Fine-mesh photodetectors for CMS Endcap Electromagnetic Calorimeter

The behaviour of fine-mesh vacuum phototriodes (VPTs) with the external diameters of 21 and 35 mm has been investigated in an axial magnetic field up to 4 T in view of their applications as readout devices for CMS Endcap Electromagnetic Calorimeter. The measured VPT parameters are: the photocathode's sensitivity and its homogeneity, the gain in zero and 4 T magnetic field at tilt angles corresponding to the pseudorapidity range of CMS ECAL Endcap 1.48-3.0 as a function of fine-mesh cell dimensions, excess noise factor and the stability of the photocathode response under the illumination by light emission diodes (LED) and the irradiation by 14 MeV neutrons. Phototriodes with 100 lines per mm fine-mesh and 25 mm external diameter are found to be the best candidates for coupling with rear PbWO/sub 4/ crystals by dimensions of 30*30 mm, proposed to be used in CMS ECAL Endcaps. VPTs provide a gain of the order (6-8) in a 4 T magnetic field and an excess noise factor of 2-2.5 under illumination of a full photocathode's area. (2 refs).