Astroparticle Physics

4.2 Interaction Processes Used for Particle Detection 51length’) for electrons in air is X 0 ≈ 36 g/cm 2 . For highenergyphotons (energy ≥ 10 GeV), where pair productiondominates, the cross section is 7/9 of the cross section forelectrons ([3], Chap. 1), so the radiation length for photonsis 9/7 of that for electrons, i.e., 47 g/cm 2 . The first interactionof photon-induced electromagnetic cascades thereforealso occurs in the uppermost layers of the atmosphere.The detection of cosmic-ray neutrinos is completely different.They are only subject to weak interactions (apartfrom gravitational interactions). The cross section for neutrino–nucleoninteractions is given byσ νN = 0.7 × 10 −38 E ν [GeV] cm 2 /nucleon . (4.4)Neutrinos of 100 GeV possess a tremendously large interactionlength in the atmosphere:λ ≈ 2.4 × 10 12 g/cm 2 . (4.5)The vertex for possible neutrino–air interactions in the atmosphereshould consequently be uniformly distributed.Charged and/or neutral particles are created in the interactions,independent of the identity of the primary particle.These secondary particles will, in general, be recorded bythe experiments or telescopes. To achieve this, a large varietyof secondary processes can be used.detectionof cosmic-ray neutrinos4.2 Interaction ProcessesUsed for Particle Detection“I often say when you can measure whatyou are speaking about, and express it innumbers, you know something about it;but when you cannot measure it, whenyou cannot express it in numbers, yourknowledge is of a meagre and unsatisfyingkind.”Lord Kelvin (William Thomson)Tables 4.1 and 4.2 show the main interaction processes ofcharged particles and photons, as they are typically used inexperiments in astroparticle physics. In this overview, notonly the interaction processes are listed, but also the typicaldetectors that utilize the corresponding interaction processes.The mechanism that dominates charged-particle interactionsis the energy loss by ionization and excitation.This energy-loss process is described by the Bethe–Blochformula:interaction mechanismsenergy lossof charged particlesBethe–Bloch formula