Particle Detection
4.2 Particle Detectors
4.2.5 Calorimeters
Once entering an absorbing medium, particles undergo successive interactions and decays, until their energy is degraded, as we have seen in Sect.4.1.7. Calorimeters are blocks of matter in which the energy of a particle is measured through the absorption to the level of detectable atomic ionizations and excitations. Such detectors can be used to measure not only the energy, but also the position in space, the direction, and in some cases the nature of the particle.
4.2.5.1 Electromagnetic Calorimeters
An ideal material used for an electromagnetic calorimeter—a calorimeter especially sensitive to electrons/positrons and photons—should have a short radiation length, so that one can contain the electromagnetic shower in a compact detector, and the signal should travel unimpeded through the absorber (homogeneous calorimeters).
However, sometimes materials which can be good converters and conductors of the signals are very expensive: one then usessamplingcalorimeters, where the degraded energy is measured in a number of sensitive layers separated by passive absorbers.
The performance of calorimeters is limited both by the unavoidable fluctuations of the elementary phenomena through which the energy is degraded and by the technique chosen to measure the final products of the cascade processes.
Homogeneous Calorimeters. Homogeneous calorimeters may be built with heavy (high density, highZ) scintillating crystals, i.e., crystals in which ionization energy loss results in the emission of visible light, or Cherenkov radiators such as lead glass and lead fluoride. The material acts as a medium for the development of the shower, as a transducer of the electron signal into photons, and as a light guide toward the photodetector. Scintillation light and/or ionization can be detected also in noble liquids.
Sampling Calorimeters. Layers of absorbers are typically interspersed with layers of active material (sandwich geometry). The absorber helps the development of the electromagnetic shower, while the active material transforms part of the energy into photons, which are guided toward the photodetector. Different geometries can be used: for example, sometimes rods of active material cross the absorber (spaghetti geometry).
Converters have high density, short radiation length. Typical materials are iron (Fe), lead (Pb), uranium, tungsten (W). Typical active materials are plastic scintillator, silicon, liquid ionization chamber gas detectors.
Disadvantages of sampling calorimeters are that only part of the deposited particle energy is detected in the active layers, typically a few percent (and even one or two orders of magnitude less in the case of gaseous detectors). Sampling fluctuations typically result in a worse energy resolution for sampling calorimeters.
Electromagnetic Calorimeters: Comparison of the Performance. The fractional energy resolutionΔE/Eof a calorimeter can be parameterized as
ΔE E = a
√E ⊕b⊕ c E ,
where the symbol⊕represents addition in quadrature. The stochastic termaorig- inates from statistics-related effects such as the intrinsic fluctuations in the shower, number of photoelectrons, dead material in front of the calorimeter, and sampling fluctuations—we remind that the number of particles is roughly proportional to the energy, and thus the Poisson statistics gives fluctuations proportional to√
E. Thea term is at a few percent level for a homogeneous calorimeter and typically 10% for sampling calorimeters. The systematic or constantbterm represents contributions from the detector nonuniformity and calibration uncertainty, and from incomplete containment of the shower. In the case of hadronic cascades (discussed below), the different response of the instrument to hadrons and leptons, called noncompensation, also contributes to the constant term. The constant termbcan be reduced to below one percent. Thecterm is due to electronic noise. Some of the above terms can be negligible in calorimeters.
The best energy resolution for electromagnetic shower measurement is obtained with total absorption, homogeneous calorimeters, such as those built with heavy crystal scintillators like Bi4Ge3O12, called BGO. They are used when optimal per- formance is required. A relatively cheap scintillator with relatively shortX0 is the cesium iodide (CsI), which becomes more luminescent when activated with thallium,
144 4 Particle Detection Table 4.3 Main characteristics of some electromagnetic calorimeters. Data from K.A. Olive et al.
(Particle Data Group), Chin. Phys. C 38 (2014) 090001. The accelerators quoted in the table are discussed in the next section
Technology (experiment) Depth (X0) Energy resolution (relative)
BGO (L3 at LEP) 22 2%/√
E⊕0.7%
CsI (kTeV at the FNALK beam)
27 2%/√
E⊕0.45%
PbWO4(CMS at LHC) 25 3%/√
E⊕0.5%⊕0.2%/E Lead glass (DELPHI, OPAL at
LEP)
20 5%/√
E Scintillator/Pb (CDF at the
Tevatron)
18 18.5%/√
E
Liquid Ar/Pb (SLD at SLC) 21 12%/√
E
and is called CsI(Tl); this is frequently used for dosimetry in medical applications, and in space applications, where high technological readiness and reliability are needed.
Energy resolutions for some homogeneous and sampling calorimeters are listed in Table4.3.
4.2.5.2 Hadronic Calorimeters
We have examined the main characteristics of hadronic showers in Sect.4.1.8.
Detectors capable of absorbing hadrons and detecting a signal were developed around 1950 for the measurement of the energy of cosmic rays. It can be assumed that the energy of the incident particle is proportional to the multiplicity of charged particles.
Most large hadron calorimeters are sampling calorimeters installed as part of complex detectors at accelerator experiments. The basic structure typically consists of absorber plates (Fe, Pb, Cu, or occasionally U or W) alternating with plastic scintillators (shaped as plates, tiles, bars), liquid argon (LAr) chambers, or gaseous detectors (Fig.4.23). The ionization is measured directly, as in LAr calorimeters, or via scintillation light observed in photodetectors (usually photomultipliers).
The fluctuations in the invisible energy and in the hadronic component of a shower affect the resolution of hadron calorimeters.
A hadron with energyEgenerates a cascade in which there are repeated hadronic collisions. In each of these, neutral pions are also produced, which immediately (τ ∼0.1 fs) decay into photons: a fraction of the energy is converted to a potentially observable signal with an efficiency which is in general different, usually larger, than the hadronic detection efficiency. The response of the calorimeters to hadrons is thus not compensated with respect to the response to electromagnetic particles (or to the electromagnetic part of the hadronic shower).
(a) (b)
W (Cu) absorber
LAr filled tubes
Hadrons r z
scintillator tile waveshifter
fiber PMT
Hadrons
Fig. 4.23 Hadronic calorimeters of the ATLAS experiments at LHC. Credit: CERN
Due to all these problems, typical fractional energy resolutions are in the order of 30–50%/√
E.
What is the difference between electromagnetic and hadronic calorimeters? Elec- tromagnetic calorimeters are designed to stop photons and electrons and prevent the electromagnetic shower from leaking into the hadronic calorimeter, which in complex detectors is normally located downstream the electromagnetic calorimeter.
Many hadrons still lose most of their energy in the electromagnetic calorimeter via strong interactions. Two prerequisites for a good electromagnetic calorimeter are a largeZ and a large signal. Due to intrinsic fluctuations of hadronic showers, a hadronic calorimeter, for which large mass number A is the main requirement in order to maximize the hadronic cross section, is less demanding. In principle, how- ever, you can have also a single calorimeter both for “electromagnetic” particles and for hadrons—in this case, cost will be a limitation.