On the spatial distribution of channelled ions

Abstract

Abstract The spatial distribution of channelled ions in the transverse plane is studied theoretically using models based on the continuum approximation and confirmed by computer simulation. The variation of the flux across the crystal channels is found to be quite pronounced. In particular the averaged flux at the centre of the channel can be as high as 4 or 5 times normal under experimentally attainable conditions or, due to the oscillatory nature of channelled trajectories, even greater at certain fixed depths. The dependence of the maximum flux on the multiple scattering and the channel geometry is examined in detail. A general expression for the flux at any point in the transverse plane is derived, and various simplifications are introduced which take advantage of the symmetry about either the atomic rows or the mid-channel axes. Both the ideal situation as well as perturbations due to misalignment and multiple scattering are treated. The results have a direct application to any channelling experiment which measures an interaction that is sensitive to the position of the target atoms, in particular atom location studies and the flux-peaking effect.