Production mechanism of neutron-rich transuranium nuclei inU238+U238collisions at near-barrier energies

Abstract

The improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model is applied to study the production mechanism of transuranium nuclei in the reaction of $^{238}\mathrm{U}+^{238}\mathrm{U}$ at 7.0 MeV/nucleon. The production of primary fragments in the dynamical process is simulated by the ImQMD model, and the decays of them are described by the statistical evaporation model (hivap code). The calculated isotope distributions of the residual fragments and the most probable mass number of fragments are generally in agreement with experimental data. By tracking residual fragments back to their original primary fragments with this approach, we find different mechanisms for the production of the residues: the most probable light uraniumlike residues mainly come from the decay of the most probable primary fragments, while the most probable transuranium residues mainly originate from the decay of more neutron-rich primary fragments rather than from the most probable primary ones. For neutron-rich transuranium isotopes $^{254\ensuremath{-}256}\mathrm{Cf}$, the decay channel of neutron evaporation is suppressed due to the quick drop of the fission barrier height with the increase of neutron number, which leads to the quick drop of the production cross sections for these residues.