Production of unknown neutron-rich isotopes in 238U+238U collisions at near-barrier energy

The production cross sections for primary and residual fragments with charge number from Z = 70 to 120 produced in the collision of U-238 + U-238 at 7.0 MeV/nucleon are calculated by the improved quantum molecular dynamics (ImQMD) model incorporated with the statistical evaporation model (HIVAP code). The calculation results predict that about 60 unknown neutron-rich isotopes from elements Ra (Z = 88) to Db (Z = 105) can be produced with the production cross sections above the lower bound of 10(-8) mb in this reaction. And almost all of the unknown neutron-rich isotopes are emitted at the laboratory angles theta(lab) <= 60 degrees. Two cases, i.e., the production of the unknown uranium isotopes with A >= 244 and that of rutherfordium with A >= 269, are investigated to understand the production mechanism of unknown neutron-rich isotopes. It is found that for the former case the collision time between two uranium nuclei is shorter and the primary fragments producing the residues have smaller excitation energies of <= 30 MeV and the outgoing angles of those residues cover a range of 30 degrees-60 degrees. For the latter case, a longer collision time is needed for a large number of nucleons being transferred and thus it results in higher excitation energies and smaller outgoing angles of primary fragments, and eventually results in a very small production cross section for the residues of Rf with A >= 269 which have a small interval of outgoing angles of theta(lab) = 40 degrees-50 degrees.