Fission-evaporation competition in excited uranium and fermium nuclei

The production cross sections and excitation functions for the neutron deficient isotopes have been measured in the and reactions for n and (4-7)n evaporation channels of the de-excitation of the compound nuclei and , respectively. This study considers in addition the de-excitation via the n evaporation channels of the compound nucleus formed in the reaction. The production cross sections of formed after evaporation of n and n from the and compound nuclei produced in the and reactions were also measured respectively. The evaporation residues emerging from the target were separated in-flight 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 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 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.

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