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Calorimetry for Particle Physics REVIEWS OF MODERN PHYSICS, VOLUME 75, OCTOBER 2003 Calorimetry for particle physics Christian W. Fabjan and Fabiola Gianotti CERN, 1211 Geneva 23, Switzerland (Published 15 October 2003) Calorimetry has become a well-understood, powerful, and versatile measurement method. Besides perfecting this technique to match increasingly demanding operation at high-energy particle accelerators, physicists are developing low-temperature calorimeters to extend detection down to ever lower energies, and atmospheric and deep-sea calorimeters to scrutinize the universe up to the highest energies. The authors summarize the state of the art, with emphasis on the physics of the detectors and innovative technologies. CONTENTS VI. Citius, Altius, Fortius 1280 A. Introduction 1280 B. Atmospheric calorimeters 1280 I. Introduction 1243 1. Setting the energy scale 1283 II. Electromagnetic Calorimetry 1244 2. Energy resolution 1283 A. Physics of the electromagnetic cascade 1244 C. Deep-water calorimeters 1283 B. Energy resolution of electromagnetic VII. Conclusions 1284 calorimeters 1246 Acknowledgments 1284 1. Stochastic term 1247 References 1284 2. Noise term 1247 3. Constant term 1247 4. Additional contributions 1248 C. Main techniques and examples of facilities 1249 I. INTRODUCTION 1. Homogeneous calorimeters 1249 a. Semiconductor calorimeters 1249 Calorimetry is an ubiquitous detection principle in b. Cherenkov calorimeters 1250 particle physics. Originally invented for the study of c. Scintillation calorimeters 1251 cosmic-ray phenomena, this method was developed and d. Noble-liquid calorimeters 1254 perfected for accelerator-based particle physics experi- 2. Sampling calorimeters 1256 mentation primarily in order to measure the energy of a. Scintillation sampling calorimeters 1257 electrons, photons, and hadrons. Calorimeters are b. Gas sampling calorimeters 1257 blocks of instrumented material in which particles to be c. Solid-state sampling calorimeters 1258 d. Liquid sampling calorimeters 1258 measured are fully absorbed and their energy trans- III. Hadron Calorimetry 1260 formed into a measurable quantity. The interaction of A. Physics of the hadronic cascade 1261 the incident particle with the detector (through electro- B. Energy resolution of hadron calorimeters 1263 magnetic or strong processes) produces a shower of sec- C. Monte Carlo codes for hadronic cascade ondary particles with progressively degraded energy. simulation 1267 The energy deposited by the charged particles of the 1. Shower physics modeling techniques 1267 shower in the active part of the calorimeter, which can 2. Applications: Illustrative examples 1268 be detected in the form of charge or light, serves as a D. Examples of hadron calorimeter facilities 1269 measurement of the energy of the incident particle. IV. Calorimeter Operation in Accelerator Experiments 1270 Calorimeters can be broadly divided into electromag- A. Performance requirements 1270 B. Integration 1272 netic calorimeters, used mainly to measure electrons and 1. Impact of material 1272 photons through their electromagnetic interactions (e.g., 2. Particle identification 1273 bremsstrahlung, pair production), and hadronic calorim- C. Calorimeter calibration 1273 eters, used to measure mainly hadrons through their V. Low-Temperature Calorimeters 1276 strong and electromagnetic interactions. They can be A. Introduction 1276 further classified according to their construction tech- B. Main technologies 1276 nique into sampling calorimeters and homogeneous 1. Thermal detectors 1276 calorimeters. Sampling calorimeters consist of alternat- 2. Phonon sensors 1277 ing layers of an absorber, a dense material used to de- 3. Superheated superconducting granules 1278 grade the energy of the incident particle, and an active C. Representative applications 1278 medium that provides the detectable signal. Homoge- 1. Search for dark matter 1278 2. Neutrinoless double-beta decay 1278 neous calorimeters, on the other hand, are built of only 3. Microcalorimeters for x-ray astronomy 1279 one type of material that performs both tasks, energy 4. Superconducting tunneling junctions for degradation and signal generation. ultraviolet to infrared spectroscopy in Today particle physics reaches ever higher energies of astronomy 1279 experimentation, and aims to record complete event in- 0034-6861/2003/75(4)/1243(44)/$35.00 1243 ©2003 The American Physical Society 1244 Christian W. Fabjan and Fabiola Gianotti: Calorimetry for particle physics formation. Calorimeters are attractive in this field for projects in atmospheric and water calorimeters are ana- various reasons: lyzed in Sec. VI. Section VII is devoted to the conclu- sions. • In contrast with magnetic spectrometers, in which the The success of calorimeters