Mass dependence of critical behavior in nucleus-nucleus collisions

The Z distributions of fragments emitted from central collisions of Ar40+45Sc at beam energies from 15 to 115 MeV/nucleon have been fitted to power laws σ(Z)Z-λ. The λ parameter reaches a minimum at a beam energy of 23.9±0.7 MeV/nucleon. A percolation model calculation reproduces the observed Z distributions for all beam energies, using the mean excitation energy as extracted from proton kinetic energy spectra. We extract the critical value of the deposited excitation energy for our system and make predictions for the dependence of this quantity on the size of the fragmenting system.