Time Projection Chamber (TPC)

The transverse coordinates (r phi) are determined by wire proportional chambers at the ends of the TPC while the longitudinal (z) coordinate is obtained from the time it takes charges to drift to the ends of the TPC. The wire chambers see a projection in the xy plane of all the tracks; the time of arrival of the charge from each track segment gives its z location.

How the ALEPH TPC works
For each track crossing the TPC about 20 radial coordinates are measured. A longitudinal electric field (11 kV/m) is applied along the z axis. When the charged particle ionizes the gas (argon-methane at 1 atmosphere) in the detector volume electrons drift along the electric field toward either end of the TPC. The drifting electrons retain information on the xy position of the original clump of ionization. The time required to drift to the end of the detector gives information on the z position of the original ionization. The drifting is illustrated by the following sketch.

The large number of coordinate measurements are used to measure the curvature of the track in the 1.5 T magnetic field and determine the particle momentum precisely. Both the magnetic and electric fields are in the z direction in the TPC. It is easy to show that for the fields of the magnitudes used, the drifting electrons retain information on the xy coordinates of their starting point.

Additionally the TPC provides dE/dx (energy loss) information which can be used to determine the particle velocity, and together with its momentum, indicates the particle type.

The TPC is just outside the ITC and extends in radius to about 1.8 meters from the beam line. Its sensitive length is about 4.4 meters.

TPC sectors
Both ends of the TPC are equipped with arrays of "sectors" or proportional wire detectors which detect the drifting electrons. There are 18 sectors at each end. These wire chambers amplify the arriving cloud of electrons by the usual avalanche process and permit a measurement of both the time of arrival and the amount of charge (proportional to the original ionization). The sector wire chambers have concentric rows of cathode pads (in postscript ) in proximity to the sense wires. These pads give directly the r-phi coordinate of the hit. Special rows of pads are also used as part of the trigger system.

The time of arrival of these pulses relative to the time of the beam crossing at the IP provides information on the z position of the track element in the TPC. The half length of the TPC is 2 meters and the maximum drift time is about 45 microseconds. HR>