By means of using an isospin-dependent Boltzmann-Langevin equation which in
cludes isospin-dependent symmetry energy, Coulomb energy, isospin-dependent
nucleon-nucleon cross sections, Pauli blocking, and initialization, the ra
dial expansion flow of reaction systems Ca-40 + Ni-58 and Ca-40 + Fe-58 at
53, 100, 150, and 200 MeV/u in the central collisions were studied. It has
shown that the more neutron-rich system exhibits smaller radial expansion f
low. It was found that the neutron-rich system had smaller threshold energy
which may provide a new method to determine the isospin dependent nuclear
equation of state from calculated result and linear fitting result.