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

Isospin effect of fragmentation reaction on isotopic distribution was inves tigated 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 las t when (Z(proj) - Z)/Z(proj) closes to 0.5 and becomes larger than 0.5 inde pendent of the reaction systems, where Z and Z(proj) are the atomic numbers of the produced isotop and projectile, respectively. It is pointed out tha t the disappearance of isospin effect comes mainly from the geometry effect in abrasion stage and the evaporation process later.