Distributed Optical Fiber Radiation and Temperature Sensing at High Energy Accelerators and Experiments Iacopo Toccafondo, EN-EA

Outline Introduction & Motivations Radiation effects on Silica-based optical fibers Distributed Optical Fiber Radiation Sensor (DOFRS) based on RIA 60Co characterization of selected P-doped fibers Distributed radiation measurements at CHARM Distributed temperature measurements at CHARM Conclusions & Further Work

Introduction Concerns High radiation levels Ensuring reliability and safety of the accelerator complex Equipment groups rely on accurate dosimetry for lifetime prediction Currently Monitoring systems Designing and installing radiation resistant components Regular maintenance, replacement/relocalization Limitations Monitoring systems are discrete/punctual, only partially online Frequent maintenance due to conservative life estimation approach

Motivation Provide accurate online distributed dosimetry Increasing safety Cost-effective implementation Future collider projects => Necessity of a cost-efficient, implementable and performant solution!

Concept Optical fibre based radiation and temperature sensing Control Unit Ionizing radiation Distance (km) Dose (Gy) Distance (km) Temperature (°C)

Concept - Distributed measurements Conventional (discrete)Optical Fibre Based (distributed) Online monitoring < 1m spatial resolution

Radiation effects on Silica-based optical fibers Microscopic level: Ionization and radiolytic processes Direct atomic displacements Macroscopic level: Radiation Induced Attenuation (RIA): increase in the glass linear attenuation through an increase of the linear absorption due to radiation-induced defects. Radiation Induced Emission (RIE) corresponds to light emission within the samples under irradiation. Refractive Index change (RIC) which may arise from density changes or from the RIA.

Radiation effects on Silica-based optical fibers RIA is influenced by a great variety of variables: The fiber composition an structure: Co-dopants such as germanium (Ge), phosphorus (P), aluminum (Al), fluorine (F) = > different radiation response Single or multi-mode Radiative environment characteristics: Dose dependency Dose rate dependency Temperature dependency Application parameters: Launched optical power wavelength E. Regnier et al., “Low-Dose Radiation-Induced Attenuation at InfraRed Wavelengths for P-Doped, Ge-Doped and Pure-Silica-Core Optical Fibres”, IEEE Transactions on Nuclear Science, Vol. 54., NO 4, August 2007

Radiation effects on Silica-based optical fibers P-doped (Phosphorus) Providing a linear response with respect to dose over a wide range, up to a few kGy. Saturation expected above some tens of kGy. Usable for dosimetry up to several tens of kGy. Dose-rate independency over a wide range of dose-rates Low annealing/fading Linear temperature response over a wide range of temperatures [-50°C, 70°C] S. Girard et al., "Radiation effects on silica-based optical fibers: Recent advances and future challenges," IEEE Trans. Nucl. Sci., vol. 60, no. 3, pp , 2013

Distributed Optical Fiber Radiation Sensor (DOFRS) based on RIA Optical Time Domain Reflectometry OTDR based Control Unit Optical Fibre Backscattered light Distance (km) Light Intensity (dB)

Distributed Optical Fiber Radiation Sensor (DOFRS) based on RIA Optical fibre based radiation sensing OTDR based Control Unit Ionizing radiation Conventional Distance (km) Dose (Gy) Distance (km) Light Intensity (dB)

Distributed Optical Fiber Radiation Sensor (DOFRS) based on RIA Limitations on the dynamic range and the spatial resolution Backscattered power can be increased by increasing: The pulse duration => longer pulses, more energy in the fiber  Degrades the spatial resolution The input peak power P(0)  Limited by the onset of unwanted or nonlinear effects and the possible saturation of the receiver due to Fresnel reflections  Trade-off between the dynamic range and the spatial resolution resulting in a fundamental limit of the conventional OTDR technique.

