THE EFFECT OF PULSE RATE ON VACUUM PHOTOTRIODES RESPONSE AND THE USE OF AN LED PULSER TO IMPROVE STABILITY

The Endcap Electromagnetic Calorimeter of the Compact Muon Solenoid detector (CMS) at the Large Hadron Collider (LHC) uses vacuum phototriodes (VPTs), which operate in the full 3.8T magnetic field of the experiment, to detect the scintillation light from the lead tungstate crystals. Initial measurements of the variation in response of VPTs, induced by sudden changes in the illuminating light pulse rate, prompted the inclusion of a dedicated stability pulser based on light emitting diodes (LEDs). The response of production VPTs, under simulated LHC operating conditions, has been investigated in three independent studies: in-situ tests with the installed endcaps at CERN, and separate VPT studies by groups at the University of Virginia, USA and Brunel University, UK. In this work, results are presented which illustrate the magnitude of the effect to demonstrate the expected stability of the VPTs during normal LHC operation, with a proposed regime for operating the stability pulser to minimise variations in response. It is demonstrated that a continuous signal at a rate of 100Hz is sufficient to reduce the change in the VPT response to <0.2%.