CURRENT STATUS OF THE OPAL SILICON MICROVERTEX PROJECT

For the 1991 run of the OPAL detector at LEP, a complete two layer single-sided silicon microstrip detector array was installed around the beam-pipe. The detectors employed a novel biassing technique for the capacitively read out diode strips (Field Effect Transistors integrated on the silicon wafer) and the MX5 microplex read-out chip was used for the first time. The performance measured in OPAL for the ladders of 3 daisy-chained detectors (length 180mm) is very good with signal/noise of 22:1. The resolution of the detectors (measured to be around 5mum from test beam investigations) has allowed improvement in the impact parameter resolution of OPAL by greater than a factor of two. The beauty, tau and charm physics potential that this opens up is now being exploited. OPAL has plans to further upgrade to a detector giving both co-ordinates of the track intersection with the silicon barrels. To achieve this, ladder assemblies using two detectors of orthogonal strips glued back-to-back are being used. In addition, double-sided detectors of novel design have been fabricated and are being evaluated. Results from OPAL data taking at LEP, test beam results and laboratory measurements are presented. Both the performance of the detectors currently used in OPAL and those intended for future upgrades are discussed.