Disappearance of flow as a probe of the nuclear equation of state

The disappearance of directed, collective nuclear motion (‘‘flow’’) away from the interaction region of heavy-ion collisions has been observed using the Lawrence Berkeley Laboratory Streamer Chamber. We find that flow vanishes at a beam energy near 50 MeV/nucleon for the ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$ ${+}^{139}$La system and near 60 MeV/nucleon for the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ ${+}^{93}$Nb system. The disappearance of flow may be understood as resulting from a balance between attractive and repulsive scattering strengths. Full calculations with the Boltzmann-Uehling-Uhlenbeck model show that the disappearance of flow is sensitive to the assumed nuclear equation of state (EOS) and to the in-medium scattering cross section (${\mathrm{\sigma }}_{\mathit{N}\mathit{N}}$). Also, in the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ ${+}^{93}$Nb system, the purely attractive contribution to the reduced flow does not appear to be strongly sensitive to the EOS assumptions.