Speaker
Description
The ATLAS Inner Tracker (ITk) will begin operation at the HL-LHC following LS3 for Run 4. Humidity inside the detector will be monitored using radiation-hard Fiber Optic Sensors based on Long Period Grating (LPG) and Fiber Bragg Grating (FBG), designed to operate below -20 °C and <10% humidity. Measuring relative humidity requires a complex method involving independent temperature and radiation dose readings at each sensor location. An optical interrogator captures the reflection spectrum, revealing characteristic peaks. This contribution presents the most effective method for extracting relative humidity, including spectral filtering to achieve 10 pm resolution.
Summary (500 words)
The ATLAS Inner Detector is being upgraded with the new Inner Tracker (ITk), with commissioning scheduled for mid-2030. A key concern during ITk operation is the risk of water condensation, which could damage sensitive and expensive detector electronics. To prevent this, continuous monitoring of temperature and humidity is essential. Sensors must remain functional for up to ten years while withstanding harsh conditions, including a total ionizing radiation dose of 2 MGy. The ITk will operate at low temperatures (-20 °C) and low humidity (<10%).
The proposed solution is a Fiber Optic Sensor (FOS) system, integrated into the Detector Control System (DCS). Each FOS package includes three components:
1. FBG RH – a radiation-hard Fiber Bragg Grating sensor sensitive only to temperature,
2. FBG RS – a radiation-soft FBG sensitive to both temperature and radiation dose,
3. LPG – a Long Period Grating sensor sensitive to temperature, radiation dose, and relative humidity.
These elements are housed between two neoceramic plates with a low thermal expansion coefficient (-5 × 10⁻⁷ K⁻¹), minimizing mechanical strain on the LPG sensor. A total of 52 FOS packages will be installed in the ITk.
Sensor readings are obtained using an optical interrogator, which detects characteristic transmission and absorption peaks in the 1420–1620 nm range. To extract relative humidity (RH), temperature (T) is first calculated from the FBG RH, assuming consistent T across the FBG RS and LPG. Radiation dose (D) is then derived from the FBG RS, assumed constant for the LPG. Finally, RH is extracted from the LPG.
Because the relationship between wavelength shift and both radiation dose and humidity is steep, accurate peak detection (within 10 pm) is critical. Spectral noise is reduced using a low-pass filter, followed by a 1-sigma Gaussian fit around each peak. Analysis showed that a two-parameter fit and a 5 pm cut-off wavelength—half the interrogator’s sampling interval—provided optimal results.
To assess radiation resistance, FOS packages were exposed to 2 MGy at CERN’s IRRAD facility. Post-irradiation, LPG sensors coated with 10 TiO₂ layers lost their absorption peak. However, sensors coated with 8 layers retained the peak, although they exhibited reduced sensitivity to humidity and increased temperature sensitivity. To address this, a low-pass Butterworth filter was applied to the FBG RH and RS data to improve temperature extraction and, consequently, RH calculation.
An ongoing challenge is the unexpected sensitivity of FBG sensors to both temperature and humidity, complicating RH extraction from the LPG. This issue is under active investigation.
Presenting this work offers an opportunity to receive feedback on unresolved challenges in FOS development. It also opens the door for collaboration with other experiments beyond ATLAS, potentially extending this sensing solution to additional detectors ahead of the High-Luminosity LHC (HL-LHC) upgrade.