Vacuum phototriodes for electromagnetic calorimeter applications

Abstract

Vacuum Phototriodes (VPTs) are fast, sensitive and radiation-hard photodetectors. They provide internal gain in a high magnetic field. They are used in a number of High Energy Physics experiments one of which, the Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC), is discussed here. The COMSOL Multiphysics simulation environment has been used to model the VPT. A segment that captures the characteristics of the thin anode mesh, a critical component within the VPT, has been accurately modelled in COMSOL. The electron trajectories, backscatter and induced currents on the anode are simulated using the electrostatics and charged particle tracing modules within COMSOL. This provides an insight on how an initial photoelectron and subsequent secondary electrons, behave as they travel within the VPT. Additional simulations were carried out to model the effects of the magnetic environment that the VPT experiences inside the CMS detector. When CMS was under construction extra VPTs were produced and stored in nitrogen purged refrigerators. These VPTs have been re-characterised to be used in a beam test at CERN, for a concept called double side readout. This re-characterisation process was carried out to ensure there was no severe degradation of the stored VPTs. The outcome shows there was minimal degradation with an average deviation of 6.6% from the initially measured gain during manufacture. The double-sided readout concept is discussed and its effect on reducing the effect of the non-uniformity of light produced in radiation damaged lead tungstate crystals is presented. One option to upgrade the endcap electromagnetic calorimeter of CMS for the forthcoming High Luminosity Large Hadron Collider (HL-LHC) was to replace the existing VPTs with a four-fold segmented device within the same external glass vacuum envelope. This VPT contains a 4-fold segmented anode. Two possible upgrade options are discussed. One of which would use this segmented VPT in a shashlik configuration with optical fibres, the second option being a high granularity silicon-tungsten calorimeter. The prototype Hamamatsu VPT for the first option provides a solution for space restricted environments by providing 4 independent channels. Experimental characterisation of the performance of this new VPT is presented. The photocathode uniformity response was measured along with the crosstalk that occurs between the segments as the illumination is precisely positioned along the faceplate. A COMSOL model was created for the segmented anode VPT prototype. The simulation included determining the induced signal as a function of time and the cross-talk in adjacent quadrants.