Mechanisms Suppressing Superheavy Element Yields in Cold Fusion Reactions

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of Ca48, Ti50, and Cr54 with Pb208. Moving from Ca48 to Cr54 leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity PCN (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for Ti50 and 15 for Cr54 in comparison to Ca48. The energy dependence of PCN indicates that cold fusion reactions (involving Pb208) are not driven by a diffusion process.