Role of Multichance Fission in the Description of Fission-Fragment Mass Distributions at High Energies

Fission-fragment mass distributions were measured for $^{237--240}\mathrm{U}$, $^{239--242}\mathrm{Np}$, and $^{241--244}\mathrm{Pu}$ populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction $^{18}\mathrm{O}+^{238}\mathrm{U}$ at the Japan Atomic Energy Agency tandem facility. Among them, the data for $^{240}\mathrm{U}$ and $^{240,241,242}\mathrm{Np}$ 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 multichance fission.