Optical fibres as radiation sensors at high energy accelerators  Fiber optic radiation sensors  Jochen Kuhnhenn, Stefan K. Höffgen, Udo Weinand, Raphael Wolf Fraunhofer INT, Euskirchen

Slide 2 Meeting at CERN, Jochen Kuhnhenn BU  Topics of this presentation  Radiation effects in optical fibres and principles of radiation detection with optical fibres  Fibre-optic beam-loss monitors  Fibre-optic integrating dosimeters  Other challenging uses of optical fibres at accelerators

Slide 3 Meeting at CERN, Jochen Kuhnhenn BU  Introduction of radiation effects group at Fraunhofer INT  Investigation of radiation effects in electronic and opto-electronic components since 25 years  Operating several irradiation facilities  Supports manufacturers and users (space, accelerators, medicine, nuclear facilities, …)  Specialised knowledge led to the development of several unique radiation detection systems Fraunhofer Locations in Germany  Thanks to our collaborators:  DESY Hamburg (M. Körfer)  HMI Berlin (F. Wulf / W. Goettmann)  BESSY Berlin (J. Bahrdt)

Slide 4 Meeting at CERN, Jochen Kuhnhenn BU  Topics of this presentation  Radiation effects in optical fibres and principles of radiation detection with optical fibres  Fibre-optic beam-loss monitors  Fibre-optic integrating dosimeters  Other challenging uses of optical fibres at accelerators

Slide 5 Meeting at CERN, Jochen Kuhnhenn BU  1 23 Radiation effects in optical fibres  Throughout this presentation “Radiation” means ionising radiation (X-rays,  -rays, particles)  Radiation changes all properties of optical fibres, but some are only relevant at high doses with small (practical) influence  Change of refractive index  Change of bandwidth  Change of mechanical properties (e.g. dimension, strength)  Radiation-induced luminescence light  Most important effect in this context Cherenkov radiation  Most obvious and disturbing effect is an increase of their attenuation (RIA)  Strongly depending on actual fibre and radiation environment

Slide 6 Meeting at CERN, Jochen Kuhnhenn BU  Parameter dependencies of RIA  Manufacturing influences  Fibre type (Single mode, graded index, step index)  Doping of core, doping of cladding (for SM fibres)  Preform manufacturer and used processes  Core material manufacturer  OH Content  Cladding core diameter ratio (CCDR)  Coating material  Drawing conditions  Operation conditions  Wavelength  Light power  Launch conditions  Environment  Total dose  Dose rate  Annealing periods  Temperature In combination with each other: Differences of many orders of Magnitude possible!

Slide 7 Meeting at CERN, Jochen Kuhnhenn BU  Wavelength dependence (example)

Slide 8 Meeting at CERN, Jochen Kuhnhenn BU  Example of dependencies: Fibre type differences (~830 nm)  What does that mean for injected light of 1 mW:  Wavelength: ~830 nm  Fibre length: 100 m  Pure silica fibre: 0.89 mW  F-doped fibre: 0.17 mW  Ge-doped fibre: 3  mW  P-doped fibre: mW

Slide 9 Meeting at CERN, Jochen Kuhnhenn BU  Differences between manufacturers: GI fibres and SI fibres

Slide 10 Meeting at CERN, Jochen Kuhnhenn BU  Radiation effects in optical fibres: Short summary  Huge differences between different  Fibres  Operation conditions  Difficult to compare results of different tests unless all details are known  No predictive theoretical model available  There are only very few “rules of thumb” you can trust!

Slide 11 Meeting at CERN, Jochen Kuhnhenn BU  Main advantages of optical fibres as radiation sensors  Immune against external electro-magnetic-fields  Do not disturb external high precision magnetic fields, e.g. in the undulator section of free electron lasers  Environmental conditions (temperature, vacuum, …) usually no major problem  Capable of monitoring extended areas  Extremely small sensors: diameter of much less than 1 mm

Slide 12 Meeting at CERN, Jochen Kuhnhenn BU  Topics of this presentation  Radiation effects in optical fibres and principles of radiation detection with optical fibres  Fibre-optic beam-loss monitors  Fibre-optic integrating dosimeters  Other challenging uses of optical fibres at accelerators

Slide 13 Meeting at CERN, Jochen Kuhnhenn BU  Introduction to light guiding in step-index optical fibres  Total reflection of light if angle below critical value  Different possible light paths cause dispersion  Parameters of interest:  Difference of refractive index between core and cladding  Launch conditions into fibre (angle of incident)

