Impact of nuclear structure of correlated pairs of fission fragments in mass distribution spectra of heavy-ion fusion-fission reactions

The nuclear structure effects of correlated pairs of fission fragments in the mass distribution spectrum of the fissioning nucleus are investigated using the fragmentation theory based on the asymmetric two-centre shell model. The model is applied to explain the observed structure in relative mass distribution yields of even Z-even N correlated fission fragment pairs in the low-energy heavy-ion induced fusion-fission reactions 208 Pb(18 O,f) and 238 U(18 O,f). In each of these reactions, the respective fused system is considered to utilise its excitation energy in 0-14 neutron evaporation channels before proceeding to decay. The fissioning isotopes (^226-212Th and ^256-242Fm) resulted after neutron evaporation (in the step of Δ_n = 2) contribute to the observed even-even individual fission fragments mass distribution spectra. It is shown that the most probable decay channel consisting of correlated pair of magic and deformed shell fission fragments always has a larger mass distribution yield than that containing both magic shell structure fission fragments. The pear-shaped structure that emerges from the combined magic plus deformed shell fission fragments might have relevance to an occurrence of stable octupole deformation of a fissioning nucleus before saddle configuration, which was first noticed by Johansson (S A E Johansson, Nucl. Phys. 22, 529 (1961)) and more recently, authenticated by Scamps and Simenel (G Scamps and C Simenel, Nature 564, 382 (2018)). Thus, the combined quantum shell effects of magic and deformed fission fragments both in potential and the cranking mass parameters decide the most probable decay modes (i.e., the correlated pairs of fission fragments with maximum yields) in fusion-fission reactions (²⁰⁸Pb(¹⁸O,f) and ²³⁸U(¹⁸O,f)). (author)