The TPC will be full of gas with an electric field applied across it. When charged particles pass through, they will knock electrons out of atoms in the gas and these will drift in the electric field. By measuring the arrival of electrons at the end of the chamber, the TPC will reconstruct the path of the original charged particles. It will be made of lead- tungstate crystals like these. When high energy photons strike lead tungstate, they cause it to glow, or scintillate, and this glow can be measured.
Lead tungstate is extremely dense, which means that it stops most photons that reach it.
If the low energy particles may be identified by the loss of energy, the higher ones are detected measuring the time it takes for a particle to reach from the collision point to the detector barrel which is 3.5 meters away. The sensor for the arrival of the particles will be Parrallel Plate Counters - 160000 of them distributed over 150 square meters. Using the tracking information from other detectors every track firing a sensor is identified. This detector is based on the detection of so-called Cherenkov photons emitted by the particles in a dielectric medium. The two-third prototype successfully tested at CERN in November 1997 is shown.