Astroparticle Physics

4.2 Interaction Processes Used for Particle Detection 53Fig. 4.2Energy loss of charged particles invarious targets [2]minimum of ionization, and particles with such βγ valuesare said to be minimum ionizing. For high energies, the energyloss increases logarithmically (‘relativistic rise’) andreaches a plateau (‘Fermi plateau’) owing to the density effect.The energy loss of gases in the plateau region is typically60% higher compared to the ionization minimum. Theenergy loss of singly charged minimum-ionizing particlesby ionization and excitation in air is 1.8MeV/(g/cm 2 )and2.0 MeV/(g/cm 2 ) in water (ice).Equation (4.6) only describes the average energy loss ofcharged particles. The energy loss is distributed around themost probable value by an asymmetric Landau distribution.The average energy loss is about twice as large as the mostprobable energy loss. The ionization energy loss is the basisof a large number of particle detectors.In particle astronomy, the Fly’s Eye technique takes advantageof the scintillation mechanism in air to detect particleswith energies ≥ EeV (≥ 10 18 eV). In this experiment,the atmosphere represents the target for the primary particle.The interaction products create scintillation light in the air,which is recorded by photomultipliers mounted in the focalplane of mirrors on the surface of the Earth.For high energies, the bremsstrahlung process becomessignificant. The energy loss of electrons due to this processcan be described by− dEdxZ 2∣ = 4αN AbremsA r2 eE ln183Z 1/3 = E X 0, (4.7)Landau distributionFly’s Eyebremsstrahlung