“…For A = n/2, with η = 1,2, ..., a large part of the ion beam is parallel to the channel axis, i.e., it is focused around the origin in the transmission angle plane. This is an effect of angular focusing, which is called the effect of zerodegree focusing of channeled ions [30,31]. In order to describe ion channeling in axial channels of crystals that are not necessarily very thin, Petrovic et al [27] generalized the model of Neskovic [26] and formulated the theory of crystal rainbows.…”