Cherenkov radiation is an electromagnetic reaction that occurs when charged particles pass through a transparent medium at a speed greater than the similar phase index of light in the same medium. The characteristic blue glow of an underwater nuclear reactor is due to this interaction.
Story
The radiation is named after the Soviet scientist Pavel Cherenkov, a 1958 Nobel Prize laureate. 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.
The scientist saw a faint bluish light around a radioactive drug in water during experiments. His doctoral dissertation was devoted to the luminescence of solutions of uranium salts, which were excited by gamma rays instead of less energetic visible light, as is usually done. He discovered anisotropy and concluded that this effect was not a fluorescent phenomenon.
The theory of Cherenkov radiation was later developed in the framework of Einstein's theory of relativity by colleagues of the scientist Igor Tamm and Ilya Frank. They also received the 1958 Nobel Prize. The Frank-Tamm formula describes the amount of energy emitted by the emitted particles per unit length of the path traveled per unit frequency. It is a refractive index of the material through which the charge passes.
Cherenkov radiation as a conical wave front was theoretically predicted by the English scholar Oliver Heaviside in works published between 1888 and 1889, and Arnold Sommerfeld in 1904. But both of them were quickly forgotten after limiting the theory of relativity of superparticles until the 1970s. Marie Curie observed a pale blue light in a highly concentrated solution of radium in 1910, but did not understand the details. In 1926, French radiotherapists, led by Lucien, described luminous radiation of radium having a continuous spectrum.
Physical background
Although electrodynamics believes that the speed of light in vacuum is a universal constant (C), a similar indicator with which the luminescence propagates in the medium can be significantly less than C. The speed can increase during nuclear reactions and in particle accelerators. Now scientists already understand that Cherenkov radiation occurs when a charged electron passes through an optically transparent medium.
The usual analogy is the sonic boom of an ultrafast 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 the front of the strike. Similarly, a charged particle can generate a light shock wave as it travels through some medium.
In addition, the speed that must be exceeded is phase rather than group. The first can be dramatically changed using a periodic medium, and in this case, you can even get 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 theoretical works, Tamm and Frank wrote: “Cherenkov radiation is a kind of reaction, apparently, cannot be explained by any general mechanism, such as the interaction of a fast electron with an individual atom or radiation scattering by 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 regarding Cherenkov radiation. For example, it is believed that the medium becomes polarized by the electric field of the particle. If the latter moves slowly, then the movement tends to return to mechanical equilibrium. However, when the molecule moves fast enough, the limited response speed of the medium means that equilibrium remains in its wake, and the energy contained in it is emitted 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 with sublight speeds that are not regarded as Cherenkov radiation.
Reverse phenomenon
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 dielectric constant. This means that when a charged particle passes through the medium at a speed exceeding the phase, 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-material periodic media. Here, the structure has the same scale as the wavelength; therefore, it cannot be considered as an effectively homogeneous metamaterial.
Characteristics
In contrast to fluorescence or emission spectra having characteristic peaks, Cherenkov radiation is continuous. Around the visible glow, the relative intensity per unit frequency is approximately proportional to it. That is, higher values are more intense.
That is why the visible Cherenkov radiation has a bright blue color. In fact, most processes are in the ultraviolet spectrum - only with sufficiently accelerated charges does it become visible. The sensitivity of the human eye reaches a peak in green and is very small in the violet part of the spectrum.
Nuclear reactors
Cherenkov radiation is used to detect high-energy charged particles. In aggregates such as nuclear reactors, beta electrons are produced as decay fission products. The glow continues after the chain reaction stops, dimming, as substances with a shorter lifespan decay. Also, Cherenkov radiation can characterize the remaining radioactivity of expired fuel elements. This phenomenon is used to check for spent nuclear fuel in tanks.