Multiwire proportional chamber

A multi-wire proportional chamber is a type of proportional counter that detects charged particles and photons and can give positional information on their trajectory, by tracking the trails of gaseous ionization.

The multi-wire chamber uses an array of wires at high voltage (anode), which run through a chamber with conductive walls held at ground potential (cathode). Alternatively, the wires may be at ground potential and the cathode held at a high negative voltage; the important thing is that a uniform electric field draws extra electrons or negative ions to the anode wires with little lateral motion.

The chamber is filled with carefully chosen gas, such as an argon/methane mix, such that any ionizing particle that passes through the tube will ionize surrounding gaseous atoms. The resulting ions and electrons are accelerated by the electric field across the chamber, causing a localised cascade of ionization known as a Townsend avalanche. This collects on the nearest wire and results in a charge proportional to the ionisation effect of the detected particle. By computing pulses from all the wires, the particle trajectory can be found.

Adaptions of this basic design are the thin gap, resistive plate and drift chambers. The drift chamber is also sub-divided into ranges of specific use in the chamber designs known as time projection, microstrip gas, and those types of detectors that use silicon.

In 1968, Georges Charpak, while at the European Organization for Nuclear Research in CERN, invented and developed the multi-wire proportional chamber (MWPC). The chamber was an advancement of the earlier bubble chamber rate of detection of only one or two particles every second to 1000 particle detections every second. The MWPC produced electronic signals from particle detection allowing scientists to examine data via computers. The multi-wire chamber is a development of the spark chamber.

In a typical experiment, the chamber contains a mixture of these gases:

The chamber could also be filled with:

For high energy physics experiments, it is used to observe a particle's path. For a long time, bubble chambers were used for this purpose, but with the improvement of electronics, it became desirable to have a detector with fast electronic read-out. (In bubble chambers, photographic exposures were made and the resulting printed photographs were then examined.) A wire chamber is a chamber with many parallel wires, arranged as a grid and put on high voltage, with the metal casing being on ground potential. As in the Geiger counter, a particle leaves a trace of ions and electrons, which drift toward the case or the nearest wire, respectively. By marking off the wires which had a pulse of current, one can see the particle's path.

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