Fiber Optic Sensors for relative humidity monitoring in High Energy Physics Applications

CONSTANT THERMAL AND HYGROMETRIC CONTROL OF THE AIR IS MANDATORY! ATLAS CMS LHCb ALICE 4 main experiments running in the LHC: COMPACT MUON SOLENOID The heart is the tracker Silicon sensors measure position of particles Radiations cause deterioration and heat generation in silicon Cooling at T< 0 ˚C is needed The tracker has to be kept dry to avoid condensation CONSTANT THERMAL AND HYGROMETRIC CONTROL OF THE AIR IS MANDATORY!

OUR SOLUTION: FIBER OPTIC SENSORS Humidity Sensors Issues in HEP Trackers Low mass and Small dimensions Insensitivity to magnetic field Operation down to -30 ˚C and in the range [0, 100] %RH Accuracy: better than ±3 %RH Reduced number of wires High long term stability Radiation resistance to dose up to 1 MGy (100 Mrad) Currently in use at CERN: 1) Different types of miniaturized sensors (standard capacitive) 2) Measurements with laboratory instrument on air samples transferred over long distance (remote air sniffer) PRECON HS2000 HONEYWELL HIH4030 SENSIRION SHT75 - No distributed sensing - Only time-averaged measurements Sniffer rack in remote laboratory NO-ONE satisfies the whole set of requirements from CERN!!! Vaisala DRYCAP Dewpoint Transmitter DMT242 OUR SOLUTION: FIBER OPTIC SENSORS

WHY FIBER OPTIC SENSORS @ CERN? Miniaturized dimension Minimal mass Multi parametric sensing Possibility to work in harsh environments Immunity to electromagnetic interference Water and corrosion resistance Radiation tolerant VERY APPEALING TO EXPLORE HIGH ENERGY PHYSICS ENVIRONMENTS

Fiber Bragg Grating fundamentals FBG is a permanent periodic modulation of the refraction index in the core of the fiber neff is the fiber effective refractive index (neff ~ 1.455 in silica) Λ is the grating pitch (typically ~ 500 nm) λB is the reflected Bragg’s wavelength

FBG- based Strain and Temperature Sensors ε Strain changes: Sε≈ 1 pm/με - Λ via direct deformation - neff via elasto-optic effect T ST≈10 pm/°C Temperature Changes: Λ via thermal expansion - neff via thermo-optic effect For strain measurements, discrimination between temperature and strain needed: Strain free reference FBG sensors coupled to another FBG (T-Compensation technique)

FBG AS MULTIFUNCTIONAL SENSORS Multi-parametric sensing Functionalization: integration with appropriate materials and suitable packaging to measure physical, chemical and biological parameters FBG + COATING: Multi-parametric sensing Magnetic field Humidity Cryo temperatures Acoustic waves Weight Chemical and biological analytes ….

FBG-based relative humidity sensors Bare FBGs are insensitive to humidity. Use of a sensitive material as coating of the FBG to induce a mechanical effect. Hygroscopic polymers swell upon adsorption of water molecules. Development of humidity sensor by coating the FBG with a polyimide film Water molecules absorbed by the hygroscopic coating Coating expansion (“Swelling”) Strain induced on the FBG Bragg wavelength shift (ΔλB) FBG-RH sensor Water molecules Polyimide coating Optical fiber

Temperature compensation Polyimide-coated FBGs are intrinsically sensitive to temperature: A temperature compensation scheme is required to extract RH measurements from the sensor readings Final configuration of FBG-based thermo-hygrometers developed for CMS: 2 FBGS coupled side by side (1 poly-coated and 1 bare)

PH-DT experimental set-up

Investigations @ CERN about RH FBG sensors Typical FBG RH response at 20°C λB blue shift induced by PI coating water molecules absorption Complete agreement between FOS readings and the reference Similar results obtained at 0 and -15°C Temperature and Relative humidity Sensitivities Characteristic curves SRH=2.09 pm/%RH±20% ST=10.08 pm/˚C±12% Slight dependence of SRH on T and ST on RH !! Linear response of FBG coated towards RH

INVESTIGATIONS ABOUT FBG SENSORS RADIATION RESISTANCE VERY COMPATIBLE RESULTS OVER THE 4 SAMPLES!!! Ceramic Commercial Packaged FBG Temperature sensors 4 samples belonging to 2 different batches ϒ- irradiation campaigns up to 210 kGy Negligible variations of ST Polyimide Coated FOS RH sensors SRH unaffected by ϒ-radiations! Coating thickness:10 µm (±2 µm) 4 sensors belonging to the same batch ϒ- irradiation campaigns up to 210 kGy RH response found to be linear Sensing performances unchanged after each campaign

FBG-based thermo-hygrometers in operation @ CMS CMS Tracker Bulkhead: 72 FBG thermo-hygrometers installed Commercial sensors procured (Welltech for RH/ Micron Optics for T) In operation since December 2013 (h24) Reliable system During LS1 coupled to commercial RH/T sensors (which will die when LHC goes ON) 1 fiber optic array with 8 FBG-based hygrometers (MULTIPLEXING) FBG-based thermo-hygrometer Reference T and RH sensors mounted in a special protective casing * Pictures from the installation of the FBG sensors in CMS

12 months of FBG-RH sensors’ operation in CMS Ex.1: Sensor F5-H-2 (Z+) 1 year of measurements (from Dec 2013 to Dec 2014) Some interruptions in the data acquisition Ex.2: Sensor F5-H-14 (Z+) Ex.2: Sensor F5-H-8 (Z+) Different RH measurements due to different locations (and different services around)