  Quasi-fission (QF) reactions occur in fully damped heavy-ion collisions
without the formation of an equilibrated compound nucleus, leading to the
formation of fragments with similar properties as in fission reactions. Similar
shell effects are expected to affect fragment formation in both fission and QF.
Our purpose is to investigate QF dynamics in different reactions forming the
same compound nucleus and search for possible signatures of shell effects in
fragment formation. 50Ca+176Yb and 96Zr+130Sn QF reactions are simulated with
the time-dependent Hartree-Fock code Sky3D near the Coulomb barrier. Evolutions
of the quadrupole (Q20) and octupole (Q30) moments are interpreted in terms of
features of the potential energy surface (PES) of the 226Th compound nucleus.
Both reactions encounter QF. In 50Ca+176Yb, those only occur at finite angular
momenta. In the more symmetric 96Zr+130Sn reaction with stronger Coulomb
repulsion in the entrance channel, QF also occurs in central collisions. In
agreement with earlier predictions, 50Ca+176Yb encounters partial mass
equilibration that is stopped when the heavy fragment reaches Z~54 protons, as
in the asymmetric fission mode of 226Th. 96Zr+130Sn encounters an inverse QF
also leading to similar fragments as in asymmetric fission. In both systems, QF
trajectories in the Q20-Q30 plane follow the asymmetric fission valley of 226Th
PES. The observation of an inverse QF is a clear prediction that shell effects
have a strong influence in QF. The similarity between fragments formed in
asymmetric fission and QF supports the idea that the same shell effects are at
play in both mechanisms. Interpreting QF dynamics with PES used in fission is
naturally limited by the fact that these PES are usually computed with axial
symmetry, no angular momentum and no excitation energy, thus motivating future
developments of PES for QF.
