Abstract
The role of closed and deformed proton and neutron shells in the fission of $${}^{248}$$ Cf and $${}^{254,256}$$ Fm nuclei at excitation energies from 40 to 56 MeV is studied. The mass–energy distributions of fission fragments of these nuclei formed in the $${}^{16}\textrm{O}\, + \, ^{232}$$ Th and $${}^{16,18}\textrm{O}\, + \, ^{238}$$ U reactions at projectile-ion energies close to the Coulomb barrier are measured by means of the CORSET time-of-flight spectrometer. It is shown that, in all of the reactions under study, an enhanced yield of fragments in the mass region around 100 a.m.u. is due to the effect of the $$Z\sim 38$$ deformed proton shell. A manifestation of a supershort mode is found in the fission of $${}^{256}$$ Fm at the compound-nucleus excitation energy of 40 MeV.