Searching for the Superasymmetric Fission Mode of 248Cf, 254Fm, and 260No in Reactions 22Ne + 232Th, 238U; 16O + 232Th, 238U

Resumo

The mass and energy distributions of fission fragments in reactions ¹⁶O + ²³²Th, ¹⁶O + ²³⁸U, ²²Ne + ²³²Th, and ²²Ne + ²³⁸U are measured to investigate the role of closed proton and neutron shells in the fission of ²⁴⁸Cf, ²⁵⁴Fm, and ²⁶⁰No 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 ²⁶⁰No* formed in the reaction ²²Ne + ²³⁸U, 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, ⁴⁸Ca and ²⁰⁸Pb. Superasymmetric fission at a mass yield level of 10−5 was not observed for compound nuclei ²⁴⁸Cf and ²⁵⁴Fm* formed in reactions ¹⁶O + ²³²Th, ¹⁶O + ²³⁸U, and ²²Ne + ²³²Th at an initial excitation energy of 40−45 MeV. Bimodal fission was observed for the ²⁶⁰No*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 ¹³²Sn.