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

¹ Department of Nuclear Physics and Accelerator Applications, Research School of Physics, Australian National University, Canberra, ACT 2601, Australia
² Department of Fundamental and Theoretical Physics, Research School of Physics, Australian National University, Canberra, ACT 2601, Australia
³ SHE Chemistry Department, GSI Helmholtzzentrum für Schwerionenforschung, 64291 Darmstadt, Germany
⁴ SHE Chemistry Section, Helmholtz Institute Mainz, 55099, Mainz, Germany
⁵ Department of Chemistry - TRIGA Site, Johannes Gutenberg-Universität Mainz, 55099, Mainz, Germany

^* e-mail: david.hinde@anu.edu.au
^* Present address: Defence Science and Technology Group, Canberra, Australia
^* Present address: Department of Physics, Central University of Kerala, Kasaragod 671314, India
^* Present address: Image X Institute, Sydney School of Health Sciences, University of Sydney, Sydney, Australia
^† Present address: Advanced Technology Institute, University of Surrey, Guildford, Surrey, GU2 7XH, UK
^‡ Present address: Airservices Australia, Alan Woods Building, 25 Constitution Ave, Canberra ACT 2600, Australia
^§ Present address: School of Engineering, Australian National University, ACT 2601, Australia

Published online: 18 October 2024

Abstract

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.