Fission-Evaporation Competition In Excited Uranium And Fermiun Nuclei

The production cross sections and excitation functions for the U223-226 neutron deficient isotopes have been measured in the Ne-20 + Pb-208 and Ne-22 + Pb-208 reactions for (4, 5)n and (4-7)n evaporation channels of the de-excitation of the compound nuclei U-228* and U-230*, respectively. This study considers in addition the de-excitation via the (5, 6)n evaporation channels of the U-224* compound nucleus formed in the Al-27 + Au-197 reaction. The production cross sections of Fm-247mg,Fm- 246 formed after evaporation of (5, 6)n and (7, 8)n from the Fm-252* and Fm-254* compound nuclei produced in the Ne-20 + Th-232 and Ne-22 + Th-232 reactions were also measured respectively. The evaporation residues emerging from the target were separated inflight from the projectiles and background reaction products by the electrostatic recoil separator VASSILISSA (Yeremin A V et al 1994 Nucl. Instrum. Methods A 350 608). The investigation regards the U and Fm compound nuclei in the 40-80 MeV excitation energy range. For the analysis of the (H I, xn) evaporation cross sections the advanced statistical model (D'Arrigo A et al 1994 Phys. Lett. B 334 1) calculations were used. The angular-momentum dependence of the shell correction to the fission barrier, and the effects of the nuclear viscosity and dynamical deformation for these fissile excited nuclei are considered. The (Gamma(n)/Gamma(t)) values at the initial steps of the de-excitation cascade for the U and Fm compound nuclei were derived from the measured excitation functions and discussed from the point of view of the consequences for the fission-process dynamics.