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Description
The Tile Calorimeter (TileCal) is a sampling hadronic calorimeter covering the central region of the ATLAS experiment at the CERN Large Hadron Collider (LHC). It is composed of steel absorbers and plastic scintillators as the active medium, with scintillation light collected by wavelength-shifting fibres and read out by photomultiplier tubes. The stability of the calorimeter response is essential for precise measurements of jets and missing transverse momentum in ATLAS.
The long-term performance of plastic scintillators and associated optical components in the high-radiation environment of the LHC is a key aspect for ensuring a stable calorimeter response, particularly in view of operation at the High-Luminosity LHC (HL-LHC). Radiation-induced ageing affects the light yield of scintillators and the transmission properties of optical fibres, potentially impacting the calorimeter energy scale and uniformity.
TileCal is equipped with multiple dedicated calibration systems that provide precise monitoring and calibration of the detector response over time. The calibration strategy relies on complementary systems that probe different parts of the signal chain. A movable ¹³⁷Cs source system is used to equalize and monitor the response of the full optical chain, including scintillators, wavelength-shifting fibres, and photomultiplier tubes (PMTs). A laser system provides frequent monitoring of the PMT gains, and the observed variations are corrected in the signal reconstruction. A charge injection system is used to calibrate the front-end electronics. The combination of these systems, together with test-beam measurements, enables precise calibration of the detector signals to the electromagnetic energy scale.
The response degradation is studied using integrated minimum-bias currents and calibration measurements. The loss in the light yield of the scintillators and fibres is extracted, and its dependence on the detector location and integrated luminosity is evaluated. The observed ageing effects are quantified and compared with expectations based on radiation dose simulations and previous measurements. Extrapolations to higher accumulated doses expected at HL-LHC are performed (Figure 2). These studies provide important input for the long-term operation of TileCal at the HL-LHC.
In conclusion, the TileCal calibration systems enable precise monitoring of radiation-induced ageing effects in scintillators and optical components. The detector demonstrates stable performance under LHC conditions, and the observed ageing effects provide a quantitative basis for extrapolations to HL-LHC radiation levels.