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(50 A.MeV)+Sn shows the same linear increase as a function of their mass as observed in heavy ion collisions up to the highest energies 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 it was found that at 50 A.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. Thus heating of the system is not observed. The observed binary event structure, even in central collisions, 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. (author)