The Upgraded Calibration System for the Scintillator-PMT Tile Hadronic Calorimeter of the ATLAS experiment at CERN/LHC

The ATLAS Tile Calorimeter (TileCal) is the central section of the hadronic calorimeter of the ATLAS experiment and provides important information for reconstruction of hadrons, jets, hadronic decays of tau leptons and missing transverse energy in highest energy proton-proton and heavy-ion collisions at CERN’s Large Hadron Collider. 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 photomultiplier tubes (PMTs) located on the outside of the calorimeter. The readout is segmented into about 5000 cells (longitudinally and transversally), each read out by two PMTs in parallel. A multi-component calibration system is employed to calibrate and monitor the stability and performance of each part of the readout chain during data taking. The TileCal calibration system comprises Cesium radioactive sources, laser and charge injection elements and it allows to monitor and equalize the calorimeter response at each stage of the signal production, from scintillation light to digitization. Based on LHC Run 1 experience, several calibration systems were improved for Run 2. We present the lessons learned, the modifications, and the current LHC Run 2 performance, which meets the design goals of sub-1% level accuracy as well as excellent short- and long-tern stability through constant monitoring.