Detection Technique

Very high energy gamma rays (> 100 GeV) are detected using atmospheric Cherenkov technique. Gamma ray interacts at the top of the atmosphere producing electron-positron pair. Later, through various processes shower of charged particles is generated in terrestrial atmosphere. These charged particles moving at relativistic speeds cause atmosphere to emit bluish Cherenkov light. This light comes as a flash lasting for a few ns and is spread over a circular region with radius of about 100 m at observation level. This light is detected using telescope consisting of mirrors with photomultiplier tubes at the focus.

There are two variants of atmospheric Cherenkov technique, wavefront sampling and imaging. In wavefront sampling technique there is a distributed array of small size telescopes which sample Cherenkov light, record arriaval time of shower front and Cherenkov photon density at various locations in pool. Using this information direction of shower axis and energy of the primary is estimated.

In imaging technique there is a single large reflector and cluster of PMTs at its focus, which record images of extensive air showers initiated by gamma rays and cosmic rays. Gamma ray and cosmic ray showers are distinguished based on image shape and orientation.






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