  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.
