The penetration of energetic ions through the open channels in a crystal lattice

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Abstract
In studying the transmission of 75 kev protons through thin crystals of Au, penetration was found to be greatly enhanced when ions were incident along directions. In another experiment, the reflection of 50 kev H+, He+, Ne+ and Xe− ions from the {110} surface of a Cu crystal was found to diminish sharply whenever incidence occurred along low-index directions or parallel to low-index planes. The results of these two experiments are considered as evidence for the easy passage of ion trajectories along open channels in the crystal lattice. Two types of channel are thought to exist in f.c.c. lattices, those formed in two dimensions between adjacent close-packed planes of atoms, and those between rows of atoms in a particular direction. The experiments suggest the most favoured in the first category to be between {111} or {100} planes, and in the second category between the or rows. It is shown that channelling could occur as the result of a series of glancing collisions with atoms on the lattice. These introduce an effective potential well across the channel which reflects the ion trajectory back and forth as it passes through. In the case of light ions it is predicted that a square well potential is a good approximation, and this is borne out by the observations. The slowing down of the lighter ions in channels is predominantly due to collision with electrons. Ranges in the channels are expected to be enhanced in the ratio of the average electron density in the crystal to that along the channel. Observations show that radiation damage to the lattice can block the channels and prevent long trajectories.

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