Delphi detector at CERN: Installing of the time projection chamber (track detector) inside calorimeters.

Principle
A charged particle going through matter interacts with the numerous electrons. A tiny fraction of the energy of the original particle is transferred to the electrons that will escape from the atom where they were bound. The observation of these free electrons will show where the original particle passed. Detecting many of these points results in an accurate determination of the particle trajectory.

Track detectors
Most modern particle experiments incorporate several track detectors. These detectors consist of a gas volume in which the particles leave a trail of ionisation i.e. negative electrons and positive ions. The ionisation is either collected on thin metal wires in the gas volume or the electrons are made to drift in an electric field to the end of the gas volume, where they are collected onto thin metal wires and result in an electric pulse. The position of the metal wire and the measured drift time indicate where the original particle passed. Often the ionisation is registered on small pads rather than on wires. Information on the "fired" pad and the drift time gives a point in space where the particle has passed. Several such points are needed to determine the particle trajectory. In DELPHI most track detectors are different types of drift chambers.

Momentum determination
Charged particles are deviated in a magnetic field. The larger the momentum, the smaller the deviation. By measuring this deviation the momentum can be determined. The track detectors are therefore often placed in a strong magnetic field. In DELPHI the magnetic field is created by a supraconducting solenoid. The magnetic field inside the solenoid is parallel to the beam line of the colliding particles and its strength is 1.2 Tesla.