Long term aging studies of the new PMTs used for the HL-LHC ATLAS hadron calorimeter upgrade

Abstract

The central hadron calorimeter (TileCal) of the ATLAS experiment at the Large Hadron Collider (LHC) is a sampling calorimeter read out by about 10,000 photomultipliers(PMTs). Earlier studies of performance showed a degradation in PMTs response as a function of theintegrated anode charge. At the end of the High-Luminosity LHC (HL-LHC) program, the expectedintegrated anode charge for PMTs reading out the most exposed cells is 600 C, and theirprojected response loss is 25%.These PMTs (≈ 8% of the total TileCal PMTs) willbe replaced with a newer version PMT with the same geometry.A test set-up is being used in the Pisa laboratory to study thelong term response of this new PMT modeland compare it to theold PMT model currently installed in TileCal. For the first time thisnew PMT has been tested after integrating more than 800 C of anode charge. Preliminaryresults obtained from data collected in Pisa over a period exceeding two years areshown in this paper. We also introduce a preliminary study aimed to disentangle the contribution tothe PMT response degradation due to the loss of quantum efficiency and to the change in absolutegain.