Fine-mesh photodetectors for CMS endcap electromagnetic calorimeter

The behaviour of fine-mesh vacuum phototriodes (VPTs) with the external dia meters of 21 and 25 mm has been investigated in an axial magnetic field up to 4 T in view of their applications as readout devices for CMS Endcap Elec tromagnetic 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 a nd the stability of the photocathode response under the illumination by lig ht emission diodes (LED) and the irradiation by 14 MeV neutrons. Phototriod es with 100 lines per mm fine-mesh and 25 mm external diameter are found to be the best candidates for coupling with rear PbWO4 crystals by dimensions of 30 x 30 mm, proposed to be used in CMS ECAL Endcaps. VPTs provide a gai n 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. (C) 2000 Elsevier S cience B.V. All rights reserved.