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

本文提出了一种描述核反应多步直接反应（MSD）过程的半量子模型，称为半经典畸变波模型，简称SCDW模型。本文的目的是研究核诱导反应在中能区预平衡态粒子发射的MSD机制。首先，我们开发了一种准随机数蒙特卡罗方法计算MSD截面的方法，并成功地计算了MSD前三个步骤的SCDW。该代码应用于中重核（即^<58>Ni和^<90>Zr）上的质子诱导反应，入射能量为数十MeV至200 MeV。结果与实验数据吻合较好，没有任何可调参数，只有在非常向前和向后的角度。我们发现，在大角度下看到的低估是由于局部Femi-gas模型给出的目标核子的动量分布。为了改善这种情况，我们开发了一种新的方法，即在SCDW模型中加入一维密度矩阵的Wigner变换。这种差异可以通过在谐振子或Woods-Saxson势中使用更现实的单粒子波函数而不是局部费米-气体模型来消除。此外，我们发现SCDW模型在正确考虑Q值的情况下适用于（p, nx）反应的分析。此外，用中位神经网络截面计算的SCDW结果表明，在入射能量高达200 MeV的MSD过程中，中位效应并不明显。最后，我们尝试对SCDW模型进行扩展，从而直接计算有效相互作用的矩阵元，并研究了脱壳效应对MSD截面的影响。因此，我们没有发现在MSD反应过程连续体中有很强的脱壳效应。

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.