宏观-微观模型和朗之万方法在低能核裂变研究中的应用

Nuclear Fission in Low Excitation Energy within Macroscopic-microscopic Model and Langevin Approach

基于三维朗之万模型对低能核裂变动力学过程和断点构型进行了研究,其中位能曲面采用基于双中心壳模型和有限程液滴模型的宏观-微观模型计算得到,质量张量和黏滞张量分别采用Werner-Wheeler方法和墙加窗一体模型得到。以14 MeV中子诱发^(235)U裂变为例,分别研究了拉长形变空间和壳衰减因子对裂变碎片质量分布、总动能分布及断点处核拉长与质量非对称度关联的影响,确定了模型计算中拉长形变空间边界至少应为3.5 R_(0)(R_(0)为球形核半径),以及壳衰减因子的合理取值为60 MeV。基于该模型,计算得到了14 MeV中子诱发^(233,235)U裂变碎片质量分布,与ENDF/B-Ⅷ.0评价数据符合较好,说明该模型具有定量计算裂变碎片质量分布的能力。

The three-dimensional Langevin model was used to study the fragment mass distribution and total kinetic energy(TKE)distribution and the scission configuration in low-energy nuclear fission,in which the potential energy surface was calculated by using the macroscopic-microscopic model based on the two-center shell model and the finite range liquid drop model,and the inertia tensor and the friction tensor were obtained within the Werner-Wheeler method and the wall-and-window model,respectively.Taking the case of 14 MeV n+235 U fission as an example,the influence of the elongation deformation space on the fission fragment mass distribution and the TKE distribution was investigated,and the correlation between the elongation and the mass asymmetry at the scission point was also studied.It is found that the elongation deformation space has a significant influence on the symmetric fission channel,especially for the calculation of the TKE around the symmetric mass region,which is because that the symmetric channel corresponds to the super-long deformation for the light actinide nuclei such as U,Np,Pu and so on,so that the less elongation deformation space could block the larger elongated nuclear shapes along the Langevin trajectories.In addition,the influence of the shell damping parameter on the fission fragment mass distribution and TKE distribution and the scission configuration was studied.It is found that the shell damping parameter has a larger influence on the fragment mass distribution,and the ratio of the peak height and the valley increases with the larger shell damping parameter,due to the stronger shell effect.However,it has little influence on the TKE distribution which indicates that the strength of the shell effect has little influence on the nuclear elongation.In the last,the pre-neutron and post-neutron fragment mass distributions for the 14 MeV n+233,235 U fission were calculated,and the results agree well with the evaluated data from ENDF/B-Ⅷ.0,which shows that the present model has the power of calculating the fission fragment mass distribution quantitatively.