Sequential fission and the influence of 208Pb closed shells on the dynamics of superheavy element synthesis reactions
D. Y. Jeung, D. J. Hinde, M. Dasgupta, C. Simenel, E. C. Simpson, K. J. Cook, H. M. Albers, J. Buete, I.P. Carter, Ch. E. Düllmann, J. Khuyagbaatar, B. Kindler, Nikolai R. Lobanov, B. Lommel, C. Mokry, E. Prasad, J. Runke, C. Sengupta, J. F. Smith, P. Thörle-Pospiech, Ν. Trautmann, K. Vo-Phuoc, J. Walshe, E. Williams, A. Yakushev
Abstract
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.