Coordinate Measurement and Fundamental Limits of Accuracy

Abstract

After the passage of a charged particle through the sensitive gas volume, the electrons produced in the ionization process along a trajectory segment drift toward the sense wire, where they are collected and amplified in avalanches. There are essentially four different methods to determine particle coordinates in drift chambers:

• 1. Measurement of the drift time. Using the known drift velocity of the ionization electrons along their drift trajectory, this determines the distance along the drift trajectory between wire and track.

• 2. Measurement of the pulse-height ratios on pick-up electrodes (strips, “pads”, wires) near the sense-wire avalanche. With electrode response known this determines the coordinate of the avalanche between the pick-up electrodes. If the drift trajectory is known, the corresponding coordinate is determined. In this way the coordinate along the sense wire is measurable.

• 3. Measurement of the pulse-height ratios at the two ends of a sense wire (charge division). The position of the avalanche along the wire is determined using the known damping of the pulse as it propagates along the wire.

• 4. Measurement of the difference of arrival times at the two ends of a sense wire (time difference). This has been used for the same purpose, applying the known propagation time of the signal on the wire.