Isospin effect of fragmentation reactions induced by intermediate energy heavy ions and its disappearance

Abstract

The fragments produced in the reaction of ${}^{18}\mathrm{O}{+}^{9}\mathrm{Be}$ at 60 MeV/nucleon were measured experimentally. The isotopic distribution of the fragmentation reaction products was well reproduced by using a modified statistical abrasion-ablation model. This model predicts that the fragment isotopic distribution at a fixed atomic number Z shifts towards the neutron rich side for the neutron-rich projectile. This isospin effect will decrease with decreasing the fragment atomic number Z and disappear when ${(Z}_{\mathrm{proj}}{\ensuremath{-}Z)/Z}_{\mathrm{proj}}$ becomes larger than 0.5. It was shown that the disappearance of the isospin effect of fragmentation reaction is induced by the geometry effect in abrasion stage and the evaporation effect later.