The OPAL Endcap Electromagnetic Calorimeter

Outside the tracking chambers of OPAL were placed electromagnetic calorimeters. These were made of materials with high atomic number, in which electrons and photons shower and deposit their energy. In OPAL these calorimeters were formed from lead glass blocks. The showering particles emit Cerenkov light, and the amount of light is proportional to their energy. In OPAL, we had a cylindrical barrel of 9440 lead glass blocks, built by Tokyo. The ends of this cylinder are filled by two endcaps, each comprising 1132 blocks, read out by vacuum photo-triodes (VPT), covering the region 0.81<|cos(theta)|<0.98. Each block is approximately 9.4 cm square, with lengths between 38 and 52 cm. The endcap counters were built at RAL, while Cambridge constructed the laser monitoring system. This involves feeding a pulse of laser light into each counter; by measuring the resulting signal in the VPT we can check the calibrations of each counter, and track them over the lifetime of the experiment. This allows us to maintain a high energy resolution throughout the lifetime of the experiment. Cambridge was responsible for maintaining these calibrations, and also for most of the other software associated with the endcap calorimeters.