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 fr agmentation of uranium and lead projectiles are analysed with the abra sion-ablation model taking into account fission in the deexcitation ch ain. The survival probability against fission of excited compound nucl ei is a sensitive probe for shell and collective effects in the nuclea r level density, The expected stabilization against fission for neutro n-deficient actinides near the magic number N = 126 due to large groun d-state shell effects is not found in the experimental data. With the inclusion of collective enhancement in the level density the experimen tal data can be described well. The damping of the collective enhancem ent with excitation energy has to be treated as essentially independen t of nuclear deformation, a new finding which contradicts the current theoretical picture. A schematic description of the vibrational enhanc ement in spherical nuclei is deduced from the data, The lack of shell stabilization against fission observed for the 126-neutron shell is ex pected to have consequences for the production cross sections of spher ical superheavy elements around N = 184. (C) 1998 Elsevier Science B.V .