A calorimeter is a particle detector designed to measure the energy lost by a particle. It operates by taking advantage of an effect first observed in cloud chambers in the 1930s: a single high-energy particle split into many particles in a cascade process now known as a particle shower.


The energy of this shower gives the estimation of the energy of the initial particle. The cascade process can be explained considering the special case in which the initial particle is an electron or a positron. Indeed recoiling from atoms of matter they produce photons that can produce electron-positron pairs if they have enough energy which starts the whole cycle again, doubling the total number of particles in each steps.

The energy of the photons in each step is lower than the previous one which brings an end of the pair production process. The same cannot be said of the shower, since the electrons and positrons can still have other types of interactions up to their complete absorption in the material.


This cascade process can be generalized: charged particles can create a neutral one and vice versa. Calorimeters are sensitive to any type of particle, making them widely used in particle physics. If the shower originates from electrons or positrons it is called an electromagnetic shower. However, if it originates from hadrons (proton, neutron, pion etc.) it is called hadronic shower. Accordingly there are two types, electromagnetic or hadronic calorimeters.


Either of these can be made as a sampling or homogeneous calorimeter. In the first case there is a succession of materials where the particle shower starts which is passive, where the deposited energy is measured using ordinary light sensors (photomultiplier tube, avalanche photodiode and silicon photomultiplier). A disadvantage of this kind of calorimeter is that some of the energy is deposited in the passive material and is not measured. For homogeneous calorimeters, one material combines both the properties of an absorber and a detector, meaning that its total volume is sensitive to the deposited energy.


Examples of experiments equipped with both electromagnetic and hadronic calorimeter are the two largest experiments at the Large hadron collider (LHC): the Compact Muon Solenoid (CMS) and ATLAS (A Toroidal LHC ApparatuS).


CMS and ATLAS has the goal to investigate a wide range of physics, including the search for the Higgs bosons extra dimensions, and particles that could explain the dark matter. The so called spontaneous symmetry breaking, which includes the Higgs boson, gives mass to elementary particles and explain the differences between the weak force and electromagnetism. In July 2012 both ATLAS and CMS reported the discovery of a particle consistent with the Higgs boson with a mass of 125 GeV. This new particle was detected by looking at its possible decay into two photons or four leptons. In 2013 Peter Higgs and François Englert, two of the theoretical physicists who predicted the existence of the Higgs boson, won the Nobel Prize in Physics.


The Compact Muon Solenoid (CMS) is built around the LHC's beam pipe 100 meters underground, within which the proton-proton collisions take place. In total the pipe is 21.6 meters long, 15 meters in diameter and weighs about 14000 tonnes. To record the signatures of particles produced when beams of protons are smashed together, it contains several layer of detectors designed to measure the energy and momentum of photons, electrons, muons, and other products.

The innermost is a silicon-based tracker, surrounded by an electromagnetic calorimeter, which is itself surrounded with a sampling hadronic calorimeter. The tracker and the calorimeters are located inside a solenoid which generates a powerful magnetic field, while outside the magnet there are the muon detectors and the outer calorimeter.

The Electromagnetic Calorimeter (ECAL) designed to measure the energies of electrons and photons is made up of a barrel section and two end-caps.

The barrel section consists of 61200 crystals of lead tungstate, an extremely dense and optically clear material. The end-caps seal off the barrel at either end, each containing 7324 further crystals. Hamamatsu S8148 avalanche photodiodes (APDs) and vacuum photodiodes (VPTs) are used as photodetectors in the barrel and end-caps calorimeter respectively.


The APDs have a photosensitive area of 5 mm by 5 mm and are made of opto-semiconductors to achieve optimal performance in a strong magnetic field. The Hadron Calorimeter (HCAL) consists of layers of brass or steel interleaved with plastic scintillators passed through by wavelength-shifting fibers and read out by hybrid photodiodes.

ATLAS, like CMS, sits in a cavern 100 meters underground. It is a cylindrical long 46 meters, with a diameter of 25 meters, and weighing around 7,000 tonnes.

The detector is made up of six different subsystems designed to detect some of the most energetic particles ever created on earth. They are wrapped concentrically in layers around the collision point to measure the trajectory, momentum, and energy of the particles. The four major components are the inner detector, calorimeter, muon spectrometer and the magnet system.


There two sampling calorimeters: one electromagnetic and the other hadronic. For the electromagnetic calorimeter the energy-absorbing materials are lead and stainless steel, with liquid argon as active material kept at low temperature by a cryostat. The hadronic calorimeter is realized with a variety of different sub-parts employing several techniques.

The central region is called the Tile Calorimeter and it is made up of alternating layers of iron used as an absorber and scintillating tiles as active material to measure the energy deposited. The Tile Calorimeter measures the energy and direction of the quarks and gluons produced in the proton-proton collisions, which appear as jets of charged and neutral particles.


It consists of a fine-grained steel matrix with 430 000 “tiles” of plastic scintillator. Optical fibers read the light signal and carry it to 10 000 Hamamatsu R7877 photomultiplier tubes. The Endcap region is covered by the Liquid Argon Hadronic EndCap made of lead and liquid Argon like the electromagnetic calorimeter.