5.2 The A TLAS Detector
5.2.3 The Calorimeters
Calorimeters are used to measure the energy of particles, such as photons, electrons and hadrons (neutral or charged). The principle of calorimetry is to absorb the particle fully and detect their released energy. The released energy can be measured from so-called particle showers evolving when a particle is stopped in dense material and produces either a charge signal via ionisation or a light signal in a scintillating medium. The particle showers contain secondary particles produced via interaction with the calorimeter material. One distinguishes electromagnetic and hadronic calorimeters. In an electromagnetic shower electrons or photons lose energy via Bremsstrahlung and e+e−-pair creation, respectively. The evolving shower ofN secondary particles will stop if the energy of the secondaries is not high enough to produce in turn other secondary particles.
The same occurs in hadronic showers, the difference being that hadrons interact strongly with the nuclei of the calorimeter material. The evolution of the remaining energy of a particle with initial energy E0 passing through the detector as a function of travelled distance x is given in the following:
hE(x)i=E0·e−x/X0.
The parameterX0is called radiation length and describes the average distance a particles travels in which it loses a fraction of 1/e of its original energy. The value of X0 is dependent on the detector material. The particle energy can only be measured if the shower is fully contained in the calorimeter and thus X0 should be a small number. Electromagnetic calorimeters typically have a length of several X0 (∼20·X0).
In hadronic showers, the radiation length is replaced by the parameterλ, which is a scale for the shower length. Hadrons lose their energy in nuclear interactions andλis typically larger thanX0 which explains why hadronic calorimeters are bigger and are placed behind the electromagnetic calorimeters. Hadronic calorimeters have a lower response than electromagnetic calorimeters due to the different nature of hadronic showers: not all the energy can by transformed into a measurable signal when muons or neutrinos are produced in an hadronic shower, that can not be absorbed and carry away their energy fraction. Energy can also get lost in nuclear reactions that do not lead to scintillation light or charge production. A hadronic shower will always contain an electromagnetic component due to the production of neutralπ0mesons which decay instantly via π0 →γγ. Since the response to electromagnetic and hadronic components is different this leads to a non-linear response of the calorimeter. The energy resolution of the calorimeter improves
with increasing particle energy and can be described by Eq. 5.4b. σ(E)
E = a
√ E ⊕ b
E ⊕c. (5.4)
The first term proportional toais the stochastic term stemming from Poissonian fluctuations of the particle numberN in a shower, where√
N ∼√
E holds. The second term withbstems from noisy detector components. The constant term caccounts for dead detector material (support structure, electronic readout), calibration uncertainties, non-uniformity and non-compensation of the calorimeter.
Fig. 5.5: Cut-away view of the electromagnetic and hadronic calorimeter that are located outside the solenoid magnet that houses the ID [95].
Both electromagnetic and hadronic calorimeters of Atlasaresamplingcalorimeters: this means they consist of alternating layers of stopping material and active material that produces a measurable signal outputc. The electromagnetic and hadronic calorimeters are also divided into a barrel region and two end-cap regions. They exhibit a high granularity in order to also provide position information. Additional forward calorimeters exist that cover high-|η|
regions. All calorimeter parts are depicted in Fig. 5.5. The electromagnetic calorimeter shown in copper-colour is composed of alternating lead and liquid Argon layers (LAr). The lead stops particles, their secondaries produce charge signals in the LAr via ionisation. An external electric field is applied and charge signal read out by accordion-shaped Kapton electrodes. The LAr calorimeter is assembled from modules, segmented in φ and η with a high granularity as can be seen in Fig. 5.6. The modules are also segmented in three layers in y-direction. The first one is called presampler which provides identification and position information. The second layer is the longest (16X0) and contains the shower maximum. Fast readout from the electrodes is used for triggering (see Sec. 5.2.7). The barrel and end-cap components cover the region
|η| < 3.2. An additional forward calorimeter covers the region of 3.2 < |η| < 4.9. The first layer is made of copper/LAr for electromagnetic measurements and the second and third layer
b⊕: add the terms in quadrature:a⊕b→√ a2+b2.
cAhomogeneouscalorimeter consists of one material that can stop particles and at the same time create a signal output.
∆ϕ= 0.0245
∆ η = 0.025 37.5mm∆ η = 0.0/8 = 4.69031mmm
∆ϕ=0.0245x4 36.8mmx
Trigger Tower
∆ϕ= 0.0982
∆ η = 0.1
16X0
4.3X0
2X0
1500 mm
470 m m
η ϕ
η =0
Stri p cel l s i n L ay er 1
Square cel l s i n L ay er 2 1.7X0
Cells in Layer 3
∆ϕ×∆η = 0.0245× 0.05
Cells in PS
∆η×∆ϕ= 0.025 × 0.1
Trigger Tower
=147.3mm4
Photomultiplier
Wave-length shifting fiber Scintillator Steel
Source tubes
Fig. 5.6: Concept of LAr (left) and Tile (right) modules: length measures are given, also in terms of X0 and the accordion shape is displayed for the LAr module. The layers of a Tile module and their readout is shown. Also tubes for moving a137Cs source for calibration is shown.
use tungsten as stopping material with LAr as active material for hadronic measurements. The hadronic calorimeter is made of low-carbon steel plates alternating with plastic scintillator tiles as active material. The tiles will produce scintillation light that is read out by photo multiplier tubes, connected from both sides in a module via wavelength shifting fibres (see Fig. 5.6 right).
There are 64 modules each in long barrel and extended barrels covering the full azimuthal angle.
They are segmented in three layers and additionally in η. The central region extends to about 11λ for full shower containment and avoiding of punch-through effects into the muon system.
Long barrel and extended barrel cover a region in pseudo-rapidity of up to |η|= 1.7. Tile cells are also installed in the transition region between barrel and end-cap of the electromagnetic calorimeter. In this so-called crack region they provide information for the electron energy calibration and improve the resolution. Additional tile calorimeter paddles are installed closer to the beam axis covering a region of 2.12< η < 3.85. They are called MBTS (Minimum Bias Trigger Scintillators), used for triggering. There is also a hadronic end-cap calorimeter made of lead/LAr covering 1.5<|η|<3.2. Theηcoverage of the mentioned components of the hadronic calorimeter is summarised in Tab. 5.1 and the ∆η×∆φ granularity of each is specified.
Hadronic Calorimeter
Scintillator Tile LAr Hadronic Barrel Extended Barrel End-cap
|η|coverage <1.0 0.8-1.7 1.5-2.5 2.5-3.2
Number of layers 3 3 4 4
Granularity (∆η×∆φ) 0.1×0.1 0.1×0.1 0.1×0.1 0.2×0.2 (last layer) 0.2×0.1 0.2×0.1
Tab. 5.1: Granularity andη coverage of of the sub-components of the hadronic calorimeter.