Angular momentum limit for fusion of halo and weakly bound systems

Abstract

Recent experimental results for the fusion of the proton-halo system ${}^{8}\text{B}\phantom{\rule{-0.16em}{0ex}}+\phantom{\rule{-0.16em}{0ex}}{}^{58}\text{Ni}$ are discussed. A strong enhancement is observed both below and above the barrier. This is qualitatively different from the usual sub-barrier enhancement observed for many heavy-ion systems. Arguments are given that relate the enhancement to a static halo effect directly associated to the extended size of the proton-halo state. A simple single-barrier model is proposed which describes the fusion cross sections in terms of a maximum angular momentum for fusion, ${L}_{f}$. It is shown that a linear energy dependence for ${L}_{f}$ gives a surprisingly good description of the experimental fusion excitation function not only for ${}^{8}\text{B}\phantom{\rule{-0.16em}{0ex}}+\phantom{\rule{-0.16em}{0ex}}{}^{58}\text{Ni}$ but also for the neutron-halo systems ${}^{6}\text{He}\phantom{\rule{-0.16em}{0ex}}+\phantom{\rule{-0.16em}{0ex}}{(}^{209}\text{Bi},{}^{64}\text{Zn})$. The model is also applied to several weakly bound systems.