The Layers of a Detector

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.

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

Part of 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.

Most experiments have gigantic cylindrical magnets called solenoids embedded inside them. In magnetic fields, charged particles follow curved paths. By measuring the curvature of the track as seen by the tracking detectors, physicists can calculate the momenta of the particles. The straighter the track, the higher the momentum. Also, the direction of the curved path, clockwise or anticlockwise, reveals the sign of the charge of the particle.

This particular magnet, being transported from England, where it was made, to CERN belongs to the DELPHI experiment.

A charged particle is deviated by a magnetic field.