Astroparticle Physics

6.3 Gamma Astronomy 1136.3.3 Measurement of γ RaysIn principle the inverse production mechanisms of γ rayscan be used for their detection (see also Chap. 4). For γ rayswith energies below several hundred keV the photoelectriceffect dominates,photoelectric effect,E γ< ∼ 100 keVγ + atom → atom + + e − . (6.68)The photoelectron can be recorded, e.g., in a scintillationcounter. For energies in the MeV range as it is typicalfor nuclear decays, Compton scattering has the largest crosssection,Compton effect, E γ< ∼ 1MeVγ + e − at rest → γ ′ + e − fast . (6.69)In this case the material of a scintillation counter can alsoact as an electron target which records at the same time thescattered electron. For higher energies (≫ 1 MeV) electron–positron pair creation dominates,γ + nucleus → e + + e − + nucleus ′ . (6.70)electron–positron pairproduction, E γ ≫ 1MeVFigure 6.37 shows the dependence of the mass attenuationcoefficient µ for the three mentioned processes in a NaI scintillationcounter.This coefficient is defined through the photon intensityattenuation in matter according toI(x) = I 0 e −µx (6.71)(I 0 – initial intensity, I(x) – photon intensity after attenuationby an absorber of thickness x).Since pair production dominates at high energies, thisprocess is used for photon detection in the GeV range. Figure6.38 shows a typical setup of a satellite experiment forthe measurement of γ rays in the GeV range.Energetic photons are converted into e + e − pairs in amodular tracking-chamber system (e.g., in a multiplate sparkchamber or a stack of semiconductor silicon counters). Theenergies E e + and E e − are measured in an electromagneticcalorimeter (mostly a crystal-scintillator calorimeter, NaI(Tl)or CsI(Tl)) so that the energy of the original photon isFig. 6.37Mass attenuation coefficient forphotons in a sodium-iodidescintillation countercrystal calorimeterE γ = E e + + E e − . (6.72)