in modern experiments momentum resolution deteriorates linearly with the rests also on remarkable developments in the field of particle momentum, in most cases the calorimeter en- high-performance readout electronics that have allowed ergy resolution improves with energy as 1/ͱE, where optimum exploitation of the intrinsic potential of these E is the energy of the incident particle. Therefore detectors. A discussion of calorimeter readout tech- calorimeters are very well suited to high-energy phys- niques is beyond the scope of this paper. A very good ics experiments. review has been made by de La Taille (2000). • In contrast with magnetic spectrometers, calorimeters are sensitive to all types of particles, charged and neu- tral (e.g., neutrons). They can even provide indirect detection of neutrinos and their energy through a II. ELECTROMAGNETIC CALORIMETRY measurement of the event missing energy. • They are versatile detectors. Although originally con- In this section we discuss the physics and the perfor- ceived as devices for energy measurement, they can mance of electromagnetic calorimeters. The main tech- be used to determine the shower position and direc- niques used to build these detectors are also reviewed, tion, to identify different particles (for instance, to dis- and their merits and drawbacks are described. Examples tinguish electrons and photons from pions and muons of calorimeters operated at recent or present high- on the basis of their different interactions with the energy physics experiments, or under construction for detector), and to measure the arrival time of the par- future machines, are given as illustration. ticle. Calorimeters are also commonly used for trigger A. Physics of the electromagnetic cascade purposes, since they can provide fast signals that are easy to process and to interpret. In spite of the apparently complex phenomenology of • They are space and therefore cost effective. Because shower development in a material, electrons and pho- the shower length increases only logarithmically with tons interact with matter via a few well-understood energy, the detector thickness needs to increase only QED processes, and the main shower features can be logarithmically with the energy of the particles. In parametrized with simple empirical functions. contrast, for a fixed momentum resolution, the bend- The average energy lost by electrons in lead and the ing power BL2 of a magnetic spectrometer (where B photon interaction cross section are shown in Fig. 1 as a is the magnetic field and L the length) must increase function of energy. Two main regimes can be identified. linearly with the particle momentum p. For energies larger than ϳ10 MeV, the main source of electron energy loss is bremsstrahlung. In this energy Besides perfecting this technique to match the physics range, photon interactions produce mainly electron- potential at the major particle accelerator facilities, re- positron pairs. For energies above 1 GeV both these markable extensions have been made to explore new processes become roughly energy independent. At low energy domains. Low-temperature calorimeters, sensi- energies, on the other hand, electrons lose their energy tive to phonon excitations, detect particles with unprec- mainly through collisions with the atoms and molecules edented energy resolution and are sensitive to very-low- of the material thus giving rise to ionization and thermal energy deposits which cannot be detected in excitation; photons lose their energy through Compton conventional devices. The quest to understand the ori- scattering and the photoelectric effect. gin, composition, and spectra of energetic cosmic rays As a consequence, electrons and photons of suffi- has led to imaginative applications in which the atmo- ciently high energy (у1 GeV) incident on a block of sphere or the sea are instrumented over thousands of material produce secondary photons by bremsstrahlung, cubic kilometers. or secondary electrons and positrons by pair production. A regular series of conferences (CALOR, 2002) and a These secondary particles in turn produce other par- comprehensive recent monograph (Wigmans, 2000) tes- ticles by the same mechanisms, thus giving rise to a cas- tify to the vitality of this field. cade (shower) of particles with progressively degraded In this paper we review major calorimeter develop- energies. The number of particles in the shower in- ments with emphasis on applications at high-energy ac- creases until the energy of the electron component falls celerators. First, the physics, the performance, and prac- below a critical energy ⑀, where energy is dissipated tical realizations of electromagnetic calorimetry are mainly by ionization and excitation and not in the gen- discussed (Sec. II). Next, the physics
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