Cherenkov radiation: description, basic concepts

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Cherenkov radiation: description, basic concepts
Cherenkov radiation: description, basic concepts
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Cherenkov radiation is an electromagnetic reaction that occurs when charged particles pass through a transparent medium at a speed greater than the same phase index of light in the same medium. The characteristic blue glow of an underwater nuclear reactor is due to this interaction.

History

Cherenkov radiation, concepts

The radiation is named after Soviet scientist Pavel Cherenkov, 1958 Nobel Prize winner. It was he who first discovered it experimentally under the supervision of a colleague in 1934. Therefore, it is also known as the Vavilov-Cherenkov effect.

A scientist saw a faint bluish light around a radioactive drug in water during experiments. His doctoral dissertation was on the luminescence of solutions of uranium s alts, which were excited by gamma rays instead of the less energetic visible light, as is usually done. He discovered anisotropy and concluded that this effect was not a fluorescent phenomenon.

Cherenkov's theoryradiation was later developed within the framework of Einstein's theory of relativity by the scientist's colleagues Igor Tamm and Ilya Frank. They also received the 1958 Nobel Prize. The Frank-Tamm formula describes the amount of energy emitted by radiated particles per unit length traveled per unit frequency. It is the refractive index of the material through which the charge passes.

Cherenkov radiation as a conical wavefront was theoretically predicted by the English polymath Oliver Heaviside in papers published between 1888 and 1889, and by Arnold Sommerfeld in 1904. But both were quickly forgotten after the limitation of superparticle relativity until the 1970s. Marie Curie observed pale blue light in a highly concentrated solution of radium in 1910, but did not go into details. In 1926, French radiotherapists led by Lucien described the luminous radiation of radium, which has a continuous spectrum.

Physical Origin

Cherenkov radiation effect

Although electrodynamics considers that the speed of light in vacuum is a universal constant (C), the rate at which light propagates in a medium can be much less than C. The speed can increase during nuclear reactions and in particle accelerators. It is now clear to scientists that Cherenkov radiation occurs when a charged electron passes through an optically transparent medium.

The usual analogy is the sonic boom of a super-fast aircraft. These waves, generated by reactive bodies,propagate at the speed of the signal itself. Particles diverge more slowly than a moving object, and cannot advance ahead of it. Instead, they form an impact front. Similarly, a charged particle can generate a light shock wave when it passes through some medium.

Also, the speed to be exceeded is a phase speed, not a group speed. The former can be changed drastically by using a periodic medium, in which case one can even obtain Cherenkov radiation without a minimum particle velocity. This phenomenon is known as the Smith-Purcell effect. In a more complex periodic medium, such as a photonic crystal, many other anomalous reactions can also be obtained, such as radiation in the opposite direction.

What happens in the reactor

In their original papers on the theoretical foundations, Tamm and Frank wrote: "Cherenkov radiation is a peculiar reaction that apparently cannot be explained by any general mechanism, such as the interaction of a fast electron with a single atom or radiative scattering into nuclei On the other hand, this phenomenon can be explained both qualitatively and quantitatively, if we take into account the fact that an electron moving in a medium emits light, even if it moves uniformly, provided that its speed is greater than that of light."

However, there are some misconceptions about Cherenkov radiation. For example, it is considered that the medium becomes polarized by the electric field of the particle. If the latter moves slowly, then the movement tends back tomechanical balance. However, when the molecule is moving fast enough, the limited response speed of the medium means that equilibrium remains in its wake, and the energy contained in it is radiated in the form of a coherent shock wave.

Such concepts have no analytical justification, since electromagnetic radiation is emitted when charged particles move in a homogeneous medium at subluminal speeds, which are not considered as Cherenkov radiation.

Reverse phenomenon

Cherenkov radiation, description

The Cherenkov effect can be obtained using substances called metamaterials with a negative index. That is, with a subwavelength microstructure, which gives them an effective "average" property that is very different from the others, in this case having a negative permittivity. This means that when a charged particle passes through a medium faster than the phase velocity, it will emit radiation from its passage through it from the front.

It is also possible to obtain Cherenkov radiation with an inverse cone in non-metamaterial periodic media. Here, the structure is on the same scale as the wavelength, so it cannot be considered an effectively homogeneous metamaterial.

Features

Cherenkov radiation, basics

Unlike fluorescence or emission spectra, which have characteristic peaks, Cherenkov radiation is continuous. Around the visible glow, the relative intensity per unit frequency is approximatelyproportional to her. That is, higher values ​​are more intense.

This is why visible Cherenkov radiation is bright blue. In fact, most of the processes are in the ultraviolet spectrum - only with sufficiently accelerated charges does it become visible. The sensitivity of the human eye peaks in green and is very low in the violet part of the spectrum.

Nuclear reactors

Cherenkov radiation, basic concepts

Cherenkov radiation is used to detect high-energy charged particles. In units such as nuclear reactors, beta electrons are released as fission decay products. The glow continues after the chain reaction stops, dimming as shorter-lived substances decay. Also, Cherenkov radiation can characterize the remaining radioactivity of spent fuel elements. This phenomenon is used to check for the presence of spent nuclear fuel in tanks.

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