  The density-dependent symmetry energy is a hot topic in nuclear physics. Many
laboratories over the world are planning to perform related experiments to
probe the symmetry energy. Based on the semiclassical
Boltzmann-Uehling-Uhlenbeck (BUU) transport model, we study the effects of
nuclear symmetry energy in the central reaction 40Ca+124Sn at 140MeV/nucleon in
the laboratory system. It is found that the rapidity distribution of free
nucleon's neutron-to-proton ratio is sensitive to the symmetry energy,
especially at large rapidities. The free neutron-to-proton ratios at small or
large rapidities may reflect high or low density behavior of nuclear symmetry
energy. To probe the density dependence of nuclear symmetry energy, it is
better to give the kinetic distribution and the rapidity distribution of
emitted nucleons at the same time.
