Radiation damage of the optical components of the ATLAS TileCal calorimeter at the High-Luminosity LHC

TileCal, a sampling hadronic calorimeter, is an essential component of the ATLAS detector at the LHC. The active material, made of plastic scintillating tiles, produces light when traversed by charged particles. The light is transmitted to photomultiplier tubes by wavelength shifting fibres. The High Luminosity-LHC (HL-LHC) program will extend the TileCal lifetime for 20 years more than originally designed. The detector performance is affected by the increased exposure to radiation that will degrade the TileCal optics and by natural ageing. Since the TileCal's optical components cannot be replaced, their radiation hardness must be evaluated with precision. In addition, the experience gained with a real detector under harsh radiation conditions for long time will be invaluable for the design of future detectors at FCC or other detectors. The TileCal scintillators and fibres' response was determined by exploring information about dedicated calibration systems that employs a cesium source and laser light pulses. The uncertainties associated with the Laser calibration were studied to evaluate the optics' ageing. Run 2 data were analysed, indicating that cells of layer A, and B11 and C10 cells have already lost about 5% of light yield. No significant changes were found for the other cells. The results were extrapolated to the end of Run 3 and end of the HL-HLC, suggesting a light loss of about 20% for most cells, while the most irradiated may lose up to 80% of light yield. Nevertheless, the extrapolation uncertainty is large, so more data needs to be explored to reach better precision.