Cherenkov radiation (also spelled Cerenkov or (Cerenkov) is an electromagnetic radiation emitted when a beta particle passes through a dielectric medium at a speed greater than the velocity of light in that medium. It was discovered by Cherenkov in 1934, when he studied the radiation of radium salts in an aqueous solution. The experience was interpreted by I. M. Frank and I. E. Tamm. The gamma radiation of radium produces many secondary electrons with high energy (e. g., Compton electrons, discussed in Section 5.4.3), which pass through the medium (water) polarizing the molecules and arranging the dipoles. After passing the beta particle, the molecules rapidly revert to their ground state, emitting electromagnetic radiation. When the velocity of the beta particle (v) is greater than the velocity of light in the given medium (v > c/n, where c is the velocity of light in vacuum, n is the refractive index of the medium), there is an angle (0) where the waves of the electromagnetic radiation emitted at 0 and dt times interfere (Figure 5.12).

The angle of the interference is:

For example, with water (n = 1.337):

c 3 X 108 . о.

= m/s^2.2 X 108m/s

n 1.337 1 1

This velocity is equal to 0.26 MeV. Therefore, to have Cherenkov radiation, the beta particles must have at least 0.26 MeV. In practice, however, to be observed easily, beta energies must be above ~ 0.5 MeV.

Place of the light emission

Place of the light emission

The relation of the intensity (I(v)) and the frequency (f) of the Cherenkov radia­tion can be expressed as:

The maximal intensity is:

This means that the intensity is proportional to the frequency; therefore, the Cherenkov radiation is blue.

This interaction of the beta radiation can be applied to the direct measurement of the beta radiation by light detectors, for example, by photo multipliers. Cherenkov light can be observed in the nuclear reactors.