studies are extended with the isospin asymmetry of the system and the incident energy [@Li03; @Wolt09; @Li09; @Feng10]. In recent years, when the pion ratio has been compared with the FOPI data by using the two well known models IBUU04 and ImIQMD, in terms of isospin asymmetry and incident energy, the predictions for the symmetry energy are found to be totally opposite. ImIQMD predicts stiff symmetry energy ($\gamma_i=2$) [@Feng10], while IBUU04 predicts soft symmetry energy (x=1) [@Li09].

In the present era, the $\pi^-/\pi^+$ ratio is supposed to be a strong candidate for predicting the high-density behavior of symmetry energy. Just as for $\pi^-$ and $\pi^+$, neutrons and protons are also produced in large amounts up to 1 GeV/nucleon. Even around 400 MeV/nucleon, the production of neutrons and protons is greater than the production of pions. Unfortunately, the neutron-to-proton ratio parameter in most studies is restricted only with the transverse momentum and kinetic energy dependencies [@Li02; @Li04; @Li07]. To draw a fruitful conclusion in the near future, first, it is very important to study the isospin asymmetry and incident energy dependencies of the neutron-to-proton ratio, just as in the recent $\pi^-/\pi^+$ study, and then compare the sensitivity to the symmetry energy 