Mass number scaling in ultra-relativistic nuclear collisions from a hydrodynamical approach

We study the different nucleus-nucleus collisions, O+Au, S+S, S+Ag, S+Au an d Pb+Pb, at the CERN-SPS energy in a one-fluid hydrodynamical approach usin g a parametrization based on baryon stopping in terms of the thickness of c olliding nuclei. Good agreement with measured particle spectra is achieved. We deduce the mass number scaling behaviour of the initial energy density. We find that the equilibration time is nearly independent of the size of t he colliding nuclei.