Projectile-fragment yields as a probe for the collective enhancement in the nuclear level density

Production cross sections of several hundred nuclei produced in the fragmentation of uranium and lead projectiles are analysed with the abrasion-ablation model taking into account fission in the deexcitation chain. The survival probability against fission of excited compound nuclei is a sensitive probe for shell and collective effects in the nuclear level density, The expected stabilization against fission for neutron-deficient actinides near the magic number N = 126 due to large ground-state shell effects is not found in the experimental data. With the inclusion of collective enhancement in the level density the experimental data can be described well. The damping of the collective enhancement with excitation energy has to be treated as essentially independent of nuclear deformation, a new finding which contradicts the current theoretical picture. A schematic description of the vibrational enhancement in spherical nuclei is deduced from the data, The lack of shell stabilization against fission observed for the 126-neutron shell is expected to have consequences for the production cross sections of spherical superheavy elements around N = 184. (C) 1998 Elsevier Science B.V.