OPAL Vertex Detector

The OPAL Central Vertex Detector

The central detector of OPAL has the task of making precise measurements of the trajectories of charged particles. It consists of four components; the innermost is a silicon microvertex detector, immediately outside which lies the vertex drift chamber. The vertex chamber was built in Carleton University, Ottawa, while Cambridge, in collaboration with QMW London and the DRAL electronics group designed built the readout electronics and data acquisition software.

The vertex chamber is a 1 m long, 0.47 m diameter cylindrical drift chamber which is 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 provide a precise measurement of the position (50 microns) in the r-phi plane. A coarse measurement of the z-coordinate can 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 provides a precise z measurement for charged particles close to the interaction region.