Mass Distributions Of Fission Fragments Investigated By GEF Model

Nowadays,nuclear energy has become an important part of energy sources,and nuclear fission is a possible way to utilize nuclear energy.An accurate evaluation of fission product yields in the fission process is of great importance in many areas,including the estimation of the decay heat,operation of nuclear reactors,and management of the nuclear waste and spent fuels.At the same time,the distribution of fission yield is of great significance to the study of nuclear fission mechanism and nuclear structure.In this work,we introduced the GEF?GEneral Fission?model proposed by K.-H.Schmidt et al.The GEF model is a semi-empirical phenomenological model describing fission process,which combines physical concepts from quantum mechanics and statistical mechanics with specific experimental information to adjust a set of parameters suitable for different fission systems.This model avoids the limitation of microscopic calculation,simplifies the calculation process,and can give reliable predictions for a variety of fission observables of a large number of nuclei.In this paper,the mass distributions of neutron induced fission and spontaneous fission for actinides are studied by GEF model.The mass distributions of fission fragments of ²³²Th,²³³U,²³⁵U,²³⁸U and ²³⁹Pu in neutron induced fission were studied,the results show that with the increase of incident neutron energy,the shell effect becomes weaker,the contribution of S1 and S2 mode decreases gradually,and the symmetric SL mode increases.The average mass of heavy fragments remains around A=140,which is due to the influence of the deformation shell at N=88???0.6?.In addition,the positions of the S1 and S2 modes are almost invariable,which are also generally thought to be associated with shells in the heavy fission fragments.With the increase of incident neutron energy,if the excitation energy is high enough that the first-chance fission?n,f?,second-chance fission?n,nf?,third-chance fission?n,2nf?and so on,will occur successively.As the incident neutron energy increases,the contribution of?n,f?fission channel gradually decreases and that of the other fission channels increases.The variation trend of yield with energy is related to the position located on the mass distribution.The total yields of two peak regions decrease with the increase of incident neutron energy,while the yields of valley and two wing regions increase.There are four intersections in the mass distribution,which is very important for judging the variation trend of the yield with incident neutron energy.The results of CYFP code and GEF code for several fission systems are compared with experimental data.The GEF results are in good agreement with most experimental data?including newly acquired experimental data?,which can provide a more reliable prediction for nuclei lacking experimental data.Except for neutron induced fission,we calculated the mass distribution of spontaneous fission for ²⁴⁰Pu with GEF model,which reproduced the experimental data very well.Besides,we calculated the mass distributions of ^256-258Fm in spontaneous.The GEF model can reproduce the observed transition from double-humped distribution to single-humped distribution with the increase of neutron number,which is usually explained as ²⁵⁸Fm tends to split into two doubly magic fragments close to Z=50 and N=82 under the combined action of macroscopic potential and shell effect,and symmetric fission takes an obvious advantage.