Probing the symmetry energy and the degree of isospin equilibrium

The rapidity dependence of the singleand doubleneutron to proton ratios of nucleon emission from isospin-asymmetric but mass-symmetric reactions Zr+Ru and Ru+Zr at energy range 100 ∼ 800 A MeV and impact parameter range 0 ∼ 8 fm is investigated. The reaction system with isospinasymmetry and mass-symmetry has the advantage of simultaneously showing up the dependence on the symmetry energy and the degree of the isospin equilibrium. We find that the beam energyand the impact parameter dependence of the slope parameter of the double neutron to proton ratio (FD) as function of rapidity are quite sensitive to the density dependence of symmetry energy, especially at energies Eb ∼ 400 A MeV and reduced impact parameters around 0.5. Here the symmetry energy effect on the FD is enhanced, as compared to the single neutron to proton ratio. The degree of the equilibrium with respect to isospin (isospin mixing) in terms of the FD is addressed and its dependence on the symmetry energy is also discussed. PACS numbers: 24.10.Lx, 25.75.Dw, 25.75.-q