Role of multichance fission in the description of fission-fragment mass distributions at high energies

Hirose, Kentaro  ; Nishio, Katsuhisa   ; Tanaka, Shoya*; Lguillon, R.*; Makii, Hiroyuki   ; Nishinaka, Ichiro*; Orlandi, R.  ; Tsukada, Kazuaki  ; Smallcombe, J.*; Vermeulen, M. J.; Chiba, Satoshi*; Aritomo, Yoshihiro*; Otsuki, Tsutomu*; Nakano, Keita*; Araki, Shohei*; Watanabe, Yukinobu*; Tatsuzawa, Ryotaro*; Takaki, Naoyuki*; Tamura, Nobuyuki*; Goto, Shinichi*; Tsekhanovich, I.*; Andreyev, A. N.

Fission-fragment mass distributions were measured for U, Np and Pu populated in the excitation-energy range from 10 to 60 MeV by multi-nucleon transfer channels in the reaction O + U at the JAEA tandem facility. Among them, the data for U and Np were observed for the first time. It was found that the mass distributions for all the studied nuclides maintain a double-humped shape up to the highest measured energy in contrast to expectations of predominantly symmetric fission due to the washing out of nuclear shell effects. From a comparison with the dynamical calculation based on the fluctuation-dissipation model, this behavior of the mass distributions was unambiguously attributed to the effect of multi-chance fission.