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 RL 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.