wire chamber

Suppose you want to know not just that something passed through a box of gas, but exactly where. One clever trick is to fill the box with gas and string it with many fine wires, each held at a high voltage. When a charged particle passes through and ionizes the gas, the freed electrons drift toward the nearest wire, and very close to that thin wire the electric field is so strong that each electron triggers an avalanche of more electrons — a tiny built-in amplifier. The wire that fires tells you where the particle went. That is the idea behind the wire chamber.

More precisely, in a multiwire proportional chamber, hundreds of parallel sense wires sit in a gas-filled volume between charged plates. A charged particle's ionization releases electrons that drift to the wires; the avalanche near a wire produces an electrical pulse large enough to read out, with the pulse size roughly proportional to the original ionization (hence 'proportional'). Knowing which wire fired gives the particle's position in one direction. A refinement called the drift chamber goes further: by also measuring how long the electrons took to drift to the wire, and knowing their steady drift speed in the gas, you compute their starting distance and locate the track far more precisely than the wire spacing alone would allow.

Invented by Georges Charpak in 1968 (a feat that won him the 1992 Nobel Prize in Physics), the wire chamber transformed particle physics: for the first time, tracks could be read out electronically and fed straight to a computer, replacing the slow, photograph-by-photograph analysis of bubble chambers. Wire chambers and their descendants still cover large volumes cheaply in many experiments. Their limit is speed and granularity: in the extreme collision rates of the LHC's inner regions, silicon trackers are preferred, while gas chambers remain excellent for larger, less crowded outer regions such as muon systems.