ИСТИНА

The photofission of 238U at the excitation energy range of the fissioning nucleus up to 20 MeV is studied. Gamma-activation analysis was used to measure the independent and cumulative yields of fission fragments after the release of fast neutrons. The experiment was performed on a bremsstrahlung gamma-ray beam of the RTM-55 electron accelerator with the upper limit of spectrum 55.6 MeV. The behavior of the symmetrical and asymmetric fission modes of 238U(γ,F) and 235U(n,F) depending on the average excitation energy of the fissioning nucleus is compared on the basis of these data and data from other experiments. An analysis of the obtained data shows that the contribution of the mode responsible for the symmetric fission of the fragments increases approximately by a factor of 10 with an increase of the 238U excitation energy from 9 to 20 MeV. The contribution of the asymmetric mode STI decreases approximately by a factor of 2 in this energy range. The contribution of the asymmetric mode STII slightly increases in the region of the excitation energies of the nucleus up to 20 MeV.The work was supported by a grant from the President of the Russian Federation (MK-2693.2017.2).