Astroparticle Physics

60 4 Physics of Particle and Radiation DetectionOwing to the low electron and positron masses, the thresh-old energy for this process is only about ≈ 10 15 eV. Theattenuation of primary photons (by interactions with blackbodyphotons), as a function of the primary photon energy, isshown in Fig. 4.9 for several distances to the γ -ray sources.A potentially competing process, γγ → γγ, is connectedwith a very small cross section (it is proportional to thefourth power of the fine-stucture constant). In addition, theangular deflection of the photons due to this process is extremelysmall.galactic photon absorptionFig. 4.9Attenuation of the intensity ofenergetic primary cosmic photonsby interactions with blackbodyradiationThe mean free path for energetic photons (energy> 10 15 eV) is limited to a few tens of kiloparsecs by thisprocess. For higher energies, γγ processes with differentfinal states (µ + µ − , ...) also occur. For lower energies, photonsare attenuated by interactions with infrared or starlightphotons.4.7 Characteristic Features of Detectors“Detectors can be classified into threecategories – those that don’t work, thosethat break down, and those that getlost.”AnonymousThe secondary interaction products of astroparticles are detectedin an appropriate device, which can be a detector on