The central detector of OPAL had the task of making precise measurements of the trajectories of charged particles. It consisted of four components; the innermost is a silicon microvertex detector, immediately outside which was the vertex drift chamber. The vertex chamber was built in Carleton University, Ottawa, while Cambridge, in collaboration with QMUL and the RAL electronics group designed built the readout electronics and data acquisition software.
The vertex chamber was a 1 m long, 0.47 m diameter cylindrical drift chamber which was segmented radially into an inner layer of 36 cells with axial wires and an outer layer of 36 small angle (4 degree) stereo cells.
The axial cells provided a precise measurement of the position (50 microns) in the r-phi plane. A coarse measurement of the z-coordinate could be made by measuring the time difference between the signals from the two ends of the anode wire. The combination of axial and stereo cell information provided a precise z measurement for charged particles close to the interaction region.