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) = 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 = 269 which have a small interval of outgoing angles of theta(lab) = 40 degrees-50 degrees.
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