FISSION-EVAPORATION COMPETITION IN EXCITED URANIUM AND FERMIUN NUCLEI

The production cross sections and excitation functions for the U223-22 6 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 channe ls of the de-excitation of the compound nuclei U-228 and U-230*, resp ectively. This study considers in addition the de-excitation via the ( 5, 6)n evaporation channels of the U-224 compound nucleus formed in t he 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-25 2 and Fm-254* compound nuclei produced in the Ne-20 + Th-232 and Ne-2 2 + Th-232 reactions were also measured respectively. The evaporation residues emerging from the target were separated inflight from the pro jectiles 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 th e 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-m omentum dependence of the shell correction to the fission barrier, and the effects of the nuclear viscosity and dynamical deformation for th ese fissile excited nuclei are considered. The (Gamma(n)/Gamma(t)) val ues at the initial steps of the de-excitation cascade for the U and Fm compound nuclei were derived from the measured excitation functions a nd discussed from the point of view of the consequences for the fissio n-process dynamics.