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The Inner Detector of the ATLAS experiment will be completely replaced by the Inner Tracker (ITk) in preparation for the High-Luminosity upgrade of the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. The ITk will target an operational dew point of −60 °C, requiring precise environmental monitoring to ensure reliable detector performance. With normal operation at −20 °C, and fault conditions potentially leading to local temperatures as low as −55 °C, the monitoring system must protect the detector and prevent condensation on sensitive electronics through accurate temperature and humidity measurements—a critical challenge in high-radiation and high-electromagnetic-field environments.
This paper details the mechanical design, calibration framework, and quality-assurance protocols for radiation-tolerant fibre-optic sensor (FOS) packages developed for this purpose. Each integrated FOS package combines a radiation-hard fibre Bragg grating (FBG) for temperature measurement, a radiation-sensitive FBG for temperature and radiation measurement, and a long-period grating (LPG) sensitive to humidity, temperature, and radiation. A novel ultra-low-thermal-expansion Neoceram package body houses the sensors, while a custom PCB provides electrical conductivity between the mounting bolts for EMI/ESD protection. Structural integrity was validated through thermal cycling between −30 °C and 70 °C to ensure robustness under extreme operational conditions.
A data-driven calibration procedure addresses the high-accuracy requirements of the ITk using a climatic chamber equipped with precision humidity control and a chiller. A PID-controlled system automates data collection across temperatures from −20 °C to 25 °C and relative humidity from 0% to 100% RH. FOS outputs are calibrated against electronic reference sensors, while a dedicated algorithm decouples radiation and temperature effects from the LPG humidity response. Rigorous QA/QC protocols ensure measurement reproducibility through an established procedure designed to maintain measurement accuracy within the ITk specifications.
This work establishes a high-reliability framework that combines radiation-tolerant mechanical design with algorithmic calibration and quality-assurance methodologies, providing a comprehensive approach for the deployment of fibre-optic environmental monitoring systems in extreme high-energy physics environments.