Distributed Optical Fiber Radiation Sensor (DOFRS) based on RIA OTDR traces with singles pulse OTDR traces with coded pulse D. Lee et al. “Optimization of SNR Improvement in the Noncoherent OTDR Based on Simplex Codes, Journal of Lightwwave Technology, Vol. 24, NO.1, January 2006

Distributed Optical Fiber Radiation Sensor (DOFRS) based on RIA SNR enhancement De-coding achieved by linear processing of coded traces

Distributed Optical Fiber Radiation Sensor (DOFRS) based on RIA

Performance requirements for a DOFRS Total Ionizing Dose: from a few tens of Gy up to 100 kGy Dose resolution: 10% of the TID. Dose rate: from a few mGy/s up to a few Gy/s Spatial resolution: ≤ 1 m Some requirements also exist on the sensing fiber: Ideally a linear increase of the attenuation with dose or simply modelled response Independence of the measured dose on the dose rate Negligible annealing Weak dependence on the radiation type and energy Independence on the environmental parameters, such as temperature Temperature: 15 °C – 35 °C, hot spots: 200 °C

60 Co characterization of selected P-doped fibers Selection of potentially suitable MMF and SMF radiation sensitive P-co-doped fibers chosen to be irradiated and characterized Dose rates: 0.5 mGy/s – 1.7 Gy/s (2014) set by placing the fiber samples at different distances from the radiation source in its ejected position

60 Co characterization of selected P-doped fibers J.Kuhnhen and U.Weinand, “Quality assurance for irradiation tests of optical fibers: Uncertainty and reproducibility“, IEEE Transactions on nuclear science, vol. 56, No. 4, August 2009 Two sample irradiated at the same time Each sample can be tested at two wavelengths at the same time 830 nm and 1312 nm for the MMF 1312 nm and 1570 nm for the SMF Minimizing photobleaching effects: launched light power between 7 µ W and 15 µ W. Light power recorded as a function of time both for the samples under test and reference fiber

60 Co characterization of selected P-doped fibers MMF j-fiber: GI 50/125 µm, P-co-doped 830 nm Dose rate: 0.5 mGy/s – 1.7 Gy/s Total dose: ~ 60 kGy Linear range: ~150 Gy up to 2.2 kGy Sensitivity: ~ 3.5 mdB/m/Gy Saturation around 20 kGy Max- slope variation: 15%

60 Co characterization of selected P-doped fibers MMF j-fiber: GI 50/125 µm, P-co-doped Dose rate: 0.5 mGy/s – 1.7 Gy/s Total dose: ~ 60 kGy Linear range: ~150 Gy up to 1.6 kGy Sensitivity: ~ 1 mdB/m/Gy Saturation around 20 kGy Max- slope variation: 15% 1312 nm

60 Co characterization of selected P-doped fibers SMF Draka: GI 50/125 µm, P-co-doped 1312 nm Dose rate: 0.5 mGy/s – 1.1 Gy/s Total dose: ~ 55 kGy Linear range: ~ few Gy up to 1.8 kGy Sensitivity: ~ 100 µ dB/m/Gy Saturation around 30 kGy Max- slope variation: factor 10%

60 Co characterization of selected P-doped fibers SMF Draka: GI 50/125 µm, P-co-doped 1570 nm Dose rate: 0.5 mGy/s – 1.1 Gy/s Total dose: ~ 55 kGy Linear range: ~ few Gy up to 1 kGy Sensitivity: ~ 200 µ dB/m/Gy Saturation around 30 kGy Max- slope variation: factor 15%

60 Co characterization of selected P-doped fibers Linear and non-linear ranges both usable for dosimetry Non-linear range fitted by existing models

Calibation of a set of radiation sensitive fibres Distributed OTDR radiation sensing at CHARM  Need to demonstrate the working principle in a representative environment Cern High energy AcceleRator Mixed field facility (CHARM)

