Characterizing the effect of neutron-richness on the reaction dynamics in chromium and tungsten systems

Superheavy elements are primarily formed through heavy-ion fusion reactions [1, 2, 3]. Formation of a fully equilibrated compound nucleus is a critical step in the heavy-ion fusion reaction mechanism but can be hindered by orders of magnitude by quasifission, a process in which the dinuclear system breaks apart prior to full equilibration [1, 4, 5, 6, 7, 8]. To provide a complete description of heavy-ion fusion it is important to characterize the quasifission process. The interplay between the fusion-fission and quasifission reaction channels was explored by measuring mass distributions in eight different combinations of Cr +W reactions, with varying neutron-richness, at the Australian National University. The reactions were measured in two energy regimes: one at center-of-mass energies (Ec.m.) 13% above the Bass interaction barrier [9] and one at 52.0 MeV of excitation energy in the compound nucleus (E*CN). For the systems measured at the higher energies at Ec.m./ V Bass = 1.13 the dependence on the neutron-richness is clear and an increase in the neutron-richness of the entrance channel decreases the likelihood of quasifission [10]. However, for the reactions at E*CN = 52.0 MeV, the dependence is less clear and additional factors are shown to play a vital role, especially the influence of deformation on the effective fusion barrier. The present work demonstrates that quasifission is an important process in competition with fusion in reactions with intermediate mass projectiles, particularly with more neutron-rich systems.