ANALYSIS OF DISSIPATIVE EFFECTS IN NUCLEAR-FISSION OBSERVED IN THE FRAGMENTATION OF U-238 PROJECTILES

The abrasion-ablation model widely used previously to describe periphe ral nuclear reactions at relativistic energies has been generalized to be applicable to heavy reaction partners for which it is important to account for nuclear fission. It is shown that the changes of the fiss ion widths and the transient times of the competing decay modes caused by the nuclear viscosity must be included to consistently describe th e available experimental data. The model predictions are in good agree ment with measured total fission cross sections for high-energy U-238 projectiles impinging on different targets. Furthermore, recently meas ured production cross sections of neutron-deficient U and Pa isotopes in the fragmentation of U-238 projectiles at 950A MeV are well describ ed by the model, if a nuclear viscosity parameter of beta = 1 x 10(21) s(-1) is chosen. The model predicts rather high production cross sect ions for a wide range of fissile exotic nuclei which may be produced b y the fragmentation of U-238.