We elucidated the reason why the average total kinetic energy (TKE) of fission fragments decreases when the
excitation energy of the fissioning systems increase 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 236U∗ and 240Pu∗ excited
above respective fission barriers, and compared the results with experimental data for n + 235U and n + 239Pu
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.
各種検索
サポート
ヘルプ