Abstract
The so-called super-short fission mode, in which a nucleus divides nearly symmetrically into two unusually energetic fragments, competes favorably with the standard asymmetric fission mode for spontaneous fission of a limited number of nuclei near $^{264}\mathrm{Fm}$ but it quickly fades away at finite excitations. We investigate the energy-dependent competition between those two fission modes for even fermium isotopes from $^{254}\mathrm{Fm}$ to $^{268}\mathrm{Fm}$, using the Metropolis method to simulate the strongly damped fission dynamics being driven by shape- and energy-dependent level densities. The origin of the super-short mode is discussed and its effects on the fragment mass distribution, the total fragment kinetic energy, and the neutron multiplicity are calculated. Generally good agreement with the available data is obtained.
Highlights
Nuclear fission presents a rich variety of challenging theoretical problems in which structure and dynamics are interwoven [1]
We have extracted the fragment mass distribution, the total fragment kinetic energy (TKE) distribution, and ν, the average total number of neutrons evaporated from the fragments
We have studied the fission properties of fermium isotopes by simulating the diffusive shape evolution and analyzing the resulting scission configurations
Summary
Nuclear fission presents a rich variety of challenging theoretical problems in which structure and dynamics are interwoven [1]. A special challenge is presented by the presence of classically forbidden regions through which the shape must tunnel at the lowest energies considered For this problem, we employ a modified Metropolis walk ( E method) in which the total energy is initially increased by an amount E sufficient to overcome the lowest barrier and we reduce it back to its physical value once it becomes possible. We employ a modified Metropolis walk ( E method) in which the total energy is initially increased by an amount E sufficient to overcome the lowest barrier and we reduce it back to its physical value once it becomes possible In this way, the diffusive paths divide into two different shape ensembles, one for each fission mode, whose further evolution can be continued until scission by use of the standard Metropolis method. Valley is very low, less than 1 MeV, and the branching ratio is practically 100%
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