Astroparticle Physics

7.4 Extensive Air Showers 159precise. The shower develops in the atmosphere which actsas a calorimeter of 27 radiation lengths thickness. The informationon this shower is sampled in only one, the last layerof this calorimeter and the coverage of this layer is typicallyon the order of only 1%. The direction of incidence of theprimary particle can be obtained from the arrival times ofshower particles in the different sampling counters.energy measurementprimary particlethickness ~1 mdetectorsshower axiszenith angleFig. 7.28Air-shower measurement withsampling detectorsIt would be much more advantageous to measure thetotal longitudinal development of the cascade in the atmosphere.This can be achieved using the technique of the Fly’sEye (Fig. 7.29). Apart from the directional Cherenkov radiationthe shower particles also emit an isotropic scintillationlight in the atmosphere.For particles with energies exceeding 10 17 eV the fluorescencelight of nitrogen is sufficiently intense to berecorded at sea level in the presence of the diffuse backgroundof starlight. The actual detector consists of a systemof mirrors and photomultipliers, which view the whole sky.An air shower passing through the atmosphere near sucha Fly’s Eye detector activates only those photomultiplierswhose field of view is hit. The fired photomultipliers allowto reconstruct the longitudinal profile of the air shower.The total recorded light intensity is used to determine theshower energy. Such a type of detector allows much moreFly’s Eyefluorescence technique