Mass dependence of the disappearance of flow in nuclear collisions

The disappearance of collective flow in nucleus-nucleus collisions occurs at an incident energy (Ebal) where the attractive scattering dominant at low energies balances the repulsive scattering dominant at high energies. We have performed the first systematic study of the entrance-channel mass dependence of the disappearance of flow and hence Ebal. The new data presented for the C+C, Ne+Al, Ar+Sc, and Kr+Nb systems show that Ebal scales as A-1/3 where A is the mass of the combined system. Boltzmann-Uehling-Uehlenbeck model calculations show trends which are in qualitative agreement with these new results.