Particle accelerators
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How to build a particle detector: calorimeters
What is a calorimeter and why do they matter?
Calorimeters measure the energy of a particle which helps identify what particle it is. There are two key types, defined by what particles they can measure: electromagnetic calorimeters, which measure electrons and photons, and hadronic calorimeters, which measure hadrons (such as protons, neutrons, and pions). When a high energy particle approaches matter, like that of a detector, it is converted into more particles of lower energy. This process repeats to produce many low-energy particles, known as a "shower".
The two types of showers
The type of shower is defined by the process that causes more particles to be created. There are two types:
Electromagnetic showers. These showers involve (i) bremsstrahlung, where electrons continuously emit photons as they are slowed down by the electric field of a charged particle (usually an atomic nucleus), and (ii) pair production, where a photon is converted into an electron-positron pair near an atomic nucleus.
Hadronic showers. This process involves hadrons being converted into other hadrons or photons with lower energy. This is, however, more difficult to measure energy precisely since some of the energy of the original particle is stored in "invisible sources, such as the energy that holds the nuclei that are formed together.
These showers convert one high-energy particle into many lower energy particles. Low energy particles are more likely to be absorbed, which is essential for measuring their energy.
Measuring the energy
The energy of the original particle is given by the sum of the energies of the lower energy particles. The measurement signal is built up through one of two main ways:
Scintillating materials (crystals or plastic scintillator tiles): shower particles excite atoms or molecules which then de-excite by emitting visible/UV photons. These photons then hit photodetectors which convert this light into an electrical pulse that is proportional to the original particle's energy.
Ionisation calorimeters (such as liquid argon): charged particles shower free electrons which are then directed to electrodes by an electric field. These electrodes have a current flowing through them that is proportional to the energy of the original particle.
Design choices: homogeneous vs sampling
For building a calorimeter, there are two key design types: homogeneous and sampling. A homogeneous calorimeter uses a single material that both produces the shower and provides the signal (usually a scintillating crystal. This can give excellent energy resolution for electromagnetic particles because most of the shower energy is measured directly. A sampling calorimeter separates these roles. Dense absorber layers (lead, tungsten, steel) force the shower to develop, while thin active layers (scintillator or liquid argon) sample the shower as it passes through. Sampling designs are robust and can cover larger areas economically, but the resolution is limited by sampling fluctuations.
How thick is “thick enough”?
To measure the energy well the shower needs to be contained. Electromagnetic showers are characterised by the radiation length and are typically contained within tens of radiation lengths. Hadronic showers are larger and leak more easily, so hadronic calorimeters are made several interaction lengths deep and often sit behind the electromagnetic calorimeter.