Super radiation hard vacuum phototriodes for the CMS endcap ECAL

The energy resolution sigma/E of the electromagnetic calorimeter (ECAL) in the energy range of 50-500 GeV is defined mainly by two terms: stochastic alpha/ root E and constant C. The photoreadout of the CMS Endcap ECAL consists of vacuum phototriodes (VPT), which are broadening a signal from n //p photoelectrons characterized by the excess noise factor F = n //p(sigma/E)**2. The technical specification of the CMS ECAL requires the value of F to be smaller than 4 in the CMS LHC environment during 10 years of detector operation. In this paper we present results of the VPT performance study in a magnetic field up to 4 T, in a gamma radiation field of 0-50 kGy and in a neutron fluence of 7 multiplied by 10**1**5n/cm**2. The standard phototriodes FEU-188 with faceplates from UV glass used in CMS ECAL as well as VPTs with super radiation hard cerium-doped glasses were investigated at the **6**0Co gamma facility, a neutron generator and a nuclear reactor in the Petersburg Nuclear Physics Institute (PNPI). The dependence of the VPT gain and the excess noise factor in magnetic fields on the fine-mesh plane orientation has also been studied.