  Using isospin dependent quantum molecular dynamical model, the studies of the
isospin effect on preequilibrium nucleon emission in heavy ion collisions under
different entrance channel conditions show that the ratio of preequilibrium
neutron number to proton number depends strongly on symmetry potential, beam
energy, and the ratio of neutron to proton of the colliding system, but weakly
on isospin dependent in-medium nucleon-nucleon cross sections, impact
parameter, Pauli potential, and momentum dependent interaction in the energy
region from 45MeV/u up to 150 MeV/u where the dynamics is dominated by
nucleon-nucleon collisions. In addition, the ratio of preequilibrium neutron
number to proton number for a neutron-rich colliding system is larger than the
initial value of the ratio of the colliding system, but the ratio for a
neutron-deficient system is less than the initial value.
