COLLECTIVE ENCHANCEMENT AND SHELL EFFECTS IN THE LEVEL DENSITY OF SPHERICAL NUCLEI

Production cross sections of thorium, actinium, and radium fragments f rom U-238 projectiles are analyzed on the basis of the abrasion-ablati on model taking into account the fission of heavy nuclei. It is found that the survival probability of highly excited compound nuclei agains t fission is a sensitive probe for the shell and collective effects in the nuclear level density of neutron-deficient actinides near the mag ic number N = 126. With the inclusion of collective enhancement in the level density the calculated survival probability is able to describe the experimental data. The lack of shell-stabilization against fissio n observed for the N = 126 shell is expected to have consequences for the production cross sections of spherical superheavy nuclei around N = 184.