Fission of Hg-180,Hg-182,Hg-183* and Pt-178* nuclei at intermediate excitation energies

Background: The nature of asymmetric fission of preactinides is not yet understood in detail, despite intense experimental and theoretical studies carried out at present. Purpose: The study of asymmetric and symmetric fission of Hg180,182,183 and Pt178 nuclei as a function of their excitation energy and isospin. Methods: Mass-energy distributions of fission fragments of Hg180, Pt178 (two protons less than Hg180), and Hg182 (two neutrons more than Hg180) formed in the Ar36+Sm144, Nd142, and Ca40+Nd142 reactions were measured at energies near and above the Coulomb barrier. Fission of Hg183 obtained in the reaction of Ca40 with Nd143 was also investigated to see if one extra neutron could lead to dramatic changes in the fission process due to the shape-staggering effect in radii, known in Hg183. The measurements were performed with the double-arm time-of-flight spectrometer CORSET. Results: The observed peculiarities in the fission fragment mass-energy distributions for all studied nuclei may be explained by the presence of a symmetric fission mode and three asymmetric fission modes, manifested by the different total kinetic energies and fragment mass splits. The yield of symmetric mode grows with increasing excitation energy of compound nucleus. Conclusions: The investigated properties of asymmetric fission of Hg180,182,183 and Pt178 nuclei point out the existence of well-deformed proton shell at Z≈36 and a less deformed proton shell at Z ≈ 46.