Slide 14 Meeting at CERN, Jochen Kuhnhenn BU  Wavelength dependencies  Light guiding and signal detection dependent on the following contributions  Fibre attenuation as a function of wavelength  Photon efficiency of selected photomultiplier  Wavelengths of interest:  400 nm to 800 nm

Slide 15 Meeting at CERN, Jochen Kuhnhenn BU  Refractive index of pure silica for wavelength of interest  Refractive index in fibre core:  1.45 to 1.47  Refractive index difference to cladding:   Calculation of numerical aperture and angle of total reflection: Pure silica Refractive Index Wavelength [µm]

Slide 16 Meeting at CERN, Jochen Kuhnhenn BU  Simplified 2-D model of Cherenkov-light guiding in fibre  Angular dependency of particle path to light guiding axis Electron =47°  18°  Max =55°  Min =38°

Slide 17 Meeting at CERN, Jochen Kuhnhenn BU  More realistic 3-D view  Full light cone has to be taken into account  Possibility for grazing incident and spiral light propagation G. Anzivino et. al., NIMA(357)380

Slide 18 Meeting at CERN, Jochen Kuhnhenn BU  Numerical simulation in 3-D  Results obtained for d=1 mm, NA=0.37,  =1  Improved calculations also considering grazing trajectories, spiral light paths and end-face reflections P. Gorodetzky et. al., NIMA(361)161

Slide 19 Meeting at CERN, Jochen Kuhnhenn BU  Detected signals as a function of fibre length  Decrease of signal due to (intrinsic) attenuation in the fibre  Comparable signals at different locations if events are within ~20 m

Slide 20 Meeting at CERN, Jochen Kuhnhenn BU  Selection of optical fibres used as Cherenkov detector  Radiation resistance  Pure silica core fibres with F-doped cladding  Large core diameter  Maximise sensitive volume and therefore generated light signal  Shield ambient light  Reduced signal background  Reasonable costs  Finally selected fibre for most of the projects:  300 µm core / 330 µm cladding diameter step-index fibre  Acrylate coating and black nylon buffer

Slide 21 Meeting at CERN, Jochen Kuhnhenn BU  Installation at accelerators  Version 2: PMT looks downstream Beam pipe Beam PMT Beam pipe Beam PMT  Version 1: PMT looks upstream  Advantages: Higher signal due to better geometry  Advantages: Better resolution (“velocity” for time scaling: 0.4 c) Always correct order of events recorded in PMT

Slide 22 Meeting at CERN, Jochen Kuhnhenn BU  Examples of uses at accelerators: DELTA  Installation at DELTA (Uni Dortmund)  Injection efficiency was poor DELTA Dortmund

Slide 23 Meeting at CERN, Jochen Kuhnhenn BU  Examples of results at DELTA: Injection efficiency Kuhnhenn, doi: /

Slide 24 Meeting at CERN, Jochen Kuhnhenn BU  Installation at FLASH at DESY Hamburg

Slide 25 Meeting at CERN, Jochen Kuhnhenn BU  Radial arrangement of 4 sensor fibres Beam pipe  Asymmetric signals can detect directed losses Fibres

Slide 26 Meeting at CERN, Jochen Kuhnhenn BU  Schematic of Cherenkov system at FLASH (DESY) PMT1 ADuC1 PMT2 PMT3 ADuC2 PMT4 PreAm1 PreAm2 PreAm3 PreAm4 Control-PC Ums. RS485 RS232 PCI Scope- System 1x4 Switch Spectrometer Beam Whitelightsource Parallel connection Ext. USB2 TCP/IP ADuC3 SMA FC ST Cer.-Fibre

Slide 27 Meeting at CERN, Jochen Kuhnhenn BU  Results of the Cherenkov system at FLASH undulators

Slide 28 Meeting at CERN, Jochen Kuhnhenn BU  Other examples: University Lund and DESY Zeuthen J. Bahrdt, FEL 2008 Grabosch, SEI Herbst 2007

Slide 29 Meeting at CERN, Jochen Kuhnhenn BU  Advantages of fibre optic Cherenkov detectors  “Real time” commissioning and optimisation  Prevents damages due to high undetected beam losses  Simple to install and covers whole accelerator areas  Proven and used routinely at several installations

Slide 30 Meeting at CERN, Jochen Kuhnhenn BU  Topics of this presentation  Radiation effects in optical fibres and principles of radiation detection with optical fibres  Fibre-optic beam-loss monitors  Fibre-optic integrating dosimeters  Other challenging uses of optical fibres at accelerators

