On the origin of the radial flow in low energy heavy ion reactions

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.