Inner Tracking Chamber (ITC)

The ITC, located between the vertex detector and the TPC, is used to measure the r phi position of a charged particle track with high precision. It is a conventional cylindrical drift chamber with 960 sense wires arranged in 8 concentric cylindrical layers. Each layer consists of 96 or 144 hexagonal drift cells, each of which contains a sense wire on which a pulse will be induced when a charged particle passes through the cell. The active length of the chamber is 2 meters and extends in radius between 16 and 26 cm from the beam line.

How a drift chamber works
When a charged particle originating at the Interaction Point (IP) passes through the ITC, the argon gas within one or two drift cells in each of the eight layers is ionized along the particle track. As the sense wire is held at a positive potential relative to the field wires which define the cell boundaries, electrons drift toward the sense wire. Within a few wire diameters (30 microns) of a sense wire the electric fields are very large and the drifting electrons are accelerated sufficiently to eject other electrons from gas atoms. Thus an avalanche develops and very large amounts of charge are present very close to the sense wire. The negative charge cloud drifts toward the sense wire and the heavier positive charges drift more slowly away from the wire. These charge motions induce a negative electrical pulse on the sense wire. As a consequence of the multiplication of charge (by a factor of 10⁵ or 10⁶) in the avalanche, the induced pulses are large enough to be visible on an oscilloscope but are amplified for further processing using conventional electronics.

The resolution of the ITC
The time of arrival of these pulses relative to the time of the beam crossing at the IP provides information on the precise position of the track in the drift cell. The drift velocity is approximately 5 cm/microsecond or 50 microns/nanosecond so that a time measurement at the nanosecond level is adequate.

The measured drift time depends essentially on the perpendicular distance of the track from the sense wire and is available for each cell that the track passes through. At an early stage in the analysis of data, the track parameters are found which best fit the measured positions as determined from the hit cells and their respective drift times. The r phi resolution of the ITC is about 150 microns averaged over the drift cell.

The position of tracks along the beam direction, z, is also determined by measuring the difference in arrival time of the signals at each end of the wires. This z measurement has a resolution of a few cm and is not used in the standard tracking but is used for a track trigger. HR>