Slide 31 Meeting at CERN, Jochen Kuhnhenn BU  Historical perspective S. Kronenberg and C. Siebentritt, Nucl.Instr.Meth. 175 (1980)

Slide 32 Meeting at CERN, Jochen Kuhnhenn BU  A long way from the idea to the real application  Gaebler, 1983: „... fibres exhibit properties, which are excellent suited for their application as radiation detectors.“  Lyons, 1985: „... P-doped fibers... might be... suitable for... dosimetry.“  Henschel, 1992: „... radiation induced loss... has been investigated with respect to the suitablility for radiation dosimetry purposes.“  Borgermans, 1999: „The... fibre may be used for dosimetry applications...“  West, 2001: „ response of P-doped fibres is reviewed... [for] their possible use in dosimetry.“  van Uffelen, 2002: „Feasibility study for distributed dose monitoring...“

Slide 33 Meeting at CERN, Jochen Kuhnhenn BU  Principle: Measuring RIA in P-doped optical fibres  Properties of radiation induced attenuation (RIA) in P-co-doped optical fibres:  Strong effect  High sensitivity  Linear dose response  Quantitative results  Slow annealing  Dose rate independence  High reproducibility  Only one calibration per fibre sample necessary

Slide 34 Meeting at CERN, Jochen Kuhnhenn BU  Calibration and dose rate dependency  One fit covers all dose rates (4 orders of magnitude difference)  Nearly linear dose-attenuation function  Saturation of induced attenuation above ~1000 Gy  Calibration for this fibre:  D[Gy]  A[dB/m] * 27

Slide 35 Meeting at CERN, Jochen Kuhnhenn BU  Two implementations to use RIA for dosimetry  High precision optical power meters  Optical Time Domain Reflectometry (OTDR)

Slide 36 Meeting at CERN, Jochen Kuhnhenn BU  Implementation of power meter system at FLASH Control-PC ADuC Trans. RS485 RS232 Further Modules GPIB/ TCP/IP ST-Connector E2000-Connector Rad. resistant Dosimetry fibre Splice

Slide 37 Meeting at CERN, Jochen Kuhnhenn BU  Power meter system for FLASH (DESY, Hamburg)

Slide 38 Meeting at CERN, Jochen Kuhnhenn BU  Power meter system at FLASH (DESY, Hamburg)

Slide 39 Meeting at CERN, Jochen Kuhnhenn BU  Sensor modules for dosimetry fibre

Slide 40 Meeting at CERN, Jochen Kuhnhenn BU  Power meter system at FLASH (DESY, Hamburg)

Slide 41 Meeting at CERN, Jochen Kuhnhenn BU  Exemplary results for power meter measurements at TTF1 Dose [Gy] Date Accumulated dose since compared to TLD measurements Easter

Slide 42 Meeting at CERN, Jochen Kuhnhenn BU  Installation of power meter system at MAXlab, Sweden  Test setup at Fraunhofer INT: stability, correct connections

Slide 43 Meeting at CERN, Jochen Kuhnhenn BU  Power meter system at MAXlab, Sweden

Slide 44 Meeting at CERN, Jochen Kuhnhenn BU  Power meter system at MAXlab, Sweden: Results

Slide 45 Meeting at CERN, Jochen Kuhnhenn BU  Power meter system at MAXlab, Sweden: Results

Slide 46 Meeting at CERN, Jochen Kuhnhenn BU  Second system based on RIA: OTDR measurements  Commercially available test system to measure attenuation along the fibre  Advantages:  Easy to install and operate  Only one fibre end needed

Slide 47 Meeting at CERN, Jochen Kuhnhenn BU  Local resolution of OTDR system  Test irradiation to investigate effects of exposed fibre parts of shorter length

Slide 48 Meeting at CERN, Jochen Kuhnhenn BU  Local resolution of OTDR system  Top graph: Acquired OTDR trace after irradiation with 400 Gy  Bottom graph: Analysed dose data derived from top graph  At least 2 m of fibre need to be irradiated to obtain quantitative resuls

Slide 49 Meeting at CERN, Jochen Kuhnhenn BU  Installation at TTF1 (DESY)

Slide 50 Meeting at CERN, Jochen Kuhnhenn BU  Some results obtained at TTF1

Slide 51 Meeting at CERN, Jochen Kuhnhenn BU  OTDR system at ELBE (FZ Rossendorf) Sensor fibre

