A wire chamber (also called multi-wire proportional chamber ) is a detector for ionizing radiation, which is used among other things in accelerator experiments in elementary particle physics. In addition to the indication of the presence of ionizing radiation (such as the Geiger- Müller counter ) and the trajectories of the particles are determined in the wire chamber.
The advantage of the wire chamber from the cloud chamber and bubble chamber is located in the electronic readability of the data collected. The detour via photographic or video recordings, as in the fog and bubble chamber and in the spark chamber used previously omitted. Also many more events per unit of time are recorded.
For the development of the wire chamber at CERN, the Nobel Prize in Physics 1992 was awarded to Georges Charpak.
The principle of the wire chamber is similar to the proportional counter. In a gas-filled chamber are parallel wires that lie at positive high voltage. A particle that is flies through the chamber along its path, the ionized gas, it is characterized between the nearest adjacent wire and a cathode to an electrical current. Whereas in the conventional counter tube only one wire is present, and the cathode as a pipe concentrically surrounding the wire, one wire chamber of many parallel wires between two cathode plates. On each wire, the current pulses are taken separately; thus can be determined, in which wire flew past the particle. Dividing the cathode into narrow strips that run perpendicular to the wires, and you measure also the flow of this strip, it can be seen near where the crossing point of a wire and a cathode strip the particle is flown. Another way to determine the position of the particle in two dimensions, is to be fitted with each other crossed wires of two -wire chambers, one above the other. With multiple layers of such detectors can also reconstruct the particle trajectories in three dimensions. Although the distances between the wires and the cathode strips in the region of several millimeters, the particle trajectories can be determined with an accuracy of some tenths of a millimeter by the pulses of adjacent strips or wires are compared.
Wire chambers are called proportional counters (English: multi -wire proportional chamber, MWPC ) operated: In the high electric field strength near the anode wires, the electrons are accelerated; when they hit the gas atoms, ionize them. Thereby electrons are released again, and it is so (depending on the applied voltage and the gas pressure) to a gain of the current by a factor of 103 to 106 ( avalanche charge ). The current pulse is proportional to the originally generated charge. It can be concluded on the particle type or energy. The applied voltage is in contrast to the Geiger- Müller counter is too low ( and the gas pressure is too high ) to ignite an independent gas discharge. As a gas mixture of the inert gas argon (main component ) and a further gas such as CO2 or methane is used in a pressure close to 1 bar mostly. The addition of such a gaseous compound causes shorter pulses, makes the detector that is "faster" by the drifting free electrons removed by inelastic scattering on the molecules of some energy and its temperature is lowered so. It also suppresses ultraviolet radiation, which could lead to supernumerary pulses.
The electrons need a certain time to the location of ionization in the particle to the anode to migrate ( to " drift "). If you determined in special wire chambers this time, the distance of the particle measured from the wires and thus determine the particle trajectory with high accuracy. Such an arrangement is referred to as a drift chamber.
To measure the velocity of the electrons in the gas ( drift velocity ), there are special drift chambers, Velocity Drift Chambers ( VDC) called at known locations where gas molecules are ionized and then the drift time is measured.
External magnetic field
Next to the path of the particles and the pulse (and hence the energy for a known type of particle ) be able to determine precisely a homogeneous magnetic field is applied perpendicular to the direction of movement. The particles are deflected by the Lorentz force in the magnetic field. From the curvature of the tracks of the momentum of the particle is determined (see cyclotron ). Since the radius grows with the particle momentum, the curvature with increasing momentum (and thus high particle energy ) can be determined with less accuracy. Therefore, this method is not applicable in general, to determine the kinetic energy of high-energy particles exactly.