Time projection chamber
In physics, a time projection chamber (or TPC) is a particle detector invented by David R. Nygren, an American physicist, at Lawrence Berkeley Laboratory in the late 1970s. Its first major application was in the PEP-4 detector, which studied 29 GeV electron-positron collisions at the PEP storage ring at SLAC.
A time projection chamber consists of a gas-filled detection volume in an electric field with a position-sensitive electron collection system. The original design (and the one most commonly used) is a cylindrical chamber with multi-wire proportional chambers (MWPC) as endplates. Along its length, the chamber is divided into halves by means of a central high-voltage electrode disc, which establishes an electric field between the center and the end plates. Furthermore, a magnetic field is often applied along the length of the cylinder, parallel to the electric field, in order to minimize the diffusion of the electrons coming from the ionization of the gas. On passing through the detector gas, a particle will produce primary ionization along its track. The z-coordinate (along the cylinder axis) is determined by measuring the drift time from the ionization event to the MWPC at the end. This is done using the usual technique of a drift chamber. The MWPC at the end is arranged with the anode wires in the azimuthal direction, θ, which provides information on the radial coordinate, r. To obtain the azimuthal direction, each cathode plane is divided into strips along the radial direction.
In recent years other means of position-sensitive electron amplification and detection have become more widely used, especially in conjunction with the increased application of time projection chambers in nuclear physics. These usually combine a segmented anode plate with either just a Frisch grid  or an active electron multiplication element like a Gas Electron Multiplier. These newer TPCs also depart from the traditional geometry of a cylinder with an axial field in favour of a flat geometry or a cylinder with a radial field.
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- Spencer Klein (27 January 2004). "The time projection chamber turns 25". CERN Courier 44 (1).
- Jeffery Kahn (22 February 1999). "Back to the Beginning, The Time Projection Chamber". Sciencebeat. Lawrence Berkeley National Laboratory.
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