Shell Stabilization in Compound Nucleus Survival.

The formation of evaporation residues from compound nuclei is ultimately limited by the competition of fission and particle evaporation. In the super-heavy elements (SHE) nuclear structure effects play a paramount role in the stabilization against spontaneous fission, e.g. the fission barrier consists entirely of the ground-state shell effect. This work investigates the influence of this stabilization on the formation of SHE. This shell stabilization against fission for the predicted SHE island around N=184 cannot be investigated directly. The best probing ground reachable is the transition region from spherical to deformed actinides around N=126. There, the liquid-drop part of the fission barrier and shell-effects are of comparable magnitude. The production of these nuclei has been thoroughly investigated at GSI in fusion-evaporation reactions of different projectile-target combinations leading to thorium isotopes, in projectile fragmentation of uranium, and in electromagnetic-induced fission of proton-deficient actinides between astatine and uranium. In all these experiments the expected stabilization against fission through the spherical 126-neutron shell is not found. Collective effects in the level density tend to cancel the shell stabilization. We conclude that spherical SHE loose at least great part of their stability against fission at rather low excitation energies and angular momenta. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]