Mass dependence of the disappearance of flow in nuclear collisions

The disappearance of collective flow in nucleus-nucleus collisions occurs at an incident energy ([ital E][sub bal]) 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 [ital E][sub bal]. The new data presented for the C+C, Ne+Al, Ar+Sc, and Kr+Nb systems show that [ital E][sub bal] scales as [ital A][sup [minus]1/3] where [ital A] is the mass of the combined system. Boltzmann-Uehling-Uehlenbeck model calculations show trends which are in qualitative agreement with these new results.