MASS DEPENDENCE OF DIRECTED COLLECTIVE FLOW

Sidewards directed fragment flow has been extracted for Kr-84 + Au-197 collisions at E/A = 200 MeV, using techniques that are free of reacti on plane dispersion. The fragment flow per nucleon increases with mass , following a thermal or coalescencelike behavior, and attains roughly constant limiting values at 4 less than or equal to A less than or eq ual to 12. Comparisons of the impact parameter dependences of the meas ured coalescence-invariant proton flow to Boltzmann-Uehling-Uhlenbeck calculations clearly favor a momentum dependent nuclear mean field.