Electromagnetic and Hadronic Showers: Calorimeters

Abstract

The ionisation mechanism presented in Chap. 6 is only one of the processes by which charged particles interact with matter, and is the dominant for heavy particles in thin media. Electrons and photons are subject to a number of other processes which cannot be neglected and are exploited in the development of calorimeters. Hadrons too loose energy by more mechanisms than just ionisation. The additional processes are the topic of this chapter. Following a review of the characteristics of electromagnetic and hadronic showers, the later sections present the design of calorimeters, both electromagnetic and hadronic, which exploit this type of phenomenology.

Glossary

Bhabha scattering Positron scattering off atomic electrons with energy loss per collision below 0.255 MeV

Bremsstrahlung Emission of photons by a charged particle consequent to the acceleration of an electric charge

Compton scattering Scattering of a photon off an atomic electron

Critical energy (electron) Energy of an electron for which the rate of energy loss by bremsstrahlung equals the rate of energy loss by ionisation

Critical energy (positron) Energy of a positron for which the rate of energy loss by bremsstrahlung equals the rate of energy loss by ionisation

Electron–positron annihilation The reaction $e^+e^-\rightarrow \gamma \,\gamma$, in which matter and anti-matter is turned into photons

Electromagnetic calorimeter Detector designed to measure the energy of electrons, positron or photons by total absorption

Electromagnetic shower Chain reaction initiated by an energetic electron, positron or photon into a substance, in which multiple electrons and photons are created or liberated

Energy resolution (calorimeter) Ratio between the Gaussian width of the energy response and the value of the input energy

Hadronic shower Chain reaction initiated by an energetic hadron through the inelastic collisions with nuclei, and nuclear interactions.

Homogeneous calorimeter Calorimeter designed such that the whole detector volume is filled by a high-density material working as both absorber and sensing medium

Interaction length Nuclear interaction length is the distance in a given substance after which an incident number of hadrons diminishes by a factor 1 / e by nuclear interaction

Møller scattering Electron scattering off atomic electrons with energy loss per collision below 0.255 MeV

Pair production Production of one electron and one positron form the disappearance of one photon, in the field of an atomic nucleus

Photoelectric effect Absorption of a photon by an atom and the subsequent emission of an electron

Radiation length Mean thickness of a material (in g/cm$^2$) after which an electron or positron energy is reduced by a fraction of 1/e of its initial energy by bremsstrahlung only

Radiative energy loss see bremsstrahlung

Sampling calorimeter Calorimeter designed alternating layers of absorber (e.g. iron, lead or uranium) and active material plastic scintillator, silicon, liquid or gaseous ionisation medium)

Stochastic coefficient Contribution to the resolution of a calorimeter due to stochastic effects (typically the energy deposition and detection)