Evolution of fission properties in Fermium region.

In this paper, we model the fission process by assuming that at certain elongation, after crossing the fission barrier, a fissile nucleus can be treated as a superposition of dinuclear systems (DNS). The distribution of primary fission fragments is described as a result of competition between evolution of initially formed DNS and its decay in relative distance. The level densities required for the calculations were microscopically derived accounting for deformation and excitation energy effects. The calculations performed for even 2 4 4 − 2 6 0 Fm isotopes give overall good description of mass and neutron multiplicity distributions. To describe sudden onset of symmetric fission in 2 5 8 Fm, the fissile nucleus is treated as superposition of DNS at smaller elongations than for lighter Fm isotopes, which is in line with significant reduction of half-life for 2 5 8 Fm. Our results indicate the presence of bimodality due to coexistence of spherical and deformed mass symmetric fission modes. [ABSTRACT FROM AUTHOR]