Astroparticle Physics

54 4 Physics of Particle and Radiation Detectionenergy loss [ MeV / (g/cm )]210 310 210where α is the fine-structure constant (α −1 ≈ 137). The def-inition of the radiation length X 0 is evident from (4.7). Theother quantities in (4.7) have the same meanings as in (4.6).Energy loss due to bremsstrahlung is of particular importancefor electrons. For heavy particles, the bremsstrahlungenergy loss is suppressed by the factor 1/m 2 . The energyloss, however, increases linearly with energy, and is thereforeimportant for all particles at high energies.In addition to bremsstrahlung, charged particles can alsolose some of their energy by direct electron–positron pairproduction, or by nuclear interactions. The energy loss dueto these two interaction processes also varies linearly withenergy. Muons as secondary particles in astroparticle physicsplay a dominant rôle in particle-detection techniques, e.g.,in neutrino astronomy. Muons are not subject to strong in-teractions and they can consequently travel relatively largedistances. This makes them important for particle detectionin astroparticle physics. The total energy loss of muons canbe described by:− dEdx ∣ = a(E) + b(E) E , (4.8)muonradiation lengthdirect pair productionnuclear interactionsenergy loss of muonsmuons in rock2110 10 2 10 3 10 4 10 5muon energy [GeV]Fig. 4.3Energy loss of muons in standardrockmuon calorimetrywhere a(E) describes the ionization energy loss, and b(E)Esummarizes the processes of muon bremsstrahlung, directelectron pair creation, and nuclear interaction. The energyloss of muons in standard rock depends on their energy. It isdisplayed in Fig. 4.3.For particles with high energies, the total energy loss isdominated by bremsstrahlung and the processes that dependlinearly on the particles’ energies. These energy-loss mechanismsare therefore used as a basis for particle calorimetry.In calorimetric techniques, the total energy of a particle isdissipated in an active detector medium. The output signal ofsuch a calorimeter is proportional to the absorbed energy. Inthis context, electrons and photons with energies exceeding100 MeV can already be considered as high-energy particlesbecause they initiate electromagnetic cascades. The mass ofthe muon is much larger than that of the electron, makingmuon calorimetry via energy-loss measurements only possiblefor energies beyond ≈ 1 TeV. This calorimetric techniqueis of particular importance in the field of TeV neutrinoastronomy.