Channelling (physics)

Channelling is the process that constrains the path of a charged particle in a crystalline solid.

Many physical phenomena can occur when a charged particle is incident upon a solid target, e.g., elastic scattering, inelastic energy-loss processes, secondary-electron emission, electromagnetic radiation, nuclear reactions, etc. All of these processes have cross sections which depend on the impact parameters involved in collisions with individual target atoms. When the target material is homogeneous and isotropic, the impact-parameter distribution is independent of the orientation of the momentum of the particle and interaction processes are also orientation-independent. When the target material is monocrystalline, the yields of physical processes are very strongly dependent on the orientation of the momentum of the particle relative to the crystalline axes or planes. Or in other words, the stopping power of the particle is much lower in certain directions than others. This effect is commonly called the "channelling" effect. It is obviously related to other orientation-dependent effects, such as particle diffraction. These relationships will be discussed in detail later.

Mechanism

From a simple, classical standpoint, one may qualitatively understand the channelling effect as follows: If the direction of a charged particle incident upon the surface of a monocrystal lies close to a major crystal direction, the particle with high probability will suffer a small-angle scattering as it passes through the several layers of atoms in the crystal. If the direction of the particle's momentum is close to the crystalling plane, but it is not close to major crystalling axes, this phenomenon is called "plane channelling". Negatively-charged particles like antiprotons and electrons are attracted towards the positively-charged nuclei of the plane, and after passing the center of the plane, they will be attracted again, so negatively-charged particles tend to follow the direction of one crystalline plane.

Because the crystalline plane has a high density of atomic electrons and nuclei, the channeled particles eventually suffer a high angle Rutherford scattering or energy-losses in collision with electrons and leave the channel. This is called the "dechannelling" process.

Positively-charged particles like protons and positrons are instead repulsed from the nuclei of the plane, and after entering the space between two neighboring planes, they will be repulsed from the second plane. So positively-charged particles tend to follow the direction between two neighboring crystalline planes, but at the largest possible distance from each of them. Therefore, the positively-charged particles have a smaller probability of interacting with the nuclei and electrons of the planes (smaller "dechannelling" effect) and travel longer distances.

The same phenomena occur when the direction of momentum of the charged particles lies close to a major crystalline, high-symmetry axis. This phenomenon is called "axial channelling".

At low energies the channelling effects in crystals are not present because small-angle scattering at low energies requires large impact parameters, which become bigger than interplanar distances. The particle's diffraction is dominating here. At high energies the quantum effects and diffraction are less effective and the channelling effect is present.

Applications

There are several particularly interesting applications of the channelling effects.

Channelling effects can be used as tools to investigate the properties of the crystal lattice and of its perturbations (like doping) in the bulk region that is not accessible to X-rays. This is an important variation of the Rutherford backscattering ion beam analysis technique.

At higher energies (tens of GeV), the applications include the channelling radiation for enhanced production of high energy gamma rays, and the use of bent crystals for extraction of particles from the halo of the circulating beam in an particle accelerator.

General literature

• J.W. Mayer and E. Rimini, Ion Beam Handbook for Material Analysis, (1977) Academic Press, New York
• L.C. Feldman, J.W. Mayer and S.T.Picraux, Material Analysis by Ion Channelling, (1982) Academic Press, New York

External links

• nice animation
• CERN NA43 Experiment that investigated interactions of high energy particles with crystals
• Note and reports on crystal extraction
• The future looks bright for particle channelling on CERN Courier