An Electromagnetic Calorimeter for Use in a Strong Magnetic Field at LEP Based on CEREN 25 Lead Glass and Vacuum Photo-Triodes

The end-caps of the lead glass calorimeter being built for the OPAL experiment at LEP will be required to operate in axial magnetic fields up to 1 Tesla. The technique adopted uses CEREN 25 glass instrumented with vacuum photo-triodes (single stage photomultipliers). We report results from tests on single modules in electron beams while subject to strong magnetic fields, which prove the viability of this design. The method results in excellent linearity, resolution and stability. The best energy resolution so far achieved is 5.3%//E at zero field with a degradation of less than 1.0%//E for magnetic fields up to 1 Tesla.