[Physics Class Notes] on Cherenkov Radiation Pdf for Exam

Bright blue light visible is used in movies from the deep sea to signify something mystic in that area. Have you ever wondered why? It is due to the emission of light called Cherenkov radiation. While filmmakers bright out this strange blue light using visual effects, it is actually due to a pretty interesting phenomenon. The accelerated speed of charged particles, such as electrons, in a dielectric medium, is the cause of this strange radiation. (The high speeds are greater than that of light, that is, more than 299,792,458 m/s.) 

When a charged particle passes through a dielectric medium, it emits electromagnetic radiation which is termed Cherenkov radiation. This radiation is named after the physicist Pavel Cherenkov.

Properties of Cherenkov Radiation

This radiation has a high frequency and is continuous. Due to its continuity, it does not have any characteristic peaks in its spectrum but is rather constant. Owing to its high frequencies it has short wavelengths and is also very intense. It thus emits blue light that falls in the ultraviolet region of the electromagnetic spectrum. With sufficient accelerated charged particles it becomes visible to the naked eye. A common example of Cherenkov radiation is the blue light emitted by underwater nuclear reactors.

Cherenkov Effect

The Cherenkov effect comes into picture when a positron or electron travels through a transparent medium at a speed greater than that of light in that medium. This would cause a flash of bright light known as Cherenkov light and this phenomenon is known as the Cherenkov effect. Common transparent media where this effect is observed are water and air.

The speed of light in water is approximately 200,000 km/sec and in the air, it is about 300,000 km/sec. To travel faster than light in water and exhibit this effect, a charged particle needs energy above 175 keV. Radioactive beta electrons often exhibit this effect while its an impossibility for the heavy and slow alpha particles. In the air, the energy demanded by Cherenkov light from the particles is greater than 21 MeV for a small flash of light. This is a far cry and is never fulfilled by radioactive electrons in the air.

During their journey, the electrons pass through many atoms and molecules that they encounter. The balancing of the medium is done by de-exciting photons. The de-excitation, which leads to the emission of photons, leads to the dissemination of blue light. This emission costs the photons, a mere amount of 2.5 eV energy.

Cosmic radiation in the atmosphere exhibits the Cherenkov effect as it possesses electrons, positrons, and the high energy muons that are capable of producing Cherenkov light. The flashes produced from the light is used for the detection of cosmic showers.

Uses of Cherenkov Effect

Many experiments in physics use the Cherenkov effect. These include:

Cherenkov radiation is used for the detection of high energy charged particles, such as beta particles, in nuclear fission decay. It is also used for verifying the presence of nuclear fuel spent in pools by the characteristics of the light emitted from the fuel rods.

Observations made in astrophysics have shown that using the Cherenkov effect the properties of astronomical objects with high-frequency gamma rays can be determined and cosmic showers in space can be detected.

Recently, the Cherenkov light has been used to produce images of substances in the body. This attempt was aimed at imaging for diagnostic value demonstration and the radioactive elements used were fluorine (13), iodine (131), nitrogen (13), phosphorus (32), and yttrium (90).

Selective biological molecules of low concentrations can be detected using Cherenkov radiation. On introducing radioactive elements by enzymatic and synthetic procedures, the affinity constants and dissociation rates are determined in the biomolecules.

Fun Facts

1. Cherenkov does not need a space filled medium, it can occur even in a vacuum. In a vacuum, the amplitude of the wavefronts decreases but is still comparable to that of the speed of light. This phenomenon is used in microwaves.

2. The path of the exciting beta electrons that travel in water and exhibit the Cherenkov effect is of the order of a few millimeters, that is, about the thickness of a normal toenail but the number of electrons emitted is huge.