Cerenkov radiation is emitted when a charged particle passes through a medium with a velocity greater than the velocity of light in that medium. In contrast to Bremsstrahlung emission—which is mainly due to interactions between charged particles and nuclei—the emission of Cerenkov radiation is a property of the gross structure of the absorber material. As a charged particle passes through the bound electrons of a material, it induces a polarization of the medium along its path. The time variation of this polarization can lead to radiation. If the charge is moving slowly, the phase relations will be random for radiation from different points along the path and no coherent wave front will be formed. However, if the particle velocity (v) is greater than the velocity of light in the medium (c/n), where n is the Refractive Index, then a coherent wave front can be formed and radiation emitted. The radiation will be in phase and will be emitted in a forward cone of semi-angle θ where:
As the velocity increases, cos θ decreases and θ increases; thus, the cone opens out in contrast to bremsstrahlung emission. The radiation is mainly in the blue end of the visible spectrum and in the ultraviolet.
Cerenkov radiation may be observed around the core of a water-cooled nuclear research reactor of the swimming pool type. In this case, it is caused by high energy beta particles from the beta decay of fission products in the fuel rods.
Cerenkov detectors, operating in a similar way to scintillation detectors, can be used to detect high energy beta particles.