Atmospheric Cherenkov Technique in Very High Energy Gamma-ray Astronomy :

High energy cosmic rays and gamma-rays bombarding the Earth's atmosphere generate showers of particles through nuclear interaction with atmospheric nuclei and electromagnetic cascade processes.

Charged particles in showers traveling faster than the speed of light in atmosphere (slower than that in vacuum) emitts photons known as Cherenkov radiation. These photons reach the ground in a cone of about 1 degree and can be detected as a light flash by collecting them with a reflector and taking its image by an ultra-fast camera.

Though showers may originate from cosmic ray nuclei which form background for gamma-ray sky, gamma-rays can be selected using image parameters since nucler showers show more diffuse images and come from random direction while gamma-ray showers are expected to come from a point source (in most cases) and have preferred direction.

* Air Cherenkov Detectors (NASA Imagine the Universe!)

* Ogio-san's page (in Japanese)

* Air shower technique in the cosmic ray study (in Japanese)

* See more experiments.