Avalanche photodiodes and vacuum phototriodes for the CMS electromagnetic calorimeter

The Electromagnetic Calorimeter for the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider has lead tungstate crystals as the active medium. The measurement of scintillation light from the crystals poses substantial technical challenges. The photodetectors must be fast, sensitive, radiation-hard, and operate with significant internal gain in a magnetic field of 4 T. CMS has developed two technologies to satisfy these requirements. In the ‘Barrel’ subsystem of the electromagnetic calorimeter (ECAL), avalanche photodiodes (APDs) are used. In the ‘Endcaps’, where the radiation levels are highest, single-stage, mesh-anode phototubes (‘vacuum phototriodes’) are deployed. The design and performance characteristics of the APDs and vacuum photo-triode (VPTs) are described and illustrative test beam results are presented.