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The formation of superheavy elements (SHEs) by nuclear fusion can be conceptually divided into two steps: capture, and compound nucleus formation. Once captured, the two nuclei may reseparate before fusing to form a compact compound nucleus. This outcome is called quasifission. Fusion-fission, in many cases, leads to reactions outcomes inseparable from quasifission. Evaporation residue (ER) measurements are therefore the most reliable, direct experimental signature of fusion. ER cross section measurements forming the same compound nucleus, 220Th, using 16O, 40Ar, 48Ca, 82Se and 124Sn-induced reactions [1–4] revealed [1] that fusion was severely suppressed for the more symmetric reactions relative to the 16O-induced reaction. Here two new reactions forming 220Th using 28Si, 34S projectiles provide conclusive evidence that the ER cross section is exponentially suppressed as a function of ZpZt. The fission characteristics show no mass-angle correlation, demonstrating that here ER cross sections are suppressed by slow quasifission.

Light, weakly-bound nuclides such as 6,7Li and 9Be exhibit a diverse range of near-barrier reaction phenomena. Complete fusion is found to be suppressed by up to 35% with respect to barrier-passing model calculations. Their weak binding leads to significant breakup through excitation of the continuum, but also through production of weakly-bound or unbound neighbouring nuclides via nucleon transfer. Such systems also exhibit large yields of incomplete fusion, which is correlated empirically with cluster breakup thresholds for these stable nuclides. Disentangling these reaction phenomena and their complex interplay is one of the most interesting challenges in near-barrier reaction dynamics. Here we briefly review our recent progress in understanding the interplay between breakup and fusion. These results strongly suggest that the dominant mechanism leading to incomplete fusion and the suppression of complete fusion is direct capture of the projectile cluster constituents by the target nucleus, rather than requiring projectile breakup prior to fusion.

A novel fusion-evaporation residue separator based on a gas-filled superconducting solenoid has been developed at the Australian National University. Though the transmission efficiency of the solenoid is very high, precision cross sections measurements require this efficiency to be accurately known and vitally, requires knowledge of the angular distribution of the evaporation residues. We have developed a method to deduce the angular distribution of the evaporation residues from the laboratory-frame velocity distribution of the evaporation residues transmitted by the solenoid. The method will be discussed, focusing on benchmarking examples for 34S+89Y, where the angular distributions have been independently measured using a velocity filter (A. Mukherjee et al., Phys. Rev. C. 66, 034607 (2002)) . The establishment of this method now allows the novel solenoidal separator to be used to obtain reliable, precise fusion cross-sections.

A novel fusion product separator based on a gas-filled superconducting solenoid has been developed at the Australian National University. Though the transmission efficiency of the solenoid is very high, precision cross sections measurements require this efficiency to be estimated accurately. A Monte Carlo simulation for the effciency can be performed, but in turn requires knowledge of the angular distribution of the evaporation residues. We have developed a method to deduce the angular distribution of the evaporation residues from the laboratory-frame velocity distribution σexp(vl) of the evaporation residues measured after the solenoid. The method will be discussed, focussing on the example of 34S+89Y, where the angular distribution has been independently measured using a velocity filter [1]. The establishment of this method now allows the novel solenoidal separator to be used to obtain the most reliable, precision fusion cross-sections.

In collisions of light, stable, weakly bound nuclides, complete fusion (capture of all of the projectile charge) has been found to be suppressed by ∼30% at above-barrier energies. This is thought to be related to their low thresholds for breakup into charged clusters. The observation of fusion suppression in the neutron-rich radioactive nucleus 8Li is therefore puzzling: the lowest breakup threshold yields 7Li+n which cannot contribute to fusion suppression because 7Li retains all the projectile charge. In this work, the full characteristics of 8Li breakup in reactions with 209Bi are presented, including, for the first time, coincidence measurements of breakup into charged clusters. Correlations of cluster fragments show that most breakup occurs too slowly to significantly suppress fusion. However, a large cross section for unaccompanied α particles was found, suggesting that charge clustering, facilitating partial charge capture, rather than weak binding is the crucial factor in fusion suppression, which may therefore persist in exotic nuclides. [ABSTRACT FROM AUTHOR]

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.