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, generally attributed to enhanced binding energy causing a valley in the potential energy surface, which attracts quasifission trajectories. Measurements of binary quasifission mass spectra and cross-sections have been made for reactions of ⁵⁰Ti with actinide nuclides from ²³²Th 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 ²³²Th to only 40% for ²⁴⁹Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the peak in yield near ²⁰⁸Pb 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 observables and dynamics in superheavy element synthesis reactions.