Distributed OTDR radiation sensing at CHARM – the facility 24 GeV/c beam focused on target, generating a mixed radiation field composed of protons, neutrons, electrons, positrons, pions, photons, muons and other particles. Particle energy spectra representative of space, ground level and accelerator environments Dose rates ranging from a few µGy/s up to a few tens of mGy/s. J. Mekki et al., “CHARM: A new mixed field facility at CERN for radiation test in ground, atmospheric, space and accelerator representative environments“, RADECS2015

24 GeV/c, p Distributed OTDR radiation sensing at CHARM Target Optical Fibre Adapted from the CHARM facility webpage 130 m long fibre path Two heights around the shielding: 95 cm & 280 cm above ground Wide range of dose rates Equipment: EXFO FTB500 with two high dynamic range OTDR modules SMF (850 nm & 1300 nm) & MMF (1310 nm & 1550 nm) Available pulse duration: 5 ns up to 20 µ s Measurement time: 30 s up to 180 s

Distributed OTDR radiation sensing at CHARM 5 ns pulse duration, 180 s acquisition time, Aluminum target, 35 h of irradiation Data smoothening with Savitzky- Golay filter

Distributed OTDR radiation sensing at CHARM Possibility to acquire traces in two opposite directions j-fiber – Direction 1 j-fiber – Direction 2 The combination of the two traces allows for the monitoring of the complete path even if a major loss occurs near to the target

Distributed OTDR radiation sensing at CHARM J-fiber MMF, pulse width: 5 ns, acquisition time: 180 s Four main peaks correctly localized Main peak: 400 Gy, less than a factor 2 with FLUKA Only the main peak is localized =>about 390 Gy Low SNR doesn’t allow the detection of the other dose peaks 850 nm 1300 nm

Distributed OTDR radiation sensing at CHARM J-fiber MMF, pulse width: 5 ns, acquisition time: 180 s 850 nm1300 nm Promising results considering: FLUKA scored dose in air and with relatively high thresholds (CPU time constraints) Fibre averaging over 50 cm, FLUKA binning 5 cm Uncertainty on vertical position of the cable duct => dedicated run is currently in preparation! => new measurement campaign to optimize geometry being carried out!

Distributed OTDR radiation sensing at CHARM RadFET comparison: j-fiber MMF, pulse width: 5 ns, acquistion time: 180 s Maximum difference of 15% RadFET is punctual, DOFRS distributed Good agreement with RadFETs observed => 60 Co valid for mixed-field

Distributed OTDR radiation sensing at CHARM Draka SMF, pulse width: 5 ns, acquisition time: 180 s Results consistent with j-fiber, only 2% difference SNR too bad for event detection at 1310 nm Smaller sensitivity doesn’t allow to detect lower dose peaks

Distributed OTDR radiation sensing at CHARM J-fiber SMF, pulse width: 10 ns, acquisition time: 30 s, Al-h target Higher SNR with respect to 5 ns even though shorter acqu. time Correct detection and localization of the four major dose peaks Detecting dose variations down to Gy Less than a factor 2 at 850 nm 850 nm

Distributed OTDR radiation sensing at CHARM J-fiber SMF, pulse width: 10 ns, acquisition time: 30 s, Al-h target Higher SNR with respect to 5 ns even though shorter acqu. time Correct detection and localization of the four major dose peaks Less than a factor 2 at 1300 nm 1300 nm

Distributed temperature measurements Compensate a possible T°C dependency of the dose measurement Safety: offering a realiable, online and distributed monitoring of temperature

Distributed temperature measurements at CHARM – basic principle Raman scattering: generated by light interaction with resonant modes of molecules in the medium (vibrational modes) Raman Stokes Raman anti-Stokes Photons interact with phonons in an inelastic scattering Stokes line  photon energy is given to the phonon anti-Stoke line  phonon gives energy to photon F. Di Pasquale, Optical Amplification & Sensing Course, Scuola Superiore Sant’anna (Pisa)

