Transitions between symmetric and asymmetric modes in the region of heavy actinides

Using the improved scission-point model, the mass and charge distributions of fragments resulting from the fission of californium, fermium, and nobelium isotopes are calculated and compared with the available experimental data. For the fissioning Fm and No nuclei, the transitions from a two-peaked to a single-peaked mass distributions are predicted. The transition between asymmetric and symmetric fission modes with the variation of neutron number is shown to be related to the change of the potential energy surface at scission point.