Origin of the dramatic change of fission mode in fermium isotopes investigated using Langevin equations

Miyamoto, Yuya*; Aritomo, Yoshihiro*; Tanaka, Shoya*; Hirose, Kentaro  ; Nishio, Katsuhisa   

The fission of even-even fermium nuclides Fm at low excitation energy was studied using Langevin equations of three-dimensional nuclear-shape parameterization. The mass distributions of fission fragments show a dramatic change from an asymmetric shape for the lighter fermium isotopes to sharp symmetric fission for the heavier isotopes. The time-evolution of the nuclear shape on the potential surface reveals that the lighter fermium isotopes showing asymmetric fission are trapped in the second minimum for a substantial length of time before overcoming the second saddle point. This behavior changes dramatically for the compact symmetric fission found in the heavier neutron-rich fermium nuclei that disintegrate immediately after overcoming the first saddle point, without feeling the second barrier, resulting in a fission time two orders of magnitude shorter. The results provide an explanation for the sudden drop of the partial half-life for spontaneous fission from Fm to Fm.