THE CONDITIONS FOR THE SYNTHESIS OF HEAVY-NUCLEI

In parallel to the attempts to synthesize the heaviest nuclei, systematic studies have been made to obtain better understanding of the reaction aspects. The most comprehensive data have been taken for nearly mass-symmetric massive systems. They combine high Coulomb forces in the entrance channel with evaporation-residue cross sections which are high enough to be easily detectable. With these systems, rather cold compound nuclei can be produced, and even radiative fusion was observed. While one of the most salient features of the thoroughly studied fusion of light and medium-heavy systems is the enhanced sub-barrier fusion, the massive systems exhibit a considerable deficit of fusion above the expected potential barrier. This hindrance to fusion may be attributed to the dynamical evolution of the composite system which may lead to immediate reseparation. The experimental data reveal that the hindrance to fusion is strongly influenced by the nuclear structure of the reaction partners. The high fission competition in the evaporation cascade of fissile excited compound systems as deduced from measured evaporation-residue cross sections is compared with the expectations of the statistical model. Experimental evidence is presented that the increase of the fission barriers due to spherical shell effects in the ground state does not reduce the fission probability in the de-excitation process. A re-examination of the nuclear level density reveals essentially two explanations for this surprising effect: the temperature-induced deformation due to the collective enhancement in the nuclear level density and the mutual support of magicities due to residual neutron-proton interactions which depend on the occupation of the single-particle levels. In contrast to lighter systems, the evaporation residues of the most massive symmetric systems revealed some indication that the fusion and de-excitation processes are not decoupled by the intermediate stage of a compound nucleus but that they overlap in time and influence each other. With the results of the investigations of the nearly mass-symmetric systems in mind, several characteristics found in the synthesis of the heaviest elements are analysed.