K. Banerjee

Above-barrier complete fusion involving nuclides with low binding energy is typically suppressed by 30%. The mechanism that causes this suppression, and produces the associated incomplete fusion products, is controversial. We have developed a new experimental approach to investigate the mechanisms that produce incomplete fusion products, combining singles and coincidence measurements of light fragments and heavy residues in ^{7}Li+^{209}Bi reactions. For polonium isotopes, the dominant incomplete fusion product, only a small fraction can be explained by projectile breakup followed by capture: the dominant mechanism is triton cluster transfer. Suppression of complete fusion is therefore primarily a consequence of clustering in weakly bound nuclei rather than their breakup prior to reaching the fusion barrier. This implies that suppression of complete fusion will occur in reactions of nuclides where strong clustering is present.

A high-statistics, high-resolution, complete kinematical measurement has been performed to estimate the quantitative contributions of various direct 3$\ensuremath{\alpha}$ decay mechanisms in the decay of the famous Hoyle state, the ${0}_{2}^{+}$ resonant excited state of ${}^{12}$C at an excitation energy of 7.654 MeV, using inelastic scattering of 60-MeV $\ensuremath{\alpha}$ particles on ${}^{12}$C. The present observation of nonzero branching ratios of various direct 3$\ensuremath{\alpha}$ decay modes, which have been extracted using $\ensuremath{\sim}$20$\phantom{\rule{0.16em}{0ex}}$000 fully detected (4$\ensuremath{\alpha}$) Hoyle events, is expected to remove the ambiguities among various previous estimates, extracted from fewer ($\ensuremath{\lesssim}$5000) events.

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}Ca, ^{50}Ti, and ^{54}Cr with ^{208}Pb. Moving from ^{48}Ca to ^{54}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}Ti and 15 for ^{54}Cr in comparison to ^{48}Ca. The energy dependence of P_{CN} indicates that cold fusion reactions (involving ^{208}Pb) are not driven by a diffusion process.