effect is not much apreciable in the MSD process for incident energies up to 200 MeV.Finally, we have attempted to extend the SCDW model so that the matrix elements of sffective interaction is directly calculated, and investigated the off-shell effect on the MSD cross sections. As a result, we have not found a strong off-shell effect in the MSD reaction process continuum.