Fine--Mesh Photodetectors for CMS Endcap Electromagnetic Calorimeter

The behaviour of fine-mesh vacuum phototetrodes and phototriodes ( VPTs) in an axial magnetic field of ( 0 -- 4)T has been investigated. The measured VPT parameters are: fine-mesh cell dimensions, the photocathode sensitivity and its homogenity, the gain in zero and 4T magnetic field at tilt angles corresponding to the rapidity range of CMS ECAL Endcap 1.479 -- 3.0 and excess noise factor. Measurements have been performed on 21 and 30 mm diameter photodetectors with different fine-mesh structures: 30, 60 and 100 lines per mm under different types of photocathode illumination by green LED. Phototriodes with 30 or 60 lines per mm and an external diameter of 21 mm are found to be the best candidates for the CMS environment with the initial size of PWO crystals proposed to be used in the Endcap, by comparison with phototetrodes. They provide a gain of the order of 6 -- 8 in 4T magnetic field and an excess noise factor of 2 under full photocathode illumination.