thumb|200px|The TPC of the [[A Large Ion Collider Experiment|ALICE experiment at CERN]]
In physics, a time projection chamber (TPC) is a type of particle detector that uses a combination of electric fields and magnetic fields together with a sensitive volume of gas or liquid to perform a three-dimensional reconstruction of a particle trajectory or interaction.
The original design
The original TPC was proposed in 1974 by David R. Nygren, an American physicist, and developed 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 that total absorption calorimetry was possible in liquid argon detectors without the amplification that normally occurs in a gaseous ionization detector. This critical technology enabled the possibility of a time projection chamber based on Nygren's original design, but using liquid argon as the sensitive medium instead of gas.
In 1976, Herbert H. Chen, with collaborators at University of California, Irvine and the California Institute of Technology, proposed one of the earliest uses of liquid argon in a time projection chamber (LArTPC). Chen's initial goals with such a detector were to study neutrino-electron scattering, but the goals evolved to measure solar or cosmic neutrinos or proton decay.
In 1977, Carlo Rubbia independently, and nearly simultaneously, proposed to construct an LArTPC at CERN for rare event particle physics experiments. The fact that argon is a noble element and therefore has a vanishing electronegativity means that electrons produced by ionizing radiation will not be absorbed as they drift toward the detector readout. Argon also scintillates when an energetic charged particle passes by, releasing a number of scintillation photons that is proportional to the energy deposited in the argon by the passing particle. In contrast, signals for a collection plane wire are unipolar, since electrons do not pass by the wire but are instead "collected" by it. For both of these geometries, a larger signal amplitude implies that more drift electrons either passed by the wire (for induction planes) or were collected by it (for the collection plane).
The signal readout of all of the wires in a given anode plane can be organized into a 2D picture of a particle interaction. Such a picture is a projection of the 3D particle interaction onto a 2D plane whose normal vector is parallel to the wires in the specified anode plane. The 2D projections corresponding to each of the anode planes are combined to fully reconstruct the 3D interaction.
Dual-phase TPC
The technique itself was first developed for radiation detection using argon in the early 1970s. The ZEPLIN programme pioneered the use of two-phase technology for WIMP searches. The XENON and LUX series of detectors represent the state-of the art implementation of this instrument in physics.
Dark Matter Time Projection Chamber
The Dark Matter Time Projection Chamber is an experiment for direct detection of weakly interacting massive particles (WIMPs), one of the most favored candidates for dark matter. The experiment uses a low-pressure time projection chamber in order to extract the original direction of potential dark matter events. The collaboration includes physicists from the Massachusetts Institute of Technology (MIT), Boston University (BU), Brandeis University, and Royal Holloway University of London. Several prototype detectors have been built and tested in laboratories at MIT and BU. The collaboration took its first data in an underground laboratory at the Waste Isolation Pilot Plant (WIPP) site near Carlsbad, New Mexico in Fall, 2010.
Dark Matter Time Projection Chamber published first results from a surface run in 2010, setting a spin-dependent cross section limit.