Charged particles have been identified using a variety of techniques. All methods rely on a measurement of the momentum in a tracking chamber combined with a measurement of the velocity to determine the charged particle's mass, and therefore its identity.
Specific ionization A charged particle loses energy in matter by
ionization at a rate determined in part by its velocity. The energy loss per unit distance is typically called . The energy loss is measured either in dedicated detectors, or in tracking chambers designed to also measure energy loss. The energy lost in a thin layer of material is subject to large fluctuations, and therefore accurate determination requires a large number of measurements. Individual measurements in the low- and high-energy tails are excluded.
ΔE–E and ΔE–ΔE–E methods The
ΔE–E and
ΔE–ΔE–E methods are widely used experimental techniques for particle identification. They explore the ionizing behaviour of different particles in matter in order to distinguish between them. These methods consist of stacking a series of two (ΔE–E) or three (ΔE–ΔE–E) detectors, where the first detector is thin enough to be punched through by the ionizing particle to be identified . Detector-stacks like these are commonly called 'telescopes', and a common configuration consists of using one or two Silicon Surface Barrier Detector (SSBD) followed by a Scintilattion detector (like CsI(Tl)), where the remaiing energy of the particle will be fully deposited.
Time of flight Time-of-flight detectors determine charged particle velocity by measuring the time required to travel from the interaction point to the time-of-flight detector, or between two detectors. The ability to distinguish particle types diminishes as the particle velocity approaches its maximum allowed value, the
speed of light, and thus is efficient only for particles with a small
Lorentz factor.
Cherenkov detectors Cherenkov radiation is emitted by a charged particle when it passes through a material with a speed greater than , where is the index of refraction of the material. The angle of the photons with respect to the charged particle's direction depends on velocity. A number of Cherenkov detector geometries have been used. ==Photons==