Astroparticle Physics

116 6 Primary Cosmic RaysFig. 6.42Measurement of Cherenkov lightof photon-induced electromagneticcascades in the atmosphereA simple Cherenkov detector, therefore, consists of aparabolic mirror which collects the Cherenkov light and aset of photomultipliers which record the light collected at thefocal point of the mirror. Figure 6.42 shows the principle ofphoton measurements via the atmospheric Cherenkov technique.Large Cherenkov telescopes with mirror diameters≥ 10 m allow to measure comparatively low-energy photons(< 100 GeV) with correspondingly small shower size evenin the presence of light from the night sky (see Fig. 6.43).For even higher energies (> 10 15 eV) the electromagneticcascades initiated by the photons reach sea level and can berecorded with techniques like those which are used for theinvestigations and measurements of extensive air showers(particle sampling, air scintillation, cf. Sect. 7.4). At theseenergies it is anyhow impossible to explore larger regionsof the universe in the light of γ rays. The intensity of energeticprimary photons is attenuated by photon–photon interactionspredominantly with numerous ambient photons ofthe 2.7 Kelvin blackbody radiation. For the processγ + γ → e + + e − (6.78)twice the electron mass must be provided in the γγ centerof-masssystem. For a primary photon of energy E collidingwith a target photon of energy ε at an angle θ the thresholdenergy isFig. 6.43Photograph of the air Cherenkovtelescope CANGAROO(CANGAROO – Collaboration ofAustralia and Nippon (Japan) for aGAmma-Ray Observatory in theOutback) {15}γγ interactions2m 2 eE threshold =ε(1 − cos θ) . (6.79)For a central collision (θ = 180 ◦ ) and a typical blackbodyphoton energy of ε ≈ 250 µeV the threshold isE threshold ≈ 10 15 eV . (6.80)The cross section rises rapidly above threshold, reaches amaximum of 200 mb at twice the threshold energy, anddecreases thereafter. For even higher energies further absorptiveprocesses with infrared or starlight photons occur(γγ → µ + µ − ) so that distant regions of the universe(> 100 kpc) are inaccessible for energetic photons(> 100 TeV). Photon–photon interactions, therefore, causea horizon for γ astronomy, which allows us to explore thenearest neighbours of our local group of galaxies in the lightof high-energy γ quanta, but they attenuate the γ intensityfor larger distances so strongly that a meaningful observationbecomes impossible (cf. Chap. 4, Fig. 4.9).