A multi-wire chamber (or just wire chamber) is a type of proportional counter that detects charged particles and photons, by means of gaseous ionization detection of particles of ionizing radiation.
A proportional counter uses a wire, under high voltage, which runs through a metal or conductive enclosure whose walls are held at ground potential. The enclosure 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 around the wire, causing a localised cascade of ionization which is collected on the wire and results in an electric current proportional to the energy of the detected particle. This allows the experimenter to count particles and importantly, in the case of the proportional counter, to determine their energy.
Adaptions of 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.
The multi-wire chamber is a development of the spark chamber. 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.
In a typical experiment, the chamber contains a mixture of these gases:
- argon (about 2/3)
- isobutane (just under 1/3)
- freon (0.5%)
The chamber could also be filled with:
- liquid xenon;
- liquid tetramethylsilane; or
- tetrakis(dimethylamine)ethylene (TMAE) vapour.
Additional gases used are shown in T.Ferbel page 81.
The development of the chamber enabled scientists to study the trajectories of particles with much improved precision, in addition to which, to also for the first time observe and study the rarer interactions that occur through particle interaction. The Mw chamber is an advancement of the proportional chamber that has the capacities to measure the energies of particles instead of just the measurement of number of particles as in previous detection by counters, for example the Geiger counter. This is achieved by a change to the sensory apparatus of the instrument in that instead of one anode wire surrounded by a cathode plate, a number of equidistant wires provide sensory information from within two parallel plates.
For high energy physics experiments, it is also valuable to observe the 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, photographs were made, printed and then looked through.) 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.
If one also precisely measures the timing of the current pulses of the wires and takes into account that the ions need some time to drift to the nearest wire, one can infer the distance at which the particle passed the wire. This greatly increases the accuracy of the path reconstruction and is known as a drift chamber.
The drift chamber functions by balancing the loss of energy from particles caused by impacts with particles of gas, with the accretion of energy created with high-energy electrical fields in use to cause the particle acceleration. Design is similar to the Mw chamber but instead with central layer wires at a greater distance apart. The detection of charged particles within the chamber is possible by the ionizing of particles of gas due to the motion of the charged particle.
If two drift chambers are used with the wires of one orthogonal to the wires of the other, both orthogonal to the beam direction, a more precise detection of the position is obtained. If an additional simple detector (like the one used in a veto counter) is used to detect, with poor or null positional resolution, the particle at a fixed distance before or after the wires, a tri-dimensional reconstruction can be made and the speed of the particle deducted from the difference in time of the passage of the particle in the different part of the detector. This setup gives up the detector called Time Projection Chamber (often written just TPC)
For measuring the velocity of the electrons in a gas (drift velocity) there are special drift chambers, Velocity Drift Chambers which measure the drift time for known location of ionisation.
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- hypermail_ archive of links to CLAS drift chambers