Excitation energy dependence of fragment-mass distributions from fission of Hg formed in fusion reactions of Ar + Sm

Nishio, Katsuhisa   ; Andreyev, A. N.*; Chapman, R.*; Derkx, X.*; Dllmann, C. E.*; Ghys, L.*; Heberger, F. P.*; Hirose, Kentaro  ; Ikezoe, Hiroshi*; Khuyagbaatar, J.*; Kindler, B.*; Lommel, B.*; Makii, Hiroyuki   ; Nishinaka, Ichiro; Otsuki, Tsutomu*; Pain, S. D.*; Sagaidak, R.*; Tsekhanovich, I.*; Venhart, M.*; Wakabayashi, Yasuo*; Yan, S.*

Mass distributions of fission fragments from the compound nuclei Hg and Hg formed in fusion reactions Ar+Sm and Ar+Sm, respectively, were measured at initial excitation energies of (Hg)=,3366 MeV and (Hg)=,4871 MeV. In the fission of Hg, the mass spectra were well reproduced by assuming only an asymmetric-mass division, with most probable light and heavy fragment masses /79101. The mass asymmetry for Hg agrees well with that obtained in the low-energy /EC-delayed fission of Tl, from our earlier ISOLDE(CERN) experiment. Fission of Hg is found to proceed in a similar way, delivering the mass asymmetry of /83107, 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 Hg.