Effects of Multichance Fission on Fission Fragment Mass Distributions at High Energies

Nuclear fission is an extremely complex reaction, and still not understood completely. Especially, the behavior at high excitation energy has not been explained because of the lack of experimental data. Recently, fission-fragment mass distributions (FFMDs) were measured for ^237–240U, ^239–242Np, and ^241–244Pu populated in the excitation-energy range from 10 to 60 MeV by multinucleon transfer channels in the reaction ¹⁸O + ²³⁸U at the Japan Atomic Energy Agency tandem facility. A persistence of predominantly asymmetric FFMDs was observed up to the highest measured excitation energy for all the studied nuclides. To understand this behavior, we use the fluctuation-dissipation model and employ Langevin equations to obtain FFMDs and clarify multichance fission (MCF) effects. The MCF concept is well-known from studies of several fission observables, however FFMDs are almost not investigated. FFMDs are calculated for twenty-nine compound nuclides ^231-234Th, ^233-236Pa, ^234-240U, ^236-242Np and ^238-244Pu with the excitation energy range from 15 to 55 MeV. The calculation results show generally good agreement with experimental data. It was shown that a reliable understanding of the observed FFMDs can be obtained only by invoking MCF.