  The average transverse energy of nucleons and intermediate mass fragments
observed in the heavy ion reaction Xe(50A MeV)+Sn shows the same linear
increase as a function of their mass as observed in heavy ion collisions up to
the highest ene rgies available today and fits well into the systematics. At
higher energies this observation has been interpreted as a sign of a strong
radial flow in an otherwise thermalized system. Investigating the reaction with
Quantum Molecular dynamics simulations we find in between 50A MeV and 200A MeV
a change in the reaction mechanism. At 50A MeV the apparent radial flow is
merely caused by an in-plane flow and Coulomb repulsion. The average transverse
fragment energy does not change in the course of the reaction and is equal to
the initial fragment energy due to the Fermi motion. At 200A MeV, there are two
kinds of fragments: those formed from spectator matte r and those from the
center of the reaction. There the transverse energy is caused by the pr essure
from the compressed nuclear matter. In both cases we observe a binary event
stru cture, even in central collisions. This demonstrates as well the non
thermal character of the reaction. The actual process which leads to
multifragmentation is rather complex and is discussed in detail.
