The xenon monitor of the L3 electromagnetic calorimeter

Abstract A monitoring system was built for tracking the behaviour of each BGO crystal and associated electronics in the L3 electromagnetic calorimeter and for maintaining its calibration. The construction of the system is described in detail. This monitoring system was first used at the CERN SPS, where each crystal was calibrated in an electron beam, and then on the LEP collider. A procedure combining the information of the xenon monitor of theBhabha events allows to maintain the absolute calibration of each crystal to within 0.9%.