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 or a cylinder with a radial field. Earlier researchers in particle physics also usually made use of a more simplified box-shaped geometry arranged directly above or below the beam line, such as in the CERN NA49 and NA35 experiments.
The Liquid Argon Time Projection Chamber (LArTPC) In 1974, William J. Willis and Veljko Radeka demonstrated 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.
Detector design and properties Liquid argon is advantageous as a sensitive medium for several reasons. 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. Liquid argon is also relatively inexpensive, making large-scale projects economically feasible. However, one of the primary motivations for using liquid argon as a sensitive medium is its density. Liquid argon is around one thousand times denser than the gas used in Nygren's TPC design, which increases the likelihood of a particle interacting in a detector by a factor of around one thousand. This feature is particularly useful in neutrino physics, where neutrino–nucleon interaction cross sections are small.
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