The disappearance of collective flow in nucleus-nucleus collisions occurs at an incident energy (^bai) where the attraxitive 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 -Ebai-The new data presented for the C-j-C, Ne-hAl, Ar+Sc, and Kr+Nb systems show that JE^bai scales as A~^^^ 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.

We consider the dependence of collective Bow on the nuclear surface thickness in a Boltzmann-Uehling-Uhlenbeck transport model of heavy ion collisions. Well-de6ned surfaces are introduced by giving test particles a Gaussian density profile of constant width. Zeros of the How excitation function are as much inBuenced by the surface thickness as the nuclear equation of state, and the dependence of this effect is understood in terms of a simple potential scattering model. Realistic calculations must also take into account medium effects for the nucleon-nucleon cross section, and impact parameter averaging. We find. that balance energy scales with the mass number as A ", where y has a numerical value between 0.35 and 0.5, depending on the assumptions about the in-medium nucleon-nucleon cross section. PACS number(s): 25.70.z, 02.70.Lq, 21.65.+f

No abstract