Astroparticle Physics

114 6 Primary Cosmic RaysFig. 6.38Sketch of a satellite experiment forthe measurement of γ rays in theGeV rangee – e +e –e – e –3e – 1 bremsstrahlung2 pair production1 3 Compton scatteringe – 4 photoelectric effect242Fig. 6.39Schematic representation of anelectron cascadeCherenkov radiation134The direction of incidence of the photon is derived from theelectron and positron momenta where the photon momentumis determined to be p γ = p e + + p e −. For high energies(E ≫ m e c 2 ) the approximations |p e +|=E e +/c and|p e −|=E e −/c are well satisfied.The detection of electrons and positrons in the crystalcalorimeter proceeds via electromagnetic cascades. In theseshowers the produced electrons initially radiate bremsstrahlungphotons which convert into e + e − pairs. In alternatingprocesses of bremsstrahlung and pair production the initialelectrons and photons decrease their energy until absorptiveprocesses like photoelectric effect and Compton scatteringfor photons on the one hand and ionization loss for electronsand positrons on the other hand halt further particlemultiplication (Fig. 6.39).The anticoincidence counter in Fig. 6.38 serves the purposeof identifying incident charged particles and rejectingthem from the analysis.For energies in excess of 100 GeV the photon intensitiesfrom cosmic-ray sources are so small that other techniquesfor their detection must be applied, since sufficiently largesetups cannot be installed on board of satellites. In this contextthe detection of photons via the atmospheric Cherenkovtechnique plays a special rôle.When γ rays enter the atmosphere they produce – likealready described for the crystal calorimeter – a cascadeof electrons, positrons, and photons which are generally oflow energy. This shower does not only propagate longitudinallybut it also spreads somewhat laterally in the atmosphere(Fig. 6.40). For initial photon energies below 10 13 eV(= 10 TeV) the shower particles, however, do not reachsea level. Relativistic electrons and positrons of the cascadewhich follow essentially the direction of the originalincident photon emit blue light in the atmosphere which isknown as Cherenkov light. Charged particles whose velocitiesexceed the speed of light emit this characteristic electromagneticradiation (see Chap. 4). Since the speed of lightin atmospheric air isc n = c/n (6.73)(n is the index of refraction of air; n = 1.000 273 at 20 ◦ Cand 1 atm), electrons with velocitiesv ≥ c/n (6.74)will emit Cherenkov light. This threshold velocity of