CALIBRATION OF THE MICRON OPTICS TEMPERATURE SENSORS FOR FIBER OPTIC THEMO-HYGROMETERS FOR CMS TRACKER

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1 CALIBRATION OF THE MICRON OPTICS TEMPERATURE SENSORS FOR FIBER OPTIC THEMO-HYGROMETERS FOR CMS TRACKER SIEW YAN HOH NATIONAL UNIVERSITY OF MALAYSIA (CERN SUMMER STUDENTSHIP PROGRAMME 2013) 19 th August 2013 (DAFT VERSION) Abstract The nature of my project at European Organization for Nuclear Research (CERN) is proposed and discussed. The experiments at LHC (Large Hadron Collider) are conducted in high precision with state of the art in order to get a glimpse of the fundamental particle behaves in a re-created early universe environment by colliding particle with high energy. Trillion of data collected from the LHC has been studied by top scientists from all around the world to produce readable information and analyze the result. Optimization, data sorting, reconstruction, calibration, and fine tuning are constantly carry out at CERN in order to maintain the quality of each measurement. Therefore, tracker is primarily important and it is needed to be serviced frequently to maintain its efficiency in detecting the particle. Being a summer intern student at CERN, I was assigned to the project concerning Fiber Optic Thermo-Hygrometers which is currently deploying in CMS (Compact Muon Soleniod) detector in the upgrading period. The internship has been done in PH-DT Laboratory at CERN with the supervision of Salvatore Buontempo and co-supervision of Paolo Petagna and Gaia Berruti. INTRODUCTION CERN, the European Organization for Nuclear Research, operates the world s leading laboratory for particle physics. Its research focuses on fundamental physics, finding out what the Universe is made of and how it works. Its new flagship research facility, the Large Hadron Collider (LHC), is housed in a 27 kilometers tunnel. Here bunches of protons accelerated in opposite direction nearly at the speed of light are brought to collision at unprecedented energies, in order to recreate and study the conditions of matter immediately after the Big Bang. The large experiments at LHC (ALICE, ATLAS, CMS, LHCb) are composed of several detectors devoted to the identification of the particles created by the collisions and to the precise measurement of the energy associated to them. The innermost detector of an LHC experiment is generally called inner Tracker. In the core of the CMS experiment, the Silicon Tracker measures momentum and position of the charged particles with more than silicon pixel and micro-strip sensors hosted in a cylindrical volume of about 25. The high radiation dose resulting from the operation of LHC at full luminosity causes serious deterioration of the sensors. One of the main damage effects is an increase of the leakage current, which is a source of detector noise and of heat generation. For this reason, all Tracker subdetectors are cooled by circulating fluids at a temperature between -20 C and -30 C. To avoid

2 condensation of water, which would damage both the sensors and the complex and expensive readout electronics of the silicon modules, the volume needs to be kept dry, in particular the surrounding gas needs to have a dew point lower than the temperature on the coldest surfaces of the cooling circuit. A distributed thermal and hygrometric monitoring of the environment air mostly interests the external area of the tracker rather than the internal. In fact, the inner part of the tracker volume has been hermetically and dried conditions of the air at low temperature are guaranteed. This is not the case of the surrounding area of the tracker, which is not in a perfectly sealed environment; therefore the probability of humidity condensation is higher. Here the humidity has to be controlled by blowing in large quantities of dry gas to force out all of the water vapor. In addition, the thermal insulation of the nearby coolant pipes, which are also at low temperature because of the coolant, is not always optimal due to space and geometry constraints. For all these reason, it s important to guarantee that the detector and nearby pipework are dry, to avoid condensation and the growth of ice, which can inflict major damage. For this reason a constant and efficient thermal and hygrometric control of the air filling the tracker cold volumes is mandatory. NEEDS AND REQUIREMENTS FOR RELATIVE HUMIDITY MONITORING AT CMS All these needs and constraints together translate to the following requirements for environmental sensors: radiation hardness, miniaturized dimensions, minimal mass, minimal need of services, stable calibration unaffected by magnetic field, moderate cost, high long term stability, reliable reading across long distance, respond to full humidity range and so on. The experience deriving from the design and production of the first generation of tracking detectors at CERN has clearly shown the lack of miniaturized relative humidity sensors well-suited for local, distributed monitoring of the humidity conditions inside the characteristic harsh and virtually inaccessible environment. As a matter of the fact, all miniaturized RH Sensors presently available on the market are electronic. Despite all efforts, these sensors fail to provide a complete set of favorable characteristics required by CMS, mostly the radiation hardness capability. On the contrary, fiber optic sensors (FOS) provide many attractive characteristics among which reduced size and weight, immunity to electromagnetic interference, multiparametric sensing, water and corrosion resistance. In addition, modern fiber fabrication technologies now allow obtaining fibers with low to moderate levels of radiation hardness capability. Numerous FOS have been proposed for humidity detection over the years. Among all, Fiber Bragg Grating sensors have attracted significant attention due to their many advantageous features such as wavelength encoded information, multiplexing capability, reduction of cabling complexity, linear output, small, and multi parameter sensing. The use of FBG with moisture sensitive polymer coating as a mean of RH detection has been studied in literature [1][2]. However, RH measurements were limited to the temperature range C and the effects of ionizing radiations on the operation and reliability of such devices were completely unexplored.

