248Cf, 254Fm, and 260No in Reactions 22Ne + 232Th, 238U; 16O + 232Th, 238U]]>

摘要

The mass and energy distributions of fission fragments in reactions_(16)O +_(232)Th,_(16)O +_(238)U,_(22)Ne +_(232)Th, and_(22)Ne +_(238)U are measured to investigate the role of closed proton and neutron shells in the fission of_(248)Cf,_(254)Fm, and_(260)No nuclei at excitation energies of 40–45 MeV. Experiments are performed on the U-400 cyclotron at the Flerov Laboratory of Nuclear Reactions (Dubna, Russia) using the CORSET double-arm time-of-flight spectrometer. For compound nucleus_(260)No* formed in the reaction_(22)Ne +_(238)U, an increase in the fragment yields is observed in the superasymmetric mass region 52/208 amu, which corresponds to the formation of fissioning pairs of two double-magic nuclei,_(48)Ca and_(208)Pb. Superasymmetric fission at a mass yield level of 10?5 was not observed for compound nuclei_(248)Cf and_(254)Fm* formed in reactions_(16)O +_(232)Th,_(16)O +_(238)U, and_(22)Ne +_(232)Th at an initial excitation energy of 40?45 MeV. Bimodal fission was observed for the_(260)No*compound nucleus at an initial excitation energy of 41 MeV. In the symmetric mass split, both fission fragments are in this case close to double-magic_(132)Sn.