Investigation of Fission Modes of ^248 Cf and ^254,256 Fm Nuclei Produced in Heavy-Ion Reactions

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 ^16O + ^232 Th and ^16,18O + ^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∼ 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.