Super radiation hard vacuum phototriodes for the CMS endcap ECAL

The CMS experiment at the CERN LHC

“…The faceplates were required to show a transmission loss, integrated over the wavelength range corresponding to PbWO 4 emission, of less than 10%. Irradiation of VPTs in a nuclear reactor to 7 × 10 14 n/cm 2 showed a loss in anode sensitivity entirely consistent with discolouration of the faceplate caused by the accompanying gamma dose (100 kGy) [80]. Irradiations of tubes biased to the working voltage, with both gammas and neutrons showed no adverse effects, apart from an increase in anode current, attributable to the production of Cerenkov light in the faceplates.…”

Evolution Studies of the CMS Electromagnetic Calorimeter Endcap Signals

“…Under irradiation, color centers are formed in the glass material of the face-plates, leading to a further decrease in transparency. Transmission losses of ∼10 % under γ radiation for doses expected at |η| ≈ 2.6 have been measured [8,9]. For neutron irradiation up to 7 × 10 15 n/cm 2 , light transmission losses of about ∼10−30 % have been measured [8].…”

“…A further contribution to the VPT losses under radiation is the response loss caused by positive charge accumulation on the photo cathode, mainly due to the scintillation light showers within the photo tube [9]. This can happen if, for example, the scintillation light hits residual gas in the photo tube leading to ion collection on the photo cathode.…”