Channeling in solids
Channeling in solids
The steering of positively charged energetic particles between atomic rows or planes of a crystalline solid. The particles can be positive ions, protons, positrons, or muons. If the angle between the direction of the particle and a particular axis or plane in the crystal is within a small predictable limit (typically a few degrees or less), then the gradually changing electrostatic repulsion between the particle and each successive atomic nucleus of the crystal produces a smooth steering through the crystal lattice. See Crystal structure
An obvious consequence of this steered motion is that it prevents violent collisions of the particles with atoms on the lattice sites. Hence, as compared with a randomly directed beam of particles, the channeled beam loses energy more slowly, penetrates more deeply, creates much less damage to the crystal along its track, and is prevented from participating in all close-encounter processes (nuclear reactions, Rutherford scattering, and so forth) with lattice atoms. See Nuclear reaction, Scattering experiments (nuclei)
A related channeling phenomenon is the channeling of energetic electrons or other negative particles. In this case, the particles are attracted to the positively charged atomic nuclei, so that the probability of violent collisions with atoms on lattice sites is enhanced rather than being prevented, and the particles are steered along the rows or planes of nuclei rather than between them.
A closely related phenomenon is called blocking. In this case, the energetic positive particles originate from atomic sites within the crystal lattice by means of fission, alpha-particle decay, or by wide-angle scattering of a nonchanneled external beam in a very close encounter with a lattice atom. Those particles emitted almost parallel to an atomic row or plane will be deflected away from the row by a steering process. Consequently, no particles emerge from the crystal within a certain critical blocking angle of each major crystallographic direction. A piece of film placed some distance from the crystal provides a simple technique for recording blocking patterns. Theoretical considerations show that the same principle is involved in blocking as in channeling; hence, both phenomena exhibit an identical dependence on particle energy, nuclear charge, lattice spacing, and so forth. See Nuclear fission, Radioactivity
Applications of channeling include the location of foreign atoms in a crystal, the study of crystal surface structure, and the measurement of nuclear lifetimes. The location of foreign (solute) atoms in a crystal is one of the simplest channeling applications. It is accomplished by measuring the yields of Rutherford backscattered particles, characteristic x-rays, or nuclear reaction products produced by the interaction of channeled particles with the solute atoms. Such yields are enhanced for solute atoms that are displaced into channels of the crystal. This method has been used to determine the lattice positions of solute atoms that have been introduced into crystals, and to determine the amount of lattice damage created by ion implantation. These and similar applications have proved extremely useful in the development of semiconductor devices. See Laser-solid interactions