The tools of the trade 2

Introduction to detectors

Particle detectors are like electronic eyes. They allow physicists to see the results of particle collisions by tracking the particles that emerge.

Each of LEP's four detectors is built in the form of a giant cylinder, about three stories high and constructed like a Russian doll. Each layer of the cylinder has a specific task to perform, some measure the particles' tracks, others measure their energies. By combining the evidence from different layers, physicists are able to learn a lot about the particles emerging from collisions.

The innermost layer of a typical detector has the job of measuring particle tracks to high precision. Such detectors are frequently called vertex detectors because they are placed around the collision point, or vertex, from which new particles emerge.

The ALEPH experiment's vertex detector on the work bench.

This is surrounded by more tracking detectors whose job is to follow the tracks of emerging particles. These do not need to be so precise as the vertex detector because the density of tracks is lower as the particles fly away from the collision.

The DELPHI experiment's tracking detector being inserted.

Beyond the tracking chambers are detectors for measuring the particles' energies. A variety of different techniques is used to build these so-called calorimeters but all rely the basic principle of stopping the particles in a dense medium.

The OPAL experiment's calorimeter.

Finally, the outermost layer has the specific task of detecting particles called muons. These are the only detectable particles able to punch their way through the calorimeter and escape the detector altogether.

The L3 experiment's muon detectors.
Charged particles follow curved paths in magnetic fields.
This allows their momenta to be measured and for this reason particle detectors usually have cylindrical magnets, solenoids, embedded inside them. This one, being transported from England to CERN belongs to the DELPHI experiment.

The DELPHI experiment's magnet.

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Particle Physics Education CD-ROM 1999 CERN