Impact parameter dependence of the disappearance of flow and in-medium nucleon-nucleon cross section

The impact parameter dependence of the disappearance of nuclear flow is exa mined within the framework of the quantum molecular dynamics model. We conf ront the model with recent experimental findings of He et al. who measured the balance energy at different impact parameters for the reaction Zn-64 Al-27. We simulate the heavy ion collisions with different nucleon-nucleon cross sections which includes the energy-dependent cross section, in-medium cross section (G matrix),and several constant and isotropic cross sections . Our calculations show that the balance energy in central collisions can b e explained nicely with standard energy-dependent cross section whereas one needs a larger cross section to explain the balance energy at peripheral c ollisions. [S0556-2813(98)06009-9].