Isospin Fractionation Process in Intermediate Energy Heavy Ion Collisions

Abstract: The isospin fractionation in intermediate energy heavy ion collisions is studied by using isospin dependence quantum molecular dynamics model. At first, the reaction productions in isospin fractionation process is separated into a gas phase and a liquid phase. The gas phase includes free neutrons and protons. But there are many choices for the liquid phase, we select the fragments from the region of 1P+Z_T)/2 as the liquid phase, here Z, Z_P and Z_T are charge of the fragment, projectile and target, respectively. For considering the influence of projectile-like and target-like , we choose the upper limit of charges to (Z_P+Z_T)/2. We define the degree of isospin fractionation as a ratio of gas phase neutron-proton ratio (N/Z)_gas to liquid phase neutron-proton ratio (N/Z)_liq. The calculated results show that the degree of isospin fractionation is sensitive to the symmetry potential and insensitive to isospin dependent neucleon-neucle-on cross section. This sensitivity mainly comes from the gas part of the isospin fractionation. The liquid part is insensitive to the symmetry potential. So we may compare the theory calculation results with the experimental data directly to get the knowledge of symmetry potential. The calculated results also show that the ( N/Z)_gas is larger than the (N/Z)_sys of the colliding system but the (N/Z)_liq is less than the (N/Z)_sys for the neutron-rich system. Finally, the isospin fractionation process is not sensitive to the choices for the fragments of liquid phase.