Measures the energy a particle loses as it passes through the detector
They are designed to absorb most of the particles coming from a collision, forcing them to deposit all of their energy and stop within the detector. ATLAS calorimeters consist of layers of an “absorbing” high-density material that stops incoming particles, interleaved with layers of an “active” medium that measures their energy.
Electromagnetic calorimeters measure the energy of electrons and photons as they interact with matter. Hadronic calorimeters sample the energy of hadrons (particles that contain quarks, such as protons and neutrons) as they interact with atomic nuclei. Calorimeters can stop most known particles except muons and neutrinos.
The components of the ATLAS calorimetry system are: the Liquid Argon (LAr) Calorimeter and the Tile Hadronic Calorimeter.
Liquid Argon Calorimeter
The Liquid Argon (LAr) Calorimeter surrounds the ATLAS Inner Detector and measures the energy of electrons, photons and hadrons. It features layers of metal (either tungsten, copper or lead) that absorb incoming particles, converting them into a “shower” of new, lower energy particles. These particles ionise liquid argon sandwiched between the layers, producing an electric current that is measured. By combining all of the detected currents, physicists can determine the energy of the original particle that hit the detector.
The central region of the calorimeter is specially designed to identify electrons and photons. It features a characteristic accordion structure, with a honeycomb pattern, to ensure that no particle escapes unchallenged.
To keep the argon in liquid form, the calorimeter is kept at -184°C. Specially-designed, vacuum-sealed cylinders of cables bring the electronic signals from the cold liquid argon to the warm area where the readout electronics are located.
- Barrel 6.4m long, 53cm thick, 110,000 channels.
- LAr endcap consists of the forward calorimeter, electromagnetic (EM) and hadronic endcaps
- EM endcaps each have thickness 0.632m and radius 2.077m
- Hadronic endcaps consist of two wheels of thickness 0.8m and 1.0m with radius 2.09m
- Forward calorimeter has three modules of radius 0.455m and thickness 0.450m each
Tile Hadronic Calorimeter
The Tile Calorimeter surrounds the LAr calorimeter and measures the energy of hadronic particles, which do not deposit all of their energy in the LAr Calorimeter. It is made of layers of steel and plastic scintillating tiles. As particles hit the layers of steel, they generate a shower of new particles. The plastic scintillators in turn produce photons, which are converted into an electric current whose intensity is proportional to the original particle’s energy.
The Tile Calorimeter is made up of about 420,000 plastic scintillator tiles working in sync. It is the heaviest part of the ATLAS experiment, weighing almost 2900 tonnes!
- 2900 tonne total weight
- Central barrel made of 64 wedges, each 5.6m long; two extended barrels each with 64 wedges, each 2.6m long
- 420,000 scintillating tiles, weighing 40 tonnes
- 9,500 photomultiplier tubes