Time projection chamber (TPC)

A detector that reconstructs one coordinate from the time required for ionization electrons to drift in an electric field, with the other coordinates measured by the charge or light pattern. A two-phase xenon implementation obtains depth from the delay between prompt particle scintillation and gas-phase charge readout.

A liquid target with a gaseous charge-readout layer. An interaction produces prompt particle scintillation, conventionally S1, and ionization electrons that drift to the liquid surface. Extracted electrons produce gas electroluminescence, conventionally S2. Timing and light-pattern information give three-dimensional position; the S2/S1 distribution helps distinguish electron recoils from nuclear recoils. It does not remove neutron-induced single recoils, and the discrimination requires calibration.

Selecting an inner fiducial volume reduces externally produced radiation because outer xenon attenuates it before it reaches the selected target. This trades target mass for reduced background. It does not remove uniformly distributed internal radioactivity or every neutron recoil, so purification, veto information and calibration remain important.

An underground liquid-xenon dark-matter direct detection experiment at Gran Sasso. Its two-phase time projection chamber combined position reconstruction with light-to-charge discrimination. The 2011–2012 search used a $34\mathrm{kg}$ inner target for $224.6$ live days and found candidate counts compatible with background.^[ref]