  Quasi-fission is the primary reaction mechanism that prevents the formation
of superheavy elements in heavy-ion fusion experiments. Employing the
time-dependent density functional theory approach we study quasi-fission in the
systems $^{40,48}$Ca+$^{238}$U. Results show that for $^{48}$Ca projectiles the
quasi-fission is substantially reduced in comparison to the $^{40}$Ca case.
This partly explains the success of superheavy element formation with $^{48}$Ca
beams. For the first time, we also calculate the repartition of excitation
energies of the two fragments in a dynamic microscopic theory. The system is
found in quasi-thermal equilibrium only for reactions with $^{40}$Ca. The
differences between both systems are interpreted in terms of initial neutron to
proton asymmetry of the colliding partners.
