Comparison of clustering effects in [Formula Presented] fragmentation among [Formula Presented] and [Formula Presented] reactions: Excitation of α-cluster degrees of freedom in nuclear collisions

We examine [Formula Presented] fragmentation in [Formula Presented] and [Formula Presented] reactions using antisymmetrized molecular dynamics. We compare [Formula Presented] fragmentation among the above three kinds of reactions and find that the alpha-clustering effect in [Formula Presented] fragmentation depends on the projectile and also on the incident energy. In proton induced reactions [Formula Presented] fragments are scarcely produced at the early stage of the reaction in a wide range of incident energy. On the other hand, in [Formula Presented] and [Formula Presented] induced reactions many [Formula Presented] fragments are produced during the dynamical stage at a lower incident energy but fewer [Formula Presented] fragments are produced with increasing incident energy. We indicate that this abundance of [Formula Presented] fragments at a lower incident energy in [Formula Presented] and [Formula Presented] induced reactions is due to the excitation of [Formula Presented] to states with an excitation energy of 7–15 MeV which have the alpha-cluster structure. We see that the upper limit of the incident energy which gives rise to the dynamical production of [Formula Presented] fragments is lower in the [Formula Presented] induced reaction than in the [Formula Presented] induced reaction. We show that the excitation to the clustering states is caused by the mean field from the projectile and that nucleon-nucleon collisions work to destroy this excitation. We will see that the disappearance of the excitation to the clustering states at high incident energies is partly due to the weakened effect of the mean field of the projectile and partly due to the strengthened effect of nucleon-nucleon collision processes.