Dependence of total kinetic energy of fission fragments on the excitation energy of fissioning systems

TL;DR

This study explains why the total kinetic energy of fission fragments decreases with increasing excitation energy, highlighting the role of heavy fragment shape changes and Coulomb repulsion, using a Langevin equation-based model.

Contribution

The paper introduces a four-dimensional Langevin model to analyze how heavy fragment shapes influence TKE variation with excitation energy in fission.

Findings

Heavy fragments become more prolate at higher excitation energies.

The increased distance between charge centers reduces Coulomb repulsion.

Shell effects' washing out leads to shape changes affecting TKE.

Abstract

We elucidated the reason why the average total kinetic energy (TKE) of fission fragments decreases when the excitation energy of the fissioning systems increases as indicated by experimental data for the neutron-induced fission events. To explore this problem, we used a method based on the four-dimensional Langevin equations we have developed. We have calculated the TKE of fission fragments for fissioning systems $^{236}$U$^{*}$ and $^{240}$Pu$^{*}$ excited above respective fission barriers, and compared the results with experimental data for n + $^{235}$U and n + $^{239}$Pu reactions, respectively. From the Langevin-model analysis, we have found that the shape of the abundant heavy fragments changes from almost spherical for low excitation domain to highly prolate shape for high excitation energy, while that of the light fragments does not change noticeably. The change of the "shape" of the heavy fragments causes an increase of a distance between the charge centers of the nascent fragments just after scission as excitation energy increases. Accordingly, the Coulomb repulsion between the two fragments decreases with an increase of the excitation energy, which causes the decrease of the average TKE. In this manner, we found that the change of the shape of the heavy fragment as a function of the excitation energy is the key issue for the TKE of fission fragments to decrease as the excitation energy of the fissioning nuclei increases. In other words, washing out of the shell effects which affect the shape of the heavy fragments is the key reason for the decreasing energy dependence of the average TKE of the fission fragments.