Excitation energy dependence of fragment-mass distributions from fission of 180,190Hg formed in fusion reactions of 36Ar + 144,154Sm.

Mass distributions of fission fragments from the compound nuclei 180 Hg and 190 Hg formed in fusion reactions 36 Ar + 144 Sm and 36 Ar + 154 Sm, respectively, were measured at initial excitation energies of E ⁎ ( Hg 180 ) = 33 – 66 MeV and E ⁎ ( Hg 190 ) = 48 – 71 MeV . In the fission of 180 Hg, the mass spectra were well reproduced by assuming only an asymmetric-mass division, with most probable light and heavy fragment masses A ¯ L / A ¯ H = 79 / 101 . The mass asymmetry for 180 Hg agrees well with that obtained in the low-energy β + / EC -delayed fission of 180 Tl, from our earlier ISOLDE(CERN) experiment. Fission of 190 Hg is found to proceed in a similar way, delivering the mass asymmetry of A ¯ L / A ¯ H = 83 / 107 , throughout the measured excitation energy range. The persistence as a function of excitation energy of the mass-asymmetric fission for both proton-rich Hg isotopes gives strong evidence for the survival of microscopic effects up to effective excitation energies of compound nuclei as high as 40 MeV. This behavior is different from fission of actinide nuclei and heavier mercury isotope 198 Hg. [ABSTRACT FROM AUTHOR]