Faculty of Physics

Cherenkov radiation

In 1934, Pavel Cherenkov, performing in the laboratory Of S. I. Vavilov studies of luminescence of liquids under the influence of gamma radiation, found a weak blue radiation of unknown nature. Later it was found that this glow is caused by electrons moving at speeds greater than the phase velocity of light in the medium. Fast electrons are knocked out of the electron shells of the atoms of the medium by gamma radiation.

Already the first experiments Cherenkov radiation, undertaken at the initiative of S. I. Vavilov, revealed a number of unexplained features of the radiation: the glow is observed in all transparent liquids, and brightness is little affected by their chemical composition and chemical nature, the radiation is polarized with a preferential direction of the electric vector along the direction of propagation of the particles, while in contrast to the luminescence observed in either temperature or impurity quenching. On the basis of these data, Vavilov made a fundamental statement that the detected phenomenon is not luminescence, and light is emitted by fast electrons moving in the liquid.

The theoretical explanation of the phenomenon was given by I. Tamm and I. Frank in 1937.

In 1958 Cherenkov, Tamm, and Frank were awarded the Nobel prize in physics “for the discovery and interpretation of Cherenkov effect”. Manne Sigban of the Royal Swedish Academy of Sciences in his speech at the award ceremony noted that”the Discovery of the phenomenon, now known as the Cherenkov effect, is an interesting example of how relatively simple physical observation with the right approach can lead to important discoveries and pave new ways for further research.”

While electrodynamics asserts that the speed of light in a vacuum is a universal constant (c), the speed at which light propagates in a material can be significantly less than c. For example, the speed of light propagation in water is only 0.75 s.in nuclear reactions and in particle accelerators, matter can accelerate beyond this speed (although to less than C). Cherenkov radiation occurs when a charged particle, most often an electron, passes through a dielectric (electrically polarized) medium at a speed exceeding the speed of light propagation in the same medium.

Also, the speed to be exceeded is the phase speed of light, not the Group speed of light. The phase velocity can be drastically altered by a periodic medium, in which case one can even achieve Cherenkov radiation without a minimum particle velocity, a phenomenon known as the Smith–Purcell effect. In a more complex periodic medium, such as a photonic crystal, many other anomalous Cherenkov effects can also be obtained, such as radiation in the opposite direction (whereas ordinary Cherenkov radiation forms an acute angle at the particle velocity).

In his original work on the theoretical foundations of Cherenkov radiation Tamm and Frank wrote: “this kind of radiation, obviously, can not be explained by any common mechanism, such as the interaction of a fast electron with a single atom or as the radiation scattering of electrons on atomic 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 exceeds the speed of light in the medium.”. However, there are some misconceptions about Cherenkov radiation, for example, it is believed that the medium becomes electrically polarized electric field of the particle. If the particle moves slowly, the perturbation elastically relaxes back to the mechanical equilibrium as the particle passes. However, when a particle moves fast enough, the limited response rate of the medium means that there is a disturbance in the Wake of the particle, and the energy contained in this disturbance is emitted as a coherent shock wave. Such representations have no analytical basis, since electromagnetic radiation is emitted when charged particles move in a dielectric medium with sublight velocities, which are not considered as Cherenkov radiation.

A common analogy is the sonic boom of a supersonic aircraft or bullet. Sound waves generated by a supersonic body propagate at the speed of the sound itself; thus, the waves propagate more slowly than an accelerating object, and cannot propagate forward from the body, instead forming a shock front. Similarly, a charged particle can generate a light shock wave when passing through an insulator.

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