IACT

IACT stands for Imaging Atmospheric (or Air) Cherenkov Telescope or Technique. It is a device or method to detect very-high-energy gamma-ray photons, in the photon energy range of 50 GeV to 50 TeV. There are currently three operating IACT systems: H.E.S.S., MAGIC and VERITAS. Set to be the world's largest telescope at the highest altitude, the Major Atmospheric Cerenkov Experiment Telescope (MACE) is currently being established at Hanle, Ladakh, India. Also, currently under design is the Cherenkov Telescope Array (CTA).

Due to the rapidly falling flux of gamma-ray photons from cosmic sources in this energy regime, space-based detectors become ineffective due to their small collection areas which are often limited to some tens or hundreds of square centimeters. In the case of the IACT, the Earth's atmosphere is used as the detection medium, implying a collection area of many hundreds of square meters. This enables IACT instruments to detect gamma-ray photons in an energy regime inaccessible to space-based instruments.

The IACT works by imaging the very short flash of Cherenkov radiation generated by the cascade of relativistic charged particles produced when a very-high-energy gamma ray strikes the atmosphere. This shower of charged particles, known as an Extensive Air Shower (EAS), is initiated at an altitude of 10–20 km. The incoming gamma-ray photon undergoes pair production in the vicinity of the nucleus of an atmospheric molecule. The electron-positron pair produced are of extremely high energy and immediately undergo Bremsstrahlung or "Braking Radiation". This radiation produced is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20 ns. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes is so large.

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