Quasifission and deep inelastic collisions competing with superheavy element creation

Fusion forming superheavy elements is strongly inhibited by the faster non-equilibrium Deep Inelastic (DIC) and quasifission processes. These have often been considered as distinct processes, but recent measurements for reactions involving heavy nuclei such as ²⁰⁸Pb and lighter suggest that these two processes form a continuum. However, for reactions of heavy ions with actinide nuclei, binary mass-split spectra show reduced yields for fragments lighter than the target, resulting in a peak in yield close to ²⁰⁸Pb. This gives an apparent separation between DIC and quasifission outcomes. The ²⁰⁸Pb peak has generally been attributed to the ²⁰⁸Pb closed shells giving a valley in the potential energy surface, attracting quasifission trajectories. However, recent extensive binary and three-body cross-sections extracted for reactions of ⁵⁰Ti with actinide nuclides could not be explained in this framework. Rather, the big drop in yield observed for the heavier actinide targets is consistent with sequential fission of heavy deep inelastic/quasifission fragments. To search for shell effects in quasifission independent of sequential fission, systematics of mass spectra in non-actinide reactions forming actinide compound nuclei were studied. These showed negligible effects of the shells known to cause low energy mass-asymmetric fission of these nuclei. All these results raise questions over the understanding of the effects of closed shells on the quasifission mechanism, and quasifission mass distributions.