4.4.1 System Overview
The calorimeter system, which is divided into an electromagnetic and a hadronic part, measures the deposited energy of particles showering in the detector material. In the ATLAS detector, both parts are sampling calorimeters with active and passive material [87]. The passive material needs to have a high density to induce a shower and absorb the particle energy [13, 87], whereas the active material detects the shower. Both sub-systems are designed so that they measure the total energy of particles. The calorimeter system is illustrated and labelled in Figure 4.6.
The ECAL consists of two barrels (end to end) with a small 4 mm gap at z = 0 m. The ECAL covers a range of |η|<1.475. The end-caps of the ECALs is instrumented by two wheels (one at each end). The coverage for these wheels is 1.375 < |η| < 2.5. The ECAL uses accordion shaped lead absorbers filled with Liquid Argon (LAr) as the sampling element all housed in three separate cryostats, one for each end-cap and one for the barrel. The barrel section is named electromagnetic barrel (EMB) and the end-caps are named electromagnetic end-cap (EMEC). Both structures are split into three layers, a thin finely segmented initial layer designed to give a high precision hit position, a second layer to contain the majority of the shower and a third layer to estimate the shower leakage from the ECAL to the HCAL, to distinguish very large energy electromagnetic showers from hadronic showers. Additionally, in the region |η| < 1.8 an extra layer named the pre-sampler is used to estimate the energy losses from photons and electrons before reaching the calorimeters.
The hadronic calorimeters are designed to measure the energy of all remaining particles (except for muons), reconstruct hadronic jets and hadronic τ decays in order to perform a total energy measurement for the event. The hadronic calorimeter in the barrel region is composed of three cylinders, one long barrel (LB) which is split into two readout partitions and two extended barrels (EB), placed either side of the LB. The sections cover |η| < 1.0 for the LB and 0.8 < |η| < 1.7 for the extended barrels. These three sections are referred to as the HCAL but are also known as the TileCal because they use scintillating tiles as a sensing element. The absorber used is steel, which allows it to act as a return yoke for the magnetic field of the inner detectors solenoid. The hadronic calorimeter also features end-caps (HEC) positioned at each end directly behind the EMEC. Copper is used as the absorber in the HEC’s. Due to the positioning of the
HEC’s the same sensing elements as the electromagnetic calorimeter (LAr) is used which requires them to be enclosed in the two EMEC cryostats. The region covered by the HEC’s is 1.5<|η|<3.2.
Finally, contained in the same cryostats as the end-caps are the forward calorimeters (FCAL). The FCAL’s are located nestled inside the ECAL end-caps and the HEC close to he beam pipe, providing coverage from 3.1<|η|<4.9. Due to their relative small sizes a dense tungsten matrix is used as the absorber. Liquid Argon is used as the sensing element because of the intrinsically high radiation tolerance of the material.
Figure 4.6: The ATLAS calorimeter system showing the electromagnetic and hadronic sub- systems [85].
4.4.2 Upgrades
Phase-I - Liquid Argon (LAr) Calorimeter
An upgrade to the LAr is planned during LS1 to provide higher-granularity and higher resolution information to the L1 calorimeter trigger processors. A 10-fold increase in granularity can be seen in Figure 4.7, which compares the energy deposition of an
electron in the existing trigger readout system to that in the proposed upgraded system. The upgrade also improves the trigger energy resolution and efficiency for selected electrons, photons, leptons, jets and missing transverse energy (Emiss
T ), thus enhancing
discrimination against backgrounds and fakes from an environment with a large number of interactions per LHC bunch crossing (pile-up). As the LHC’s luminosity increases above its initial design value, the improved calorimeter trigger electronics will allow ATLAS to deploy more sophisticated algorithms, utilising the higher granularity super-cells which has the advantage of providing longitudinal shower information for the L1 trigger, and higher energy precision. Furthermore, at the start of HL-LHC operation, the current electronics is expected to be 15 - 20 years old. It is foreseen that the front-end electronic will have to be replaced due to radiation damage and the need for ATLAS to upgrade its trigger system to provide the real-time performance capabilities that the current system cannot satisfy. Replacing the front-end electronics implies that the back-end electronics will also need to be replaced [86].
Figure 4.7: An electron (with 70 GeV of transverse energy) as seen by the existing L1 Calorimeter trigger electronics (a) and by the proposed upgraded trigger electronics (b) [86].
Phase-I - Tile Calorimeter (TileCal)
ATLAS hadronic (Tile) calorimeter system will undergo minor upgrades during LS2; the majority of the upgrade is foreseen to occur in LS3. At Phase-I even the elements with high radiation exposure (the gap and cryostat scintillator systems) are not expected to suffer any significant damage. The gap and cryostat scintillators are designed to correct for energy losses in dead material between the TileCal barrel, extended barrel and the central and forward electromagnetic calorimeter cryostats. These systems will suffer from radiation damage but have been designed to be easily replaceable. The cryostat scintillators in the 1.2 < |η| < 1.6 region will suffer from significant (for scintillators) radiation exposure of up to 10 kGy/year and are expected to suffer significant light loss (up to a factor of 2) after 10 years of nominal LHC operation. The luminosity increase corresponding to HL-LHC operation will cause even greater radiation damage, so new radiation-hard scintillators and wavelength shifting fibres are being investigated [84].
Phase-II - Liquid Argon (LAr) Calorimeter
The LAr barrel electromagnetic calorimeter is expected to continue to perform well at HL-LHC luminosities up to 3000 fb−1. However, depending on running conditions and
radiation damage the FCAL, performance may degrade and it is unclear how the end-cap calorimeters (EMEC and HEC) will be affected. The present HEC cold electronics may suffer from degradation due to the increased neutron damage. As a precaution, new more radiation hard ASIC’s are under development.
For the forward region, a new FCAL detector with a reduced gap size, modified HV distribution and cooling is being investigated to replace the existing one. Another option is to install a new small calorimeter in front of the existing FCAL. This new calorimeter, called the Mini-FCAL (Figure 4.8), would reduce the particle flux in the current FCAL allowing it to operate at HL-LHC luminosities. Both options necessitate the development of new front-end and back-end electronics which is not discussed here as both options are still under development. For more details on the systems and the different variants proposed see Ref. [18].
Figure 4.8: The left hand illustration is an overview of the Mini-FCAL showing the surround- ing detectors and cryostat with part of the beam pipe still in place. The right hand diagram shows the diamond detector option for the Mini-FCAL with the first absorber removed so that the diamond detector layer can be seen. The cooling pipes are visible at the bottom [18].
Phase-II Tile Calorimeter (TileCal)
The existing scintillators used in the TileCal detectors are believed to be radiation tolerant enough to cope with the HL-LHC operation. This is due to significant shielding from the calorimeter elements located within the cryostats. However a full replacement of the readout electronics is foreseen to ensure compatibility with the new L0/L1 trigger architecture, to meet the increased radiation tolerance requirements of the front-end readout electronics and to provide higher granularity information to the trigger processors. The upgraded on-detector electronics will be organised in independent modules with separate power, cooling and monitoring services. The readout architecture is based on continuous digitisation and data transfer off-detector at each bunch crossing of all the readout channels. This design uses a optical data transfer system with built-in redundancy. More information on the planned readout architecture, replacement electronics and the demonstrator system can be found in Ref [18].