Evaporation residue formation in the de-excitation of highly fissile nuclei

The observation of evaporation residues (ER) is clear evidence of the projectile-target fusion. In the case of highly fissile nuclei, the ER production only amounts to a small fraction of the fusion cross-section but becomes a highly sensitive probe for the fission dynamics. By using an advanced statistical code calculation (that is an implementation of the Hauser-Feshbach theory), it is possible to evaluate the influence of the nuclear viscosity and the dynamic dependence of the shell correction to the fission barrier on the de-excitation cascade and residue nucleus production. Furthermore, it is possible to calculate the neutron multiplicity, and using the systematics to evaluate the neutron multiplicity. Particular attention is devoted to the determination of the level densities, which are calculated in the non-adiabatic approach allowing for the rotational and vibrational enhancements. These collective effects are gradually removed above a certain energy. Our level densities acquire a dynamic aspect through the dependence of the Coriolis force and of the rotational enhancement on the nuclear shape. As far as the fission barriers are concerned, we use the rotating droplet model predictions (angular momentum dependent) as parametrized by Sierk and allow for the temperature and angular momentum fade-out of the shell corrections. Dissipation effects, which delay fission, are treated in an approximate (time-independent) approach. These include the Kramer stationary limit and an exponential factor applied to the Kramer fission width to account for the transient time, after which the statistical regime is reached. Nuclei with mass number A > 210 were investigated. (A.A.D.)