Astroparticle Physics

7.4 Extensive Air Showers 161primary protonphotomultiplierin the focal point of a mirrorFig. 7.31Measurement of the isotropicscintillation light of extensive airshowers by Fly’s Eye detectors onboard of satellites (‘Air Watch’)the Earth’s magnetic field thereby creating synchrotron radiation.Because of the strong background in practically allwavelength ranges these attempts have not been particularlysuccessful so far. The possibility to detect large air showersvia their muon content in underground experiments has beenfollowed up in recent experiments.Apart from elementary particle physics aspects the purposeof the measurement of extensive air showers is the determinationof the chemical composition of primary cosmicrays and the search for the sites of cosmic accelerators.The arrival directions of the highest-energy particles(> 10 19 eV) which for intensity reasons can only berecorded via air-shower techniques, practically show nocorrelation to the galactic plane. This clearly indicates thattheir origin must be extragalactic. If the highest-energy primarycosmic-ray particles are protons, then their energiesmust be below 10 20 eV, if they originate from distances ofmore than 50 Mpc. Even if their original energy were muchhigher, they would lose energy by photoproduction of pionson photons of the blackbody radiation until they fallbelow the threshold of the Greisen–Zatsepin–Kuzmin cutoffmuon showers undergroundproton horizon