The response to high magnetic fields of the vacuum phototriodes for the Compact Muon Solenoid endcap electromagnetic calorimeter

The endcap electromagnetic calorimeter of the compact muon solenoid detects particles with the dense fast scintillator lead tungstate (PbWO4). Due to the low light yield of this scintillator, photodetectors with internal gain are required. Silicon avalanche photodiodes cannot be used in the endcap region due to the intense neutron flux. Following an extensive R&D programme, 26mm diameter single-stage photomultipliers (vacuum phototriodes) have been chosen as the photodetector in the endcap region. The first 1400 production devices are currently being evaluated following recent tests of a pre-production batch of 500 tubes. Tubes passing our acceptance tests have responses, averaged over the angular acceptance of the endcap calorimeter, corresponding to the range 20–55electrons/MeV deposited in PbWO4. These phototriodes operate, with a typical gain of 10, in magnetic fields up to 4T.