MASS DEPENDENCE OF CRITICAL-BEHAVIOR IN NUCLEUS-NUCLEUS COLLISIONS

The Z distributions of fragments emitted from central collisions of Ar -40 + Sc-45 at beam energies from 15 to 115 MeV/nucleon have been fitt ed to power laws sigma(Z) is-proportional-to Z(-lambda). The lambda pa rameter reaches a minimum at a beam energy of 23.9 +/- 0.7 MeV/nucleon . A percolation model calculation reproduces the observed Z distributi ons for all beam energies, using the mean excitation energy as extract ed from proton kinetic energy spectra. We extract the critical value o f the deposited excitation energy for our system and make predictions for the dependence of this quantity on the size of the fragmenting sys tem.