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The ALEPH Silicon Vertex Detector features an optical fibre laser system to monitor its mechanical stability. The operating principle and the general performance of the laser system are described. The experience obtained during 1997 and 1998 operations confirms the important role that such a system can have with respect to the detector alignment requirements. In particular, the laser system has been used to monitor short-term temperaturerelated effects and long-term movements. These results and a description of the laser-based alignment correction applied to the 1998 data are presented. * Now at Schweizerischer Bankverein, Basel, Switzerland. † Deceased.

The motivation for investigating the use of GaAs as a material for detecting particles in experiments for AbstractThe motivation for investigating the use of GaAs as a material for detecting particles in experiments for High Energy Physics (HEP) arose from its perceived resistance to radiation damage. This is a vital requirement for detector materials that are to be used in experiments at future accelerators where the radiation environments would exclude all but the most radiation resistant of detector types.