Isospin effect of fragmentation reaction and its disappearance in intermediate energy heavy ion collisions

Isospin effect of fragmentation reaction on isotopic distribution was investigated by a modified statistical abrasion-ablation model. It is shown that the isotopic distribution shifts toward the neutron rich side for neutron rich projectile, but this isospin effect will decrease and disappear at last when (Z(proj) - Z)/Z(proj) closes to 0.5 and becomes larger than 0.5 independent of the reaction systems, where Z and Z(proj) are the atomic numbers of the produced isotop and projectile, respectively. It is pointed out that the disappearance of isospin effect comes mainly from the geometry effect in abrasion stage and the evaporation process later.