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RESEARCH PRODUCT

Mechanisms Suppressing Superheavy Element Yields in Cold Fusion Reactions.

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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 $^{48}\mathrm{Ca}$, $^{50}\mathrm{Ti}$, and $^{54}\mathrm{Cr}$ with $^{208}\mathrm{Pb}$. Moving from $^{48}\mathrm{Ca}$ to $^{54}\mathrm{Cr}$ 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 ${P}_{CN}$ (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for $^{50}\mathrm{Ti}$ and 15 for $^{54}\mathrm{Cr}$ in comparison to $^{48}\mathrm{Ca}$. The energy dependence of ${P}_{\mathrm{CN}}$ indicates that cold fusion reactions (involving $^{208}\mathrm{Pb}$) are not driven by a diffusion process.