Nucleon number dependence of longitudinal radii in ion-ion collisions as a signature of the onset of collective expansion

It is shown first that nuclear geometry is responsible for only a very small part of observed nucleon number dependence of longitudinal radius R L for proton- and light-ion interactions with nuclear targets in the CERN-SPS energy region. This dependence can be described by a model based on multiple nucleon-nucleon sub-collisions provided that a sub-collision in proton-nucleus or nucleus-nucleus collisions differs from that of free nucleons. We present arguments, based on the principle of uncertainity, which ascribe this difference to short space- and time-intervals between successive sub-collisions. On the other hand the PbPb data give R L significantly higher than such a picture could accomodate, presenting an evidence for the onset of a new dynamical regime, probably connected with the expansion of matter, between OAu and PbPb collisions.