Slide 52 Meeting at CERN, Jochen Kuhnhenn BU  OTDR system at ELBE

Slide 53 Meeting at CERN, Jochen Kuhnhenn BU  OTDR system at ELBE: Results

Slide 54 Meeting at CERN, Jochen Kuhnhenn BU  Fibre optic dosimeters based on RIA measurements  Simple principle requires sophisticated measurement techniques for reliable and accurate data  Main advantages:  Integrating (even if no readout takes place)  Small sensor size (< 0.5 mm if necessary)  Quantitative dose data (tested for different dose rates and radiation energies)  High sensitivity (~ some 10 mGy/hour)

Slide 55 Meeting at CERN, Jochen Kuhnhenn BU  Dosimetry with Fibre-Bragg-Gratings = 2 n   Measuring for dosimetry

Slide 56 Meeting at CERN, Jochen Kuhnhenn BU  Test setup at FLASH (DESY)

Slide 57 Meeting at CERN, Jochen Kuhnhenn BU  First dosimetry results with Fibre-Bragg-Gratings at FLASH  Calibration curve  High dose measurement data

Slide 58 Meeting at CERN, Jochen Kuhnhenn BU  Topics of this presentation  Radiation effects in optical fibres and principles of radiation detection with optical fibres  Fibre-optic beam-loss monitors  Fibre-optic integrating dosimeters  Other challenging uses of optical fibres at accelerators

Slide 59 Meeting at CERN, Jochen Kuhnhenn BU  Using other fibre optic sensors in radiation environments  The above mentioned advantages of fibre optic sensors are attractive for other measurements at accelerators  Widely used fibre optic sensors in conventional environments  Strain (bridges, buildings, tunnels, …)  Temperature (tunnels, dams, …)  Moisture (tunnels, dams, …)  Application in radiation environments can be challenging

Slide 60 Meeting at CERN, Jochen Kuhnhenn BU  Example: Fibre optic temperature sensor  Principle:  Similar to OTDR measurement but not the original backscattered signal is analysed but two spectrally shifted peaks (stokes and anti- stokes)  Temperature information is derived by comparing the amplitudes of the two signals  Problem in radiation environment:  Radiation induced loss strongly depends on wavelength  One peak (at lower wavelength) gets more attenuated than the other on (at higher wavelength)  Radiation leads to apparent temperature change

Slide 61 Meeting at CERN, Jochen Kuhnhenn BU  Fibre optics at LHC  Part of fibre optic infrastructure is depicted to the right  Installations for LHC ring (without experiments): several km  Applications:  Communication  Control  Analogue signal transmission

Slide 62 Meeting at CERN, Jochen Kuhnhenn BU  Fibre optic infrastructure at LHC  Mean distance from surface to tunnel installations: ~ 1 km  Fast and flexible access for maintenance must be possible

Slide 63 Meeting at CERN, Jochen Kuhnhenn BU  Challenges at LHC for fibre optics  Several high radiation areas in collimator sections  Up to ~ Gy/year at nominal operation  Exposed sections up to 300 m long  No access possible without long shut down  Analogue transmission with low power budget of ~6 dB

Slide 64 Meeting at CERN, Jochen Kuhnhenn BU  INT-CERN project to find best suited optical fibre  Foreseen optical fibre  Ge-doped SM fibre  Might not operate more than some years  In collaboration with manufacturer INT identified extremely radiation hard SM fibre Kuhnhenn et al., doi /TNS

Slide 65 Meeting at CERN, Jochen Kuhnhenn BU  Irradiation test in CERN spallation field (LHC conditions)

Slide 66 Meeting at CERN, Jochen Kuhnhenn BU  Last slide  This presentation introduced four different radiation sensors using optical fibres  Cherenkov systems  Power meter systems  OTDR systems  Fibre-Bragg-Gratings  Overview of radiation effects in optical fibres was given  Finally some other aspects of using optical fibres at accelerators were presented

Slide 67 Meeting at CERN, Jochen Kuhnhenn BU  Thank you for your attention!  Contact:  Jochen Kuhnhenn Fraunhofer INT Appelsgarten Euskirchen   Tel.: Fax:

Slide 68 Meeting at CERN, Jochen Kuhnhenn BU  Backup slides  Thermal annealing of dosimetry fibres Thermal annealing of dosimetry fibres  CCDR dependence CCDR dependence  Cherenkov energy dependence Cherenkov energy dependence  Anzivino Cherenkov model Anzivino Cherenkov model  Cherenkov installation at TTF1 Cherenkov installation at TTF1  Cherenkov results at DELTA Cherenkov results at DELTA  Second dosimetry fibre calibration Second dosimetry fibre calibration  Detailed CERN tests Detailed CERN tests

