The calorimeter systems are the next thing we encounter on our journey inwards. With the obvious exception of the muons discussed in the previous section, the calorimeters are designed to stop all particles, barring neutrinos, within its volume and measure
Figure 4.5: Schematic of the Muon Spectrometer showing how the MS was designed such that particles passing through it will interact with at least three chambers. Image from [70].
their energies. The calorimeter system at ATLAS is made up of three sub-systems: the Hadronic Calorimeter (HCal), the Electromagnetic Calorimeter (ECal) and the Forward Calorimeter (FCal), and has a η coverage of−4.9 <| η |< 4.9 . These can be seen in Fig- ure 4.6.
It has been known for a long time [72] that heavy (m >> me) charged and neutral par-
ticles traversing through matter lose energy in a variety of ways4. For heavy charged
particles the effectiveness of materials in causing this energy loss is referred to as the stopping power and is given by S[E] = −dE
dx [73, p23]. For neutral particles which lose
energy via interactions with nucleons and nuclear interactions only, the average dis- tance between nuclear interaction or collision length, λ is the more appropriate figure of merit.
4Including but not limited to: Elastic nuclear scattering, Inelastic collisions of atomic electrons,
Figure 4.6: Schematic showing the Barrel Calorimeter System at ATLAS. Image from [71].
For a charged particle, the total range a particle with an energy E will travel in a par- ticular medium is therefore given by;
R = Z E
0
1
S[E] dE. (4.1)
The ATLAS hadronic and electromagnetic calorimeters are sampling calorimeters [69, p4] which means that they aim to determine the energies of particles through the usage of two types of material used in alternation: an ‘absorber material’ designed to turn the passing particles into lower energy daughter particles and a ‘sampler material’ de- signed to measure the energies of these daughter particles. This results in series of collimated particle deposits made in the calorimeters referred to as showers.
The quantity, S, and therefore R, are obviously different for different materials. To ensure that the entire energy of the particles is deposited within the volume of the calorimeters, a comparative figure of merit is needed to compare the stopping powers of different materials. The same is true, of course, for λ, but this quantity can already be directly compared to other materials.
Instead of trying to characterise materials by their stopping power, one can come up with a quantity that measures the distance a particle has to travel in a material to reduce its energy by a certain fraction (to go from E to Ep). Assuming the identity shown in
Equation 4.2 [73, p26],
S[E] = E
∆x. (4.2)
It can be shown that the partial rangeRp of the particle is equal to
Rp = Z E pE ∆x E dE = ∆x Z E pE 1 E dE = ∆x [ln E]EpE = ∆x [ln E− ln pE] = ∆x ln E pE = ∆x ln 1 p (4.3)
If the fraction of energy lost p is chosen to be 1e, then the partial range given by Equation 4.3 is only given by ∆x. It is this length that is called the radiation length X0.
Hadronic showers are usually longer and wider than electromagnetic showers as there is a strong interaction component as well as an EM one in the showering process. Hence the λ of materials tends to be much larger than X0.
The total thickness of the calorimeter system in the barrel is 22 X0 (ECal) plus 10 λ
(HCal) and in the endcaps is 24 X0 and 10 λ [69, p8]. Measurements have shown this
is enough material to limit the energy deposits to the calorimeters and not to cause ‘punch through’ to the muon spectrometer.
4.2.3.1 The Hadronic Liquid Argon Calorimeter
In the endcap region of the detector, the HCal uses copper plate absorbers and liquid Argon (LAr) a scintillating medium5 as the sampler material. It consists of two in-
dependent wheels divided into two segments, for a total of four layers per end-cap. The outer wheels are constructed from 50 mm copper plates while the inner ones are constructed from copper plates half as thick. The endcap HCal covers an η range of between 1.5 and 3.2 overlapping with the Tile Extended Barrel Calorimeter and the Forward Calorimeter respectively.
4.2.3.2 The Hadronic Tile Calorimeter
The next subsystem radially inwards is the Hadronic Calorimeter. In the barrel region of the detector, the HCal has a radial thickness of 1.97 m and uses steel as the absorbing medium and scintillating tiles as the sampler material.
The Barrel HCal has two main sections made of the same materials: one radially adja- cent to the central LAr calorimeter called the Tile Barrel covering an η of| η | < 1.0; and one radially adjacent to the endcap calorimeters called the Tile Extended Barrel servicing a η range of 0.8 <| η |< 1.7 [69, p10].
4.2.3.3 The Forward Calorimeter
The FCal is a hybrid calorimeter that only exists in the endcap region and acts as both a Hadronic and electromagnetic calorimeter within the same volume. It allows the de-
5A material that produces photons that can be converted into an electric signal when a charged parti-
termination of the energies of particles travelling between 3.1 <| η |< 4.9 [69, p10].
Given the larger particle flux incident on the detector surface at large| η | , effects that were negligible at lower η are more of a concern. One of these is the electromagnetic ra- diation reflected back into the Inner Detector from the calorimeter system by colliding neutrons (also known as neutron albedo). To reduce this effect the FCal is set back with respect to the front face of the ECal by 1.2 m. Since the same requirement of interaction lengths need to be made (FCal is about 10 λ), this calorimeter system is more densely designed.
The FCal has three layers in each endcap. All three use liquid argon as a scintillating material. The outer two layers use tungsten as an absorbing layer to primarily target the hadronic interactions and the inner layer uses copper layers to tune showering to that of electromagnetic interactions.
4.2.3.4 The Electromagnetic Calorimeter
Like the HCal, the ECal is split into barrel and endcap sections but unlike its hadronic counterpart, all parts of the ECal are made entirely of the same materials: lead absorb- ing plates and liquid argon (LAr) as a sampling material. Because of this, the ECal is often referred to as the LAr Calorimeter.
The Barrel ECal (| η | < 1.475) is made up of three layers as labelled in Figure 4.7. Layer 1 has the highest granularity and is designed to discriminate between different shower seeds that are close together. Layer 2 is the radially largest section and should contain most of the energy for the majority of electromagnetic showers. Layer 3 is designed as redundancy to help absorb energies of higher energy EM showers.
∆ ϕ = 0.0245 ∆ η = 0.025 37.5mm/8 = 4.69 mmm ∆ η = 0.0031 ∆ 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 =147.3mm4Figure 4.7: Sketch of the EM Barrel LAr Calorimeter. Image from [74].
coaxial wheels in each endcap section. They are situated directly in front of the endcap hadronic calorimeter wheels. In the endcap region devoted to precision physics ( | η | < 2.5) (we will see more of this later), the EM calorimeter is segmented in a similar fashion to the barrel with three sections in depth. For the rest of the endcap volume, the calorimeter is segmented in two sections in depth and has a coarser granularity in the η-φ plane than the rest of the acceptance [69, p9].
For optimum efficiency, the ECal needs to be operated at a cold temperature and in an environment free from dust. To this end, the ECal is housed in the LAr vacuum vessel, and is cooled in the LAr cryostat.