Radiation hardness of the ATLAS Tile Calorimeter optical components

The Tile Calorimeter (TileCal) is a sampling hadronic calorimeter and an essential part of the ATLAS experiment at the LHC. Plastic scintillating tiles are the active material. The light produced in the scintillators is transmitted to the photomultiplier tubes by wavelength shifting fibres. During the High Luminosity LHC (HL-LHC) program, the luminosity can reach a value seven times higher than the one that TileCal was designed for. Two critical points that affect the detector performance are the increased exposure to radiation that will degrade the TileCal optics and natural ageing. Since the optical components of the TileCal cannot be replaced, the radiation hardness must be evaluated. The Laser and Cesium calibration systems are used to evaluate the robustness of the TileCal optical components. These systems combined allow to isolate the response of the tiles and fibres and evaluate the evolution of the light yield with the dose. Run 2 data was analysed, indicating that cells in layer A, and B11 and C10 cells have already lost about 5\% of light yield. No significant changes were found for the other cells. This study constitutes an essential step for predicting the calorimeter performance in future HL-LHC runs. Nevertheless, the extrapolation uncertainty is large so more data needs to be explored to reach better precision on such extrapolation.