Cherenkov radiation

When a charged particle running in a material (medium) exceeds the velocity of light in the medium(c/n, c is the velocity of light in vacuum, and n is the refractive index of the medium), light is emitted from the medium. This is called Cherenkov radiation.
As shown in Fig. 1, electric polarization arises around the charged particle that enters the medium, and light is created when the electric polarization disappears after the particle passes. The light from each point along the particle trajectory forms a uniform wave-front in a specific direction and a light with a sharp directivity (Cherenkov radiation) is emitted when the particle runs faster than the velocity of light in the medium. This phenomenon is similar to the shock wave generated by a supersonic flying object. When the velocity of the charged particle is slower than the velocity of light in the medium, the phases of the light are not matched in any direction and Cherenkov radiation is not generated.
Cherenkov radiation generated by electron incidence has the following properties. Here, n is the refractive index of the medium, v the velocity of electron, c the velocity of light in vacuum, and β=v/c.

• The condition for the generation of Cherenkov radiation is given by v>c/n or nβ>1. (Critical velocity)
• The radiation angle θ is determined by cosθ=1/nβ.
• The spectral intensity of the radiation I(λ) shows the dependence of λ^-2.

For electrons accelerated at an accelerating voltage of 100 kV (200 kV), β=0.55 (β=0.70), Cherenkov radiation is generated when the refractive index of the medium is larger than 1.8 (1.4).
Fig. 2 shows the Cherenkov radiation spectra from a mica thin film observed in an electron microscope for different accelerating voltages of the incident electrons. Since the refractive index of mica is 1.59, the acceleration voltage that gives the critical velocity is 146 kV. It is seen that radiation is not observed at an accelerating voltage of 120 kV, but that strong radiation appears at 160 kV and 200 kV.
It is noted that a counting device using Cherenkov radiation is used for detecting neutrinos and other elementary particles in the field of high energy physics.

(By Dr. Naoki Yamamoto, Tokyo Institute of Technology)

Fig. 1

Generation mechanism of Cherenkov radiation.

Fig. 2

Cherenkov radiation spectra from a mica thin film at different accelerating voltages, (a) 200 kV, (b) 160 kV, and (c) 120 kV.

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