Slide 69 Meeting at CERN, Jochen Kuhnhenn BU  Experimental setup  Irradiations at INT 60 Co-Source (Activity max. 600 Ci)  Measurements at 829 nm (for OTDR system)  P-doped 50/125 µm GI fibre with polyimide-Coating  Dose rate between 2.05 Gy/min and 3.24 Gy/min  Total dose up to 1000 Gy  Heatable hose (up to 500 °C) LED 829 nm PM-Module 50/50 Coupler 

Slide 70 Meeting at CERN, Jochen Kuhnhenn BU  Temperature dependence of annealing  P-doped polyimide- fibre(829 nm)  Irradiation of 500 Gy ( 60 Co)  After irradiation heating (~200 s) and cooling down

Slide 71 Meeting at CERN, Jochen Kuhnhenn BU  Temperature dependence of annealing: Result  At temperatures of more than 300°C nearly 95% of the induced attenuation anneals  Residual attenuation (~0.2 dB/m) due to lower temperatures in fibre leads

Slide 72 Meeting at CERN, Jochen Kuhnhenn BU  Application for dosimetry fibres  Necessary temperature: 400 °C  Questions:  How many cycles possible?  Consequences for radiation sensitivity (total induced loss)?  Induced and accumulated residual attenuation?  Effect on saturation behaviour?  Effects on calibration parameters (Fit coefficients A = a  D b )?

Slide 73 Meeting at CERN, Jochen Kuhnhenn BU  Irradiation and annealing cycles: Results  8 days of continuous measurement of attenuation and temperature every 1.5 seconds

Slide 74 Meeting at CERN, Jochen Kuhnhenn BU  Irradiation and annealing cycles: Analysis  19 cycles up to 400°C  Decreasing induced loss (max. by 10%)  Cycles nearly completely annealed  Most of residual attenuation is accumulated in fibre leads  After thermal treatment of fibre leads residual attenuation below 0.3 dB/m

Slide 75 Meeting at CERN, Jochen Kuhnhenn BU  Consequences for fitting coefficients: Part 1  No change of saturation behaviour  Fit coefficients vary + / - 10%, Linearity factor (b~1) unchanged

Slide 76 Meeting at CERN, Jochen Kuhnhenn BU  Consequences for fitting coefficients: Part 2  Improved drift compensation  Better heating of fibre leads

Slide 77 Meeting at CERN, Jochen Kuhnhenn BU  Wavelength dependence of thermal annealing  Spectral annealing measured with spectrometer  Lower annealing at lower wavelengths  Results at 829 nm from former test in perfect agreement  Red curve shows smoothed data set

Slide 78 Meeting at CERN, Jochen Kuhnhenn BU  More exotic example: CCDR dependency CCDR 1:1.1 CCDR 1:1.2

Slide 79 Meeting at CERN, Jochen Kuhnhenn BU  Properties of Cherenkov light: Electron energy  Velocity in units c:  Threshold energy:  Above ~2 MeV Cherenkov angle constant

Slide 80 Meeting at CERN, Jochen Kuhnhenn BU  Properties of Cherenkov light: Wavelength  Emitted light nearly independent of particle energy Wavelength [nm] Photon yield per 300 µm and 20 nm

Slide 81 Meeting at CERN, Jochen Kuhnhenn BU  Signal dispersion as a function of core diameter  Intrinsic dispersion: ~1 ns / 20 m  Additional due to different particle paths:  For a fibre core diameter of d=300 µm, Source-Fibre-distance (in units of d) x, Energy >> 1 MeV, n core =1.46, NA=0.22 Distance x  t [ns] 100 (=3 cm)0.05 ns 1000 (=30 cm)0.5 ns 38° 55° x d

Slide 82 Meeting at CERN, Jochen Kuhnhenn BU  Numerical simulation in 3-D (1)  Takes spiral light path into account  Grazing particle trajectories taken into account Impact parameter Angle  Yield [a.u.] G. Anzivino et. al., NIMA(357)380

Slide 83 Meeting at CERN, Jochen Kuhnhenn BU  Installation at undulators of DESY TTF

Slide 84 Meeting at CERN, Jochen Kuhnhenn BU  Examples of results at DELTA: Typical loss pattern

Slide 85 Meeting at CERN, Jochen Kuhnhenn BU  Selection of dosimetry fibre

Slide 86 Meeting at CERN, Jochen Kuhnhenn BU  New fibre needs new calibration

Slide 87 Meeting at CERN, Jochen Kuhnhenn BU  Detailed specification of radiation response  Ge-doped fibre (2)  “F-doped II” fibre