  The production cross sections for primary and residual fragments with charge
number from $Z$=70 to 120 produced in the collision of $^{238}$U+$^{238}$U 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 sixty unknown neutron-rich isotopes
from element 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 unknown neutron-rich isotopes are emitted at the laboratory
angles $\theta_{lab}\leq$ 60$^\circ$. Two cases, i.e. the production of the
unknown uranium isotopes with $A\geq$ 244 and that of rutherfordium with
$A\geq$ 269 are investigated for understanding 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 $\leq$ 30 MeV and
the outgoing angles of those residues cover a range of 30$^\circ$-60$^\circ$.
For the later case, the longer collision time is needed for a large number of
nucleons being transferred and thus it results in the 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
$A\geq$ 269 which have a small interval of outgoing angles of
$\theta_{lab}$=40$^\circ$-50$^\circ$.
