Based on the isospin dependent quantum molecular dynamics model(IQMD) we investigated the dependence of isospin fractionation degree (N/Z)n / (N/Z)Nfrag on the mass of system. The (N/Z)n and (N/Z)Nfrag are the neutron proton ratio of nucleon emission (gas phase) and that of fragment emission (liquid phase) respectively. We found that the isospin fractionation degree is a scnsitive function of system mass. The (N/Z)n/ (N/Z)Nfrag, reduces with increasing the mass of colliding system when neutron proton ratio of colliding system is fixed.
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