Freeze-out in relativistic heavy ion collisions at SPS

The freeze-out conditions in the light (S+S) and heavy (Pb+Pb) colliding systems at 160 AGeV/c are analyzed within the microscopic Quark Gluon String Model (QGSM). Even for the most heavy systems particle emission takes place from the whole space-time domain available for the system evolution, but not from the thin “freeze-out hypersurface”, adopted in fluid dynamical models. Pions are continuously emitted from the whole volume of the reaction and reflect the main trends of the system evolution. Nucleons in Pb+Pb collisions initially come from the surface region. For both reactions there is a separation of the elastic and inelastic freeze-out.