Channeling of Charged Particles in Crystals

the movement of particles through the “channels” formed by parallel rows of atoms. In this case the particles undergo glancing collisions (the momentum is almost unchanged) with the rows of atoms confining them to the channels. If the particle’s trajectory is contained between two atomic planes, it is called plane channeling in contrast to axial channeling, where the particles move between adjacent rows of atoms.

The channeling of charged particles was predicted by the American physicists M. T. Robinson and O. S. Oen in 1961 and was observed by several groups of experimenters in 1963–65. The channeling of heavy particles (protons and ions) occurs at energies of more than several keV, corresponding to the length of a de Broglie wave, which is small compared to the constant of a crystal lattice. Under these circumstances the channeling can be described by the laws of classical mechanics. In order for it to take place, the angle formed by the particles’ velocity and the axis of the atomic row (or plane for plane channeling) should not exceed a certain critical value θ_cr. The larger the atomic numbers of the particles and the atom of the crystal and the smaller the separation between the atoms in the row along which channeling is taking place, the larger the angle θ_cr. For axial channeling, in some directions θ_cr = 0.1°–5° (for plane channeling it is several times smaller).

Channeled particles move along a trajectory that is further from the nuclei of the atoms of the crystal lattice than the trajectory of unchanneled particles. This has two results. (1) The path length of particles in a channel is substantially longer than that of unchanneled particles because the electron density in the channels is lower than the average in the crystal; the resulting increased path length of the ions is used in the ion implanting of semiconductors. (2) Inasmuch as channeled particles are moving relatively far from the nuclei and their inner electron shells (the K and L shells), the probability of nuclear reactions and the excitation of X rays by such particles is much less.

Particles may leave a channel as a result of scattering by defects in the crystal, and this is used to investigate such defects. The shadow effect is closely associated with channeling.

The channeling of electrons differs from the channeling of heavy particles. The features of the former are due to their wave properties and their negative charge.