  A study of medium-mass heavy-ion reactions leading to two fragments in the
exit channel from barrier to 18A MeV is proposed. A special focus is made on
fission and quasi-fission for events with two fragments ($Z\geq10$) in the exit
channel selected. Reactions induced by $^{129}Xe$ projectiles on $^{nat}Sn$ and
on $^{197}Au$ at energies ranging from $8A$ MeV to $18A$ MeV were analyzed.
Using the fragment ($Z\geq10$) multiplicity equal to 2, fission and
quasi-fission events were studied for the lowest beam energies using the
fission fragment charge distributions, the total kinetic energy distribution
(TKE) and its standard deviation $\sigma_{TKE}$. For the lighter system it is
still possible to observe fission events from incomplete fusion. At variance,
for the heavier system, Xenon on gold target only quasi-fission is evidenced.
The study of the events characterized by two fragments in the final channel
shows that fission, related to fusion in the entrance channel, disappears
around 20A MeV for both systems. At lower energies (8A, 12A and 15A MeV) for
the Sn target, an evolution with increasing energies towards an asymmetric
fission mode is displayed in the fragment charge distributions. This trend can
be attributed to the increasing angular momentum as indicated by the
out-of-plane angular distribution for light charged particles. The same effect
is also observed in the case of the gold target at 15A MeV. However, for this
heavier system, a strong memory of the entrance channel leading to
quasi-fission is evidenced. A comparison with the Viola systematics, moreover,
shows a deviation, greater for the heavier system than for $^{129}Xe+^{nat}Sn$
system. A complete understanding of the above results would certainly require
precise model calculations at these energies.
