Channeling is the process that constrains the path of a charged particle in a crystalline solid. In particle physics, an elementary particle is a particle of which other, larger particles are composed. ... Quartz crystal In chemistry and mineralogy, a crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ...

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. If 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 found to be very strongly dependent on the relative orientations of the momentum of the particle and the direction of crystalline axes or planes. This effect is commonly called the "channeling" effect. It is obviously related to other orientation-dependent effects, such as particle diffraction, and these relationships will be discussed in detail later.

Mechanism

From a simple classical standpoint, one may qualitatively understand the channeling 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 channeling". Negatively charged particles like antiprotons and electrons are attracted towards the positively charged nuclei of the plane, and after the passing the center of the plane, they will be attracted again, so negatively charged particles tend to follow the direction of one crystalline plane. The antiproton (aka pbar) is the antiparticle of the proton. ... The Electron is a fundamental subatomic particle that carries an electric charge. ... A semi-accurate depiction of the helium atom. ...

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 "dechanneling" process. Ernest Rutherford, 1st Baron Rutherford of Nelson, known as the father of nuclear physics. ...

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 smaller probability of interacting with the nuclei and electrons of the planes (smaller "dechanneling" effect) and travel a longer distances. Properties [1][2] In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... The first detection of the positron in 1932 by Carl D. Anderson The positron is the antiparticle or the antimatter counterpart of the electron. ...

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

At low energies the channeling 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 channeling effect is present.

Applications

There are several particularly interesting application of the channeling effects.

Channeling 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. In geometry and crystallography, a Bravais lattice is an infinite set of points generated by a set of discrete translation operations. ... In semiconductor production, doping refers to the process of intentionally introducing impurities into an extremely pure (also referred to as intrinsic) semiconductor in order to change its electrical properties. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz...

At higher energies (tens of GeV), the applications include the channeling 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. that the plate now repels them and they are now accelerated by it towards the next plate. ...

External links

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

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