How does a calorimeter work?

The calorimeter is made up of two parts - the electromagnetic and hadron calorimeters.

Electromagnetic Calorimeter

The electromagnetic portion of the calorimeter is the part closest to the initial high-energy collision point.

It is constructed of layers of some material with high electric charge nuclei (e.g. steel) interleaved with layers of argon sealed in cells. Let's call the steel region the absorber material and the argon the collector region.

On the right is a calorimeter module. It is on display in the Visitor Center, has alternating layers of lead sheets and lead tiles, immersed in liquid argon when in use.

Pair Production

When a high-energy electron passes into the absorber material, the absorber feels a high electric field inside the atoms. (The higher the charge of the nucleus, the higher the field close to it.)

The force on the electron from the electric field causes the electron to change direction which causes it to emit a virtual photon (so that total energy and momentum are balanced).

The virtual photon has sufficient energy and momentum that it produces a positron-electron pair. These particles have high momentum and travel roughly parallel to the direction of the initial electron. So now there are three high-energy charged particles sharing the energy of the initial electron.

Each of the electrons, positrons, or photons produces further pairs as they pass through the high field regions of atoms. Thus a cascade or "shower" of electrons and positrons develops.

As each of these charged particles passes into the collector region, it ionizes the argon and argon ions drift to the ends of the cells where they give rise to voltage pulses.

Eventually there is not enough energy to form any more pairs, and all the electrons and positrons are absorbed by the material of the calorimeter.

A high energy photon also interacts in the electromagnetic field region of an atom to produce an electron and positron pair. (Some energy is transferred to the atom, otherwise the laws of energy and momentum conservation cannot be satisfied, this is why an isolated real photon cannot produce an electron and positron pair.)

The total number of particles formed and the depth to which the shower extends into the detector can be used to calculate the energy of the initiating particle.

Is it a photon or an electron/positron?

A charged particle track in the tracking system that matches an electromagnetic shower is identified as an electron or a positron. The sign of the charge can be seen from the way the track curves in the magnetic field in the drift chamber.

An electromagnetic shower that does not have a charged particle track pointing towards it is identified as a high energy photon.

Why muons do not produce a "shower"?

Muons have exactly the same interactions as electrons. However, the behavior of high-energy muons as they through matter is quite different from that of electrons.

The reason is that the muon is about 210 times more massive than the electron, and the force on it from the electric field is the same because the charge on it is the same as the charge on an electron. That force is not large enough to cause the muon to change direction significantly. It goes right on through the field region without radiating photons to produce any pairs or slowing down significantly.

Hadron Showers

Hadrons are so much more massive than electrons, they are not significantly deflected by the atomic electric fields. However, they have strong interactions so they do interact with the material in the calorimeter to produce a different type of shower.

Hadronic Calorimeter

Hadron Energy Measurement

The hadron portion of the calorimeter actually includes the electromagnetic calorimeter but extends beyond it. It has thicker layers of more dense material. In the SLAC Large Detector (SLD) the hadron calorimeter includes both the liquid argon calorimeter and the outer layers of steel (part of the magnet yoke) that are interleaved with detector layers and referred to as the "warm iron calorimeter."

Hadronic Interactions

When a hadron passes sufficiently close to a nucleus, there are residual strong interactions between the hadron and the protons and neutrons in the nucleus. These interactions result in a variety of processes that produce additional particles and slow down the initial high energy incoming particle.

The produced particles all interact too, producing yet more particles.

As with an electromagnetic shower, each collision shares the momentum between more particles until eventually all are slowed down and stopped.

For example, pions may be produced and then absorbed. Deflections of charged particles may result in the radiation of virtual photons that produce electron-positron pairs. A proton or neutron from a nucleus may be accelerated by the collision enough to be knocked out of the nucleus and travel as part of the shower.

Shower Shape

The chance that an initial high-energy hadron will pass close enough to a nucleus to initiate a shower is smaller than the chance that a high-energy electron will pass deep enough within an atom to feel a field that could cause it to initiate an electromagnetic shower. This is just geometry, the fields of an atom extend over regions approximately 10,000 times larger in radius (and thus 100,000,000 larger in area) than the nucleus of the atom.

Thus, electromagnetic showers typically begin a shorter distance into the calorimeter than hadron showers.

The amount of material needed so that any hadronic shower stops within the detector can be calculated. The thickness needed depends on the highest energy hadron that could be produced in the initial high-energy collision.

The desire to measure energy by stopping all particles determines the overall size of the huge high-energy detectors.

More Accurate Calorimeters

The type of calorimeter displayed in the visitor center is not the only choice. When more accurate shower positions or more accurate energy determination are needed other choices may be made.

The B Factory calorimeter (in the BaBar detector) consists of an array of large crystals of cesium iodide. Each crystal is attached to a phototube. Cesium iodide is a material that acts somewhat like a plastic scintillator in that it gives off light when a charged particle travels in it.

In this type of detector, the entire volume is active. The crystals play both roles -- of the dense layers and the scintillator layers -- found in the typical calorimeter in a multipurpose detector. This detector is sensitive to low energy photons that would be stopped in the first lead layer of a traditional multi-layered calorimeter and, hence, not recorded by it.

Cesium iodide crystals are expensive, so this type of detector is only used where the total energy is not too high and when the accuracy that it provides is an important feature for the experiment.