Astroparticle Physics

4.3 Principles of the Atmospheric Air Cherenkov Technique 554.3 Principles of the Atmospheric AirCherenkov Technique“A great pleasure in life is doing whatpeople say you cannot do.”Walter BagehotThe atmospheric Cherenkov technique is becoming increasinglypopular for TeV γ astronomy since it allows to identifyphoton-induced electromagnetic showers, which develop inthe atmosphere. A charged particle that moves in a mediumwith refractive index n, and has a velocity v that exceeds thevelocity of light c n = c/n, emits electromagnetic radiationknown as Cherenkov radiation. There is a threshold effectfor this kind of energy loss; Cherenkov radiation only occursifCherenkov effectv ≥ c n or, equivalently, β = v c ≥ 1 n . (4.9)Cherenkov radiation is emitted at an angle ofθ C = arccos 1nβ(4.10)relative to the direction of the particle velocity. Due to thisprocess, a particle of charge number z creates a certain numberof photons in the visible spectral range (λ 1 = 400 nm upto λ 2 = 700 nm). The number of photons is calculated fromthe following equation:dNdx = λ 2παz2 2 − λ 1sin 2 θ Cλ 1 λ 2(4.11)≈ 490 z 2 sin 2 θ C cm −1 .These photons are emitted isotropically (about the axis) inazimuth. For relativistic particles (β ≈ 1), the Cherenkovangle is 42 ◦ in water, and 1.4 ◦ in air. In water, around 220photons per centimeter are produced by a singly chargedrelativistic particle. The corresponding number in air is 30photons per meter. Figure 4.4 shows the variation of theCherenkov angle and the photon yield, with the particlevelocity for water and air. The atmospheric Cherenkov techniquepermits the identification of photon-induced electromagneticshowers that develop in the atmosphere, and separatesthem from the more abundant hadronic cascades. Thisis possible because the recorded Cherenkov pattern is differentfor electromagnetic and hadronic cascades; also photonsFig. 4.4Variation of the Cherenkov angleand photon yield of singly chargedparticles in water and air