Robustness studies of the photomultipliers reading out TileCal, the central hadron calorimeter of the ATLAS experiment

The ATLAS Tile Calorimeter (TileCal) is the central section of the hadronic calorimeter of the ATLAS experiment. This sampling calorimeter uses steel plates as absorber and scintillating tiles as active medium. The light produced by the passage of charged particles is transmitted by wavelength shifting fibres to photo-multiplier tubes (PMTs), located in the outer part of the calorimeter. The readout is segmented into about 5000 cells, each one being read out by two PMTs in parallel. The detector readout geometry will not be changed for the Phase II of the High Luminosity Large Hadron Collider (HL-LHC) operation. A challenging goal is to understand whether the full sample of PMTs installed at the beginning of the ATLAS detector operation can be used until completion of the HL-LHC program or not. For this reason, a reliable study of the PMT robustness against ageing is required. Detailed studies modelling the PMT response variation as a function of the integrated anode charge were done. The PMT response evolution during LHC Run 2 of the PMTs mounted on the detector was studied for different amounts of anode integrated charge. At the same time, a test bench arrangement was used to study on a small PMT sample the ageing effects by LED or laser excitation. They integrated in a short time (1 year) anode charge amounts similar to those expected in the HL-LHC era for the PMTs reading out the most exposed TileCal cells. Results from on-detector PMT studies and test bench data are in agreement and led to the estimation that only the samples reading out the most exposed cells (8% of total 10,000 unit sample) may have a global response loss up to 50% (combined degradation of quantum efficiency and dynode gain). For this reason, and to preserve safe detector operation, it was decided to replace that 8% of PMTs with the latest version of the same geometry model produced by the vendor. Test bench measurements comparing old and new PMT models show that the latter type has improved performance in terms of response loss as a function of the anode integrated charge.