Pre-Scission Model Calculation of Fission Fragment Mass and Total Kinetic Energy Distributions for Even-Even Fm, No and Rf Isotopes

The main properties of the fission fragments in spontaneous fission of even-even isotopes of Fm (Z=100), No (Z=102) and Rf (Z=104) are estimated using a pre-scission point model. The underlying potential energy surfaces are calculated with Strutinsky's shell correction procedure. The parametrization of the nuclear shapes is based on Cassini ovals generalized by the inclusion of three additional shape parameters: α 1, α 4 and α 6. It represents a natural way to describe scission configurations. The corresponding fragment-mass distributions are estimated supposing they are due to thermal fluctuations in the mass asymmetry degree of freedom. A detailed comparison with all existing data for Fm, No and Rf isotopes is presented. For these three series of isotopes the experimentally observed transition from asymmetric to symmetric fission, that happens with increasing mass number A, is well reproduced. In lighter isotopes (e.g. 254 Fm and 254 Rf) two mass-asymmetric fission modes are predicted to occur with comparable yields: one having relatively compact and the other relatively elongated scission configurations. On the other hand, in heavier isotopes (e.g. 264 Fm and 264 Rf) the fragment-mass distributions are predicted to be narrow single-peaked around A/2 corresponding to essentially one compact fission mode. We call this type of fission "super-symmetric". The corresponding distributions of the total kinetic energy of the fragments are also calculated (in the point-charge approximation) and compared with measurements. Despite the fact that the dynamical effects were neglected, we have obtained a quantitative agreement with the experimental data.