Sequential fission and the influence of ²⁰⁸Pb closed shells on the dynamics of superheavy element synthesis reactions

Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic ²⁰⁸Pb. This has generally been attributed to the enhanced binding energy of ²⁰⁸Pb causing a valley in the potential energy surface, attracting quasifission trajectories. To investigate this interpretation, binary quasifission mass spectra and cross-sections have been measured at near-barrier energies for reactions of ⁵⁰Ti with actinide nuclides from ²³⁸U to ²⁴⁹Cf. Cross-sections have also been deduced for sequential fission (a projectile-like nucleus and two fragments from fission of the complementary target-like nucleus). Binary cross-sections fall from ∼70% of calculated capture cross-sections for ²³⁸U to only ∼40% for ²⁴⁹Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the ²⁰⁸Pb peak originating largely from sequential fission of heavier fragments produced in more mass-asymmetric primary quasifission events. These are increasingly suppressed as the heavy quasifission fragment mass increases above ²⁰⁸Pb. The important role of sequential fission calls for re-interpretation of quasifission characteristics and dynamics in superheavy element synthesis reactions.