Saturday, August 28, 2021

What is the Cherenkov radiation?

 Cherenkov radiation is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wave in a medium) of light in that medium. Special relativity is not violated since light travels slower in materials with refractive index greater than one, and it is the speed of light in vacuum which cannot be exceeded (or reached) by particles with mass.

A classic example of Cherenkov radiation is the characteristic blue glow of an underwater nuclear reactor. Its cause is similar to the cause of a sonic boom, the sharp sound heard when faster-than-sound movement occurs. The phenomenon is named for Soviet physicist Pavel Cherenkov, who shared the 1958 Nobel Prize in Physics for its discovery.

While the speed of light in a vacuum is a universal constant (c = 299,792,458 m/s), the speed in a material may be significantly less, as it is perceived to be slowed by the medium. For example, in water it is only 0.75c.

Matter can accelerate beyond this speed (although still less than c, the speed of light in vacuum) during nuclear reactions and in particle accelerators. Cherenkov radiation results when a charged particle, most commonly an electron, travels through a dielectric (can be polarized electrically) medium with a speed greater than light’s speed in that medium.

A reverse Cherenkov effect can be experienced using materials called negative-index metamaterials (materials with a subwavelength microstructure that gives them an effective “average” property very different from their constituent materials, in this case having negative permittivity and negative permeability).

This means that, when a charged particle (usually electrons) passes through a medium at a speed greater than the phase velocity of light in that medium, that particle emits trailing radiation from its progress through the medium rather than in front of it (as is the case in normal materials with, both permittivity and permeability positive).

Cherenkov radiation is used in:

•             Detection of labelled biomolecules- Cherenkov radiation is widely used to facilitate the detection of small amounts and low concentrations of biomolecules.

•             Medical imaging of radioisotopes and external beam radiotherapy – More recently, Cherenkov light has been used to image substances in the body.

•             Nuclear reactors- Cherenkov radiation is used to detect high-energy charged particles. In open pool reactors, beta particles (high-energy electrons) are released as the fission products decay.

•             Astrophysics experiments- When a high-energy (TeV) gamma photon or cosmic ray interacts with the Earth’s atmosphere, it may produce an electron-positron pair with enormous velocities. The Cherenkov radiation emitted in the atmosphere by these charged particles is used to determine the direction and energy of the cosmic ray or gamma ray.

•             Particle physics experiments- Cherenkov radiation is commonly used in experimental particle physics for particle identification. One could measure (or put limits on) the velocity of an electrically charged elementary particle by the properties of the Cherenkov light it emits in a certain medium. If the momentum of the particle is measured independently, one could compute the mass of the particle by its momentum and velocity, and hence identify the particle.

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