Distributed temperature measurements at CHARM – basic principle Ratio R(T) between P AS /P S or P AS /P R is used to get rid of losses F. Di Pasquale, Optical Amplification & Sensing Course, Scuola Superiore Sant’anna (Pisa)

Distributed temperature measurements at CHARM – basic principle Ratio R(T) between P AS /P S or P AS /P R is used to get rid of losses Affected by wavelength dependent losses (WDL) Long sensing range Robust against WDL Limited sensing range F. Di Pasquale, Optical Amplification & Sensing Course, Scuola Superiore Sant’anna (Pisa)

Distributed temperature measurements at CHARM Raman distributed temperature sensing (RDTS) based on OTDR Pulsed pump laser to perform the OTDR measurements Narrow band filter to discriminate between anti- Stokes and Stokes APD given the low intensity of the spontaneous Raman scattering Double-end configuration:

Distributed temperature measurements at CHARM Radiation tolerant Draka MaxCap Bendbright MM GI 50/125 µm fiber Raman DTS from INFIBRA Technologies S.r.l Operating at 1064 nm, max sensing distance 5 km Double-end configuration

Distributed temperature measurements at CHARM Expected ΔRIA between λ anti-Stokes and λ Stokes of about 16 dB/km after 1.25 kGy Dose rate = 0.23 Gy/s T = 24°C

Distributed temperature measurements at CHARM No significant temperature variation has been observed even after 1.25 kGy (pos. 50 m/180 m) Lower temperature observed in the cable duct as expected =>exposed to external daily temperature changes. Higher temperature observed in the control room as expected Meter scale spatial resolution

Distributed temperature measurements at CHARM anti-Stokes, 1017 nm ΔRIA = 14 dB/km good agreement with the spectral measurements Considering different irradiation conditions => dose rate dependency of radiation tolerance fiber (P-free) => at CHARM the fiber has already started saturating!

Distributed temperature measurements at CHARM Direct comparison with a PT100, placed at about 40 m from the control room Very good agreement: maximum difference < 0.5°C, which is within the measurement accuracy of the PT100

Distributed temperature measurements at CHARM Standard deviation estimation from the temperature measurements Absence of any significant standard deviation variation along the sensing fiber  excluding detrimental effects due to radiation  demonstrating the stability of the distributed measurement

Conclusions Successfully demonstrated the proof of concept: feasibility of distributed optical fiber radiation sensing in the challenging and complex mixed- field radiation environment at high energy physics accelerators and experiments Ability to correctly localize dose peaks with 1 m spatial resolution and estimate dose variations down to Gy Assessed that with the tested commercial P-doped fibers it is possible to estimate doses up to a few kGy => suitable for many areas in the LHC  Pointed out the early saturation, about 20 kGy of the fibers  Assessed some limitations in commercial OTDR: limited dynamic range => short acquisition time

Conclusions Successfully demonstrated that Raman-based distributed temperature sensing is feasible in a harsh mixed-field radiation environment Ability to estimate the temperature profile with <1 ̊ C temperature resolution and < 1 m spatial resolution

Further work Completing the systematic study of the interrogation and acquisition parameters of OTDR-devices Testing at CHARM of custom designed fibers in collaboration with the Laboratoire Hubert Curien of the Université Saint-Etienne => PCe, HAOF Starting the prototyping phase of a custom-designed low-cost OTDR- based radiation sensing system control unit Temperature characterization of selected radiation sensitive fibers will be carried out to fully understand the dependence of the RIA Automation and online logging (TIMBER) of readout unit data

Further work – Test installation in a machine First in-field deployment at CERN: targeting an installation in the Proton Synchrotron Booster (PSB) J.P. Saraiva and M.Brugger, “CERN-ACC-NOTE ”, 2015 Targeting full coverage (~157 m) Dose range from few tens Gy/y up to 100 kGy/y Post LS2, LINAC4 => reaching 2 GeV! Up to a factor 10 reduction

Many thanks for your attention!