3 Recent investigation at CERN have been carried out showing that FBG sensors have good potentialities to be used in CMS for relative humidity monitoring[3]. SENSING PRINCIPLE OF FBG RELATIVE HUMIDITY SENSORS A fiber Bragg grating (FBG) is a typical example of a versatile photonic component that can be applied in both optical communication and sensing systems. An FBG is a permanent periodic modulation of the refraction index in the core of a single-mode optical fiber. It behaves as a wavelength selective filter which reflects the light signal at a specific wavelength, named the Bragg wavelength ( ), that is strictly dependent on the fiber effective refractive index ( ) and the grating pitch (Λ) of the FBG: Both, the refractive index and the grating pitch can be affected by strain and temperature, thus making FBGs very popular in temperature and strain sensing applications. In particular, the Bragg wavelength shift due to the change in strain (ε) and temperature (ΔT) can be expressed as: (1) ( ) ( ) (2) where is the photo-elastic constant of the fiber, ε is the strain induced on the fiber, α is the fiber thermal expansion coefficient and ξ is the fiber-thermo-optic coefficient. Bare silica fibers are insensitive to relative humidity. Nevertheless it is possible to use an FBG as humidity sensor by coating it with a hygroscopic material that swells as a consequence of water molecules adsorption. The swelling of the moisture sensitive coating strains the fiber, thus inducing a mechanical strain to the FBG that, in turn, results in a Bragg wavelength shift. In presence of variations in relative humidity (ΔRH) and temperature (ΔT), in the linear assumption, the Bragg wavelength shift can be expressed as: where and are the sensor sensitivities to RH and temperature, respectively. The precise RH measurements can be carried out with the proposed sensors only if thermal compensation is provided since is typically one order of magnitude higher than for the class of sensors under study. A precise deconvolution of the temperature and the humidity effects from the sensor signal requires therefore an independent temperature reading carried out as close as possible to the humidity sensor position. The temperature information needed for the signal deconvolution can be, at least in principle, simply obtained through a (strain and humidity free) temperature sensor, possibly placed on the same optical fiber. This solution, that is to say a FOS (3)

4 thermo-hygrometer, made of a polyimide coated FBG sensor coupled to a bare FBGs sensor, has been chosen for temperature and humidity monitoring in CMS. Figure 1: The MO sensor (FBG1) coupled to the coated FBG2 humidity sensor in schematic diagram. Figure 2: FOS Thermo-hygrometer based on FBG technology developed for CMS EXPERIMENTAL TEST A series of experimental tests have been conducted in verifying the capability of FBG sensors, using a climatic chamber, well controlled in temperature and humidity, available in the PH-DT Laboratory at CERN. My project is concerned with the calibration of the temperature FBG sensors which will be used in CMS, coupled to the relative humidity FBG sensors. The aim was to find out the temperature sensitivity of each sensor and investigate the stability and repeatability properties. For this reasons, all the T sensors, selected from the Micron Optics producer, have been characterized in the temperature range of [-20, +20] C. During the cycles, temperature has been increased from -20 C to +20 C, with steps of 5 C and then decreased to -20 C. As theoretically expected, we observed that the temperature causes red shift of the which is due to the thermal expansion and the thermo-optic effect induced to the FBG. On the contrary, a blue shift of the have been appreciated when the temperature, inside the test camera, decrease. The chamber has been provided with temperature and humidity reference sensors (notably RTD and ARDUINO sensors) and the data from FOS FBG-sensors and analogic references have been collected in a database. During my internship at CERN, I have written a programming code by using Wolfram Mathematica software and analyzed 84 temperature FBG sensors for Fiber Optic Theromohygrometers for CMS tracker. Each temperature FBG sensor has been calibrated against the

