Basics:
A Time Projection Chamber (TPC) is a gas-filled detector used in high energy physics. It consists of an electric field forming structure, the field cage, which is concluded at the ends by readout planes. A cathode divides in general the gas volume in two equally spaced halves (the picture shows only one half of a TPC). Between the cathode (photo: grey plane)and the readout planes (photo: left plane in blue), which act as anodes, an electric field of a few hundred volts per cm is applied. A charged particle (photo: first green particle coming from the top) traversing the detector ionizes the gas molecules. The electrons and ions released in this process, called primary charges, drift afterwards to the anode (electrons (picture: green)) and the cathode (ions (picture: blue)) following the electric field lines. The electron signal is used to measure the track of the travering particle. The z-position of the track can be determined by measuring the drift time. For this the time when the track crossed the TPC is determined e.g. by external scintillators. The arrival time of the electrons at the readout plane gives then the drift time. To determine the position of the track in the rphi-plane, which is the plane parallel to anode and cathode, the readout plane is divided in a large number of small pads.
Is in addition a magnetic field
is applied parallel to the electric field also the curverture of the track can be
measured allowing the determination of the momentum of the particle. The knowledge
of the momentum in connection with the measurment of the energy released per cm,
given by the number of primary electrons per cm of the track, allows to determine on a
statistical basis which kind of particle traversed the chamber. The gas choice
is a critical parameter in the design of a TPC and depends strongly on the
experimental environment in which the TPC is used. Since the direct signal of the
primary electrons is too small to be read out directly a gas avalanche is created
close to the readout plane. For this amplification classically wires were used.
Nowadays, micro pattern gas detectors (MPGDs), mainly MicroMegas and GEMs,
are going to replace this wires for several reasons which are discussed below.High energy experiments which used and use respectively TPCs in their detectors are e.g. ALEPH, STAR and ALICE. The first TPC with MicroMegas readout is currently under construction for the T2K experiment.
MPGDs:
Gas Electron Multipliers:
Micromegas:
