The stability of vacuum phototriodes to varying light pulse loads and long term changes in response

Abstract

Mesh anode Vacuum Phototriodes (VPTs) are radiation resistant, single gain-stage photomultipliers which are designed to operate in a strong quasi-axial magnetic field. These VPTs are used in the endcap electromagnetic calorimeter of the CMS experiment at the CERN LHC to detect scintillation light from lead tungstate crystals. Short term dynamic response changes occur because of pulse rate variations during normal LHC operation cycles. Over the longer term the effect of increasing integrated charge taken from the photocathode causes an overall degradation of response. We have investigated these effects over time periods exceeding two years of simulated operation and discuss the implications for the long term performance of the VPTs in CMS.

Evolution Studies of the CMS Electromagnetic Calorimeter Endcap Signals

“…This component might be of different origin than the annealing from ionizing damage. For example, the VPT response might exhibit some recovery [17]. Another reason might be that, due to the difference between the creation and annihilation rates for color centers, the recovery for μ-intercepts could partially come from the connection between μ-slopes and μ-intercepts in the fitting procedure.…”