5 temperature reference, according to its position in the chamber. The breakdown of the temperature FBG sensors and their respective references is shown in table 1. Sensors References number MO-3-T-1-4 ARDUINO-TH 4 MO-6-T-5-8 ARDUINO-TH 4 MO-4-T-1-4 ARDUINO-TH 4 MO-4-T-5-8 ARDUINO-TH 4 MO-3-T-5-8 ARDUINO-TH 4 MO-2-T-1-26 ARDUINO-TH 26 MO-1-T-1-8 RTD 8 MO-6-T-1-4 RTD 4 MO-5-T-1-26 RTD 26 Total 84 Table 1: The breakdown of the MO sensors A time window covering the temperature interval [-20, +20] C has been selected in order to perform the calibrations. A linear fit was computed by plotting the characteristic curve (, T) of each sensors. The temperature sensitivity, defined by the slope of the fitting curve, has been evaluated. As expected, the mean value of temperature sensitivity for the analyzed sensors was close to 9pm/ C. (a) (b) Figure 4: λ Bragg variation for one of the tested sensor during a temperature test (a), temperature calibration for the selected sensor (b) Using a cubic fitting equation, described in (4), as suggested by the sensor producer, it is possible to evaluate the precision of the temperature FBG sensor.

6 ( ) ( ) (4) Figure 5: temperature calibration using a cubic fitting for the selected sensor The coefficients a, b, c and d are evaluated using a cubic fitting described in (4) as shown in figure 5. In figure 6 (a), the performances of the temperature FBG sensor can be evaluated in terms of temperature reconstruction. Fitted cubic equation was applied on the temperature full interval of data acquisition to yield a reconstructed temperature curve. The residuals, evaluated as the difference between reconstructed FOS temperature curve and reference, have been evaluated. For the sensor in analysis residuals are inside C, as shown in figure 6 (b) and figure 7 (b). (a) (b) Figure 6: Micron Optics temperature reconstruction in comparison with the reference signal on selected time window (a), on selected time window (b).

7 (a) (b) Figure 7: Micron Optics temperature reconstruction in comparison with the reference signal on full interval (a), on full interval (b). The same analysis has been conducted for all the Micron Optics temperature sensors, confirming good results in term of stability, repeatability and accuracy. SUMMARY Only part of the T-sensors was presented in this PDF report due to time constraints. The residual plots of each tested sensor were within C and this shows that the Micron Optics temperature sensors work very satisfactorily, proven that this technology is suitable to monitor the temperature changes in the environment and to clean the lambda readings from the FOS hygrometers based on FBG technology from the temperature variations. With all the FBG humidity sensors in place, I believe that the CMS detector will be able to continue its sacred task in detecting and exploring new physics generated by the particle collision at a very high precision. REFERENCES [1] Relative humidity sensor with optical fiber Bragg gratings; Optics Letters;August 15,2002; Vol. 26, No.16; P. Kronenberg, P. K. Rastogi; P. Giaccari, H. G. Limberger. [2] Characterization of a polymer-coated fibre Bragg grating sensor for relative humidity sensing; Sensors and Actuators B 110 (2005) ;Yeo, Sun,Grattan, Parry, Lade, Powell. [3] F. Berghmans, A. Gusarov, Fiber bragg grating sensors innuclear environments, in: A. Cusano, A. Cutolo, J. Albert (Eds.), Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation, Bentham Science Publishers, Ch.12 (2011)