Cherenkov glow is associated with an interesting physical phenomenon, when the speed of charged particles exceeds the phase velocity of light in a substance. A decrease in the latter value occurs due to the refraction of the rays. This effect was first discovered by studying the luminescence of liquids. Subsequently, he acquired great practical importance in nuclear physics and astronomy.
What it is?
Cherenkov glow is the emission of photons that occurs when charged particles move at a constant speed exceeding the phase velocity of light in a given medium.
This phenomenon is universal in nature, that is, under the influence of radiation with sufficient energy, all types of transparent media will “glow”, and not just liquids, as was discovered by the discoverers.
Discovery story
Superlight particles were discovered in 1934. In the scientific literature, this physical phenomenon was named after two Soviet scientists - the glow of Vavilov-Cherenkov. They studied luminescence during radioactive irradiation of liquid solutions of uranium salts.
In the course of the experiments, the following differences were revealed by the glow detected by Cherenkov:
- independence of the spectrum and intensity from the type of medium and its temperature;
- connection with the movement of charged particles;
- light wave polarization;
- the highest intensity is in the blue spectrum;
- threshold character (for example, radiation does not appear if the energy of the X-ray beam exceeds 30 KeV).
Based on these facts, Cherenkov concluded that this phenomenon is not luminescence, but the emission of photons by electrons, which appear as a result of the influence of gamma rays.
A certain difficulty in carrying out the experiments was the fact that Cherenkov did not have special equipment for registering a radioactive glow. The scientist had to adapt his eyes to complete darkness for several hours in order to notice a very weak radiation. Colleagues in science discovered the phenomenon was skeptical. Such radiation was previously noticed by the French physicist Malle and Marie Curie, but a detailed study of its properties is a merit of Cherenkov.
In 1958, the three founding scientists who proposed a theoretical justification for this effect — Cherenkov, Tamm, and Frank — were awarded the Nobel Prize.
Physical explanation
In the traditional interpretation, there are several factors that explain what the Cherenkov effect is:
- charged particles moving in a medium interact with its atoms and molecules;
- their polarization occurs;
- spontaneous coherent radiation is formed;
- as a result of the addition of the luminescence of individual atoms, the Cherenkov-Vavilov effect is observed.
In other words, this phenomenon occurs due to the interaction of the medium with charged particles.
The theoretical substantiation of the Cherenkov glow was given later on the basis of three scientific approaches:
- Tamm-Frank;
- Fermi
- quantum mechanical physics.
The latter interpretation differs from the classical one. According to this approach, this phenomenon is associated with an electromagnetic vacuum, which is changed by the substance, and not with the medium itself.
When can I observe the glow?
The pure Cherenkov glow can be observed only under ideal conditions, when a charged particle moves at a constant speed in a radiator of infinite length. In all other cases, this phenomenon is accompanied by the so-called transition radiation, which is caused by a change in the electromagnetic properties of the medium along the electron trajectory.
In opaque media, the last type of luminescence prevails, and the intensity of Cherenkov radiation decreases due to its absorption. To register an individual particle using the total flux of photons.
Practical use
Cherenkov glow is used for experimental methods in order to register elementary particles and study their properties. With the known direction of light and the refractive index of the medium, it is possible to determine the particle velocity. The received radiation is converted by a modern photoelectronic multiplier into an electrical signal, fixed by the Cherenkov counter.
Such devices are widely used in nuclear physics. The theoretical basis of the phenomenon is also associated with Mach waves in acoustics and some problems of particle accelerators. The differential type of counters makes it possible to identify the type of particle in accelerators.
Another area of application of the Cherenkov glow is gamma astronomy. Cherenkov counters with large detectors record gamma rays that enter the Earth’s atmosphere from distant stars from outer space. This allows you to better study the processes occurring in the universe.
Interesting Facts
After the recognition of works on Cherenkov's glow in experimental and theoretical physics, an active growth of research in this field began. The following interesting facts are associated with this phenomenon:
- In the depths of the oceans, radioactive isotopes also decay. There is a hypothesis that large eyes in deep-sea animals are needed in order to see in low light. Even in large depressions of the ocean floor, the water glows a little due to the Cherenkov-Vavilov phenomenon, that is, the idea of pitch darkness is erroneous.
- The described Cherenkov-Vavilov effect is used in the Baikal underwater neutrino telescope. Photomultipliers are lowered into the lake to a great depth on ropes. The purpose of the installation is to study the spectra of neutrinos and muons contained in cosmic radiation.
- There is also an interesting project for using the ice surface of Antarctica as a Cherenkov radiator.
- The international project “Array of Cherenkov Telescopes” was developed and is being gradually implemented, the main purpose of which is to study outer space in the gamma radiation spectrum. 118 dish-mounted telescopes around the world will be built.
The intense glow of Cherenkov is also observed when unloading spent nuclear fuel from power plants into the holding pool, after which the cassettes are sent for disposal.