The ATLAS calorimeter system comprises of a number of sampling detectors which offer full azimuthal coverage and are cylindrically symmetric around the beam axis.
The calorimeter system consists of electromagnetic and hadronic calorimeters, which cover the region up to|η|<3.2 and a system of Forward Calorimeters (FCal) which extend the η coverage to 3.1 < |η| < 4.9. The calorimeter system is displayed in Figure4.3
Figure 4.3:The ATLAS calorimeter system [111].
4.2.1 Electromagnetic calorimeters
Electromagnetic calorimetry is provided by the Liquid Argon (LAr) Electromagnetic Calorimeter (ECal) and the first module of the FCal. The ECal consists of two parts, the barrel (EMB) which covers the central region up to|η| <1.475 and two end-caps (EMEC) which are divided into two coaxial wheels, the outer one covering the range 1.375 <|η|<2.5 and the inner one covering the range 2.5<|η|<3.2.
The calorimeter features an accordion geometry, which provides a full azimuthal coverage with no cracks. The ECal modules consist of interleaved layers of lead/stainless steel plates, which act as the absorber and LAr, which acts as the active medium. The modules are segmented into 3 sections in depth (with a depth of 4.3, 16 and 2 X0 respectively1) with a high granularity in the region|η|<2.5 and two sections in depth with a coarser granularity in the more forward region. The first layer has the highest granularity and is used for separating single photons fromπ0particles. The second
1The radiation lengthX0is the distance at which an incident electron’s energy is reduced toE/e.
layer takes up most of the ECal volume and is where the main energy deposition from the electromagnetic cascades takes place. The third layer is used for the separation of electromagnetic cascades from hadronic ones. In the region with|η| < 1.8 there is a thin LAr layer, known as the presampler, which is used to correct for upstream energy losses. The electromagnetic component of the FCal also uses LAr as the active medium but copper was chosen as the absorbing medium, due to its higher resolution and better heat removal capability. The layout of a barrel module of the ECal is shown in Figure4.4.
Strip cells in Layer 1
Square cells in Layer 2 1.7X0
Cells in Layer 3 Δϕ×Δη = 0.0245×0.05
Figure 4.4:Sketch of a barrel module of the EM calorimeter, showing the different layers with their respective granularities inηandφ[111].
The principle of operation is based on the production of electromagnetic cascades as a result of the interaction of highly energetic particles with the absorber medium.
The main processes at play are pair production (γ → e+e−) and Bremsstrahlung (e± → e±γ). The electromagnetic cascade continues as long as the energy of the radiated photons is higher than the threshold fore+e− production (Eγ >1.022 MeV) and the energy of the radiated electrons/positrons is higher than the critical energy, at which the losses due to Bremsstrahlung become equal to the losses due to ionization.
The particles produced in the cascade ionize the LAr producing an electrical signal.
The energy resolutionσof the calorimeter is parametrized as follows σ
E = a E ⊕√b
E ⊕c, (4.1)
wherea,bandcare known as the noise, sampling and constant terms respectively. The noise term is dominant at low energies and receives contributions from electronic noise and pile-up. For sampling calorimeters it is inversely proportional to the sampling fraction, i.e. the fraction of the incident particle’s energy deposited in the active medium. The sampling term depends on the calorimeter characteristics, such as the absorber and active material properties and the thickness of the sampling layers. The constant term is dominant at high energies and depends on the calorimeter depth and other characteristics of the calorimeter layout, such as non-uniformities, cracks and the distribution of the dead material. Experimental measurements after noise subtraction giveb=10%√
GeV andc =17%.
4.2.2 Hadronic calorimeters
The ATLAS Hadronic Calorimeter (HCal) system consists of the Tile calorimeter, which covers the region with|η| < 1.7, the Hadronic End-cap Calorimeter (HEC) which covers the region with 1.5<|η|<3.2 and the second and third layers of the FCal.
The Tile calorimeter uses steel as the absorber and scintillating tiles as the active material and is segmented in depth in three layers. The tiles are connected to photo-multipliers by wavelength shifting fibers. For the HEC, LAr was chosen as the active medium for its robustness against high radiation fluxes which are expected in the forward region, while copper plates serve as absorbers. The second and third layer of the FCal, which are optimized for hadronic measurements, use LAr as active medium and tungsten as absorber. With its high atomic number, tungsten features a high nuclear interaction length which is necessary for containing the hadronic showers in the limited volume that is available to the FCal modules.
The principle of operation is similar to the ECal. Incoming hadrons induce hadronic cascades, which are composed of a hadronic component (consisting of hadrons and nu-clear fragments) and an electromagnetic component (consisting of electromagnetically decayingπ0). Due to nuclear spallation and weak hadron decays, a fraction of the cas-cade’s energy cannot be detected. As a result the calorimeter’s response to electrons is
higher than its response to hadrons, a phenomenon known as non-compensation. The electromagnetic fraction of the hadronic cascade is highly dependent on the incoming particle’s energy ranging from around 30% for energies of around 10 GeV to 50% for energies around 100 GeV.
The energy resolution of the hadronic calorimeter is parametrized by equation (4.1), withb =50%√
GeV for the sampling term andc=3% for the constant term.