Multiwire Chamber

  A detector for charged particles which essentially consists of thin parallel and equally spaced anode wires symmetrically sandwiched between two cathode planes. Cathode planes can be a set of thin equally spaced wires but also can be made of a continuous plane conductor. The gap between the plane of the anode wires and the cathode plane is normally a few (3 to 4) times the spacing between the anode wires. The cathodes are on negative voltage and the wires are grounded. This creates a homogeneous electric field in most regions, with all field lines leading from the cathode to the anode wires. Around the anode wires, the field increases rapidly. If a particle passes through the detector it ionizes the gas ( Gas Mixtures in Gaseous Detectorss) in the chamber, and the liberated electrons follow the electric field lines towards the anode wires. The strong field very close to the wire acts as a multiplication region: the energy of the electrons increases, and in turn they ionize the gas, causing an avalanche of electrons to reach the anode wire.

The principles underlying modern multiwire chambers were already shown around 1920 (Geiger-Müller counter); the first wire chamber used in high-energy physics was, in fact, a spark chamber, whose electrode plates were replaced by grids of parallel wires in order to reduce multiple scattering, energy loss and secondary interactions, and to allow the localization of particle impact points without using photographic methods. Later, the idea of the Geiger-Müller counter was taken up again and developed into modern position detectors, mostly by the work of G. Charpak and his coworkers (Nobel prize 1992), [Charpak68].

The pulses are read from the anode wire or sense wire . The pulse height depends on the gas used and the voltage applied and also geometrical parameters of the chamber like the gap, wire spacing, wire diameter, etc. If the chamber is used in proportional mode ( Gaseous Detectors, Operational Modes), the pulse height is a measure of the energy loss of the particle in the gas. This can be used for particle or momentum identification. Simple multiwire chambers are used as tracking chambers, with the anode wires only giving one bit of information for a passing particle. Multiple planes with different angles of inclination for the wires will then allow reconstruction of trajectories in space.

A wide variety of multiwire chambers of different complexity has been constructed and tested, and used successfully in experiments:

• - proportional and drift chambers of planar and cylindrical type. They provide one-dimensional measurements on a surface made by the parallel wires. The information is binary (on or off) or may contain pulse height; with drift time measured, the inter-wire distance can be subdivided, but left-right ambiguities are introduced. Planes at different angles or segmentation of the cathode ( Cathode Strips) allow one to obtain information along the wire.
• - jet and time projection chambers. These chambers are characterized by comparatively longer drift times; multiple sense wires can cover a large volume, and are usually equipped with multihit electronics, such that the passage of several tracks in the volume part associated with a wire can be recorded. These chambers give inherently two-dimensional information (the drift time is in a coordinate orthogonal to the wire plane). With added cathode instrumentation or charge division, three-dimensional points can be obtained. For more details, Drift Chamber; a good review can be found in [Blum93].
• - time expansion chambers, a special type of drift chamber.

The main parameters of a wire chamber (from the viewpoint of optimizing particle detection) are:

• - single- and multihit detection efficiency;
• - precisison and two-track separation;
• - dead time.

For more details, see [Sauli91], [Fabjan80], [Walenta71], or theproceedings of the Vienna wire chamber conferences ( [Krammer95]).