Aug 13, 2010 - Department of Biomedical, Electronic and Telecommunication Engineering, ... Optoelectronic Division â E
CMS CR -2010/100
Available on CMS information server
The Compact Muon Solenoid Experiment
Conference Report Mailing address: CMS CERN, CH-1211 GENEVA 23, Switzerland
03 July 2010 (v2, 13 August 2010)
FOS in CMS detector at CERN Salvatore Buontempo for the CMS Collaboration
Abstract Preliminary results are presented on the activity carried out by our research group on possible application of Fiber Optic Sensor (FOS) techniques to monitor high-energy physics (HEP) detectors. Assuming that Fiber Optic radiation hardness has been deeply studied for other field of application, we have applied the FOS technology to the HEP research domain. In present paper we give the experimental evidences of the solid possibility to use such a class of sensors also in HEP detector very complex environmental side conditions. In particular we present first results of FOS measurements in the Compact Muon Solenoid (CMS) experiment set up at the CERN, where we have monitored temperatures and strains in different locations by using Bragg Grating sensors during the detector operation with the Large Hadron Collider (LHC). On specific request of HEP detector experts we have also started the development of a new class of Relative Humidity (RH) sensor based on Fiber Optic technology, able to perform the monitoring of RH at low temperature, aiming to probe the dew point value with few degree precision. Preliminary results are very encouraging, letting us consider the use of FOS technique as a robust and effective solution for monitoring requirements in HEP detectors for physical and environmental parameters.
Presented at SHM: The 5th edition of European Workshop on Structural-Health-Monitoring
FOS in CMS detector at CERN G. Breglioa,b, S. Buontempo* c,d, A. Buoscioloe, M. Consalesb, A. Cusanof,b, A. Cutolof,b, M. Giordanoe,b, A. Irace a, P. Petagnad a
Department of Biomedical, Electronic and Telecommunication Engineering, University of Naples Federico II, Via Claudio, 21, 80125 Napoli, Italy b OptoSmart s.r.l.,via Pontano, 61, 80121 Napoli, Italy c Istituto Nazionale di Fisica Nucleare - Sezione di Napoli, Via Cinthia – 80126 Napoli, Italy d European Organization for Nuclear Research
CERN CH-1211
Genève 23
Switzerland e Institute for Composite and Biomedical Materials - CNR, P.le Fermi,1, 80055 Portici (NA), Italy f Optoelectronic Division – Engineering Department, “University of Sannio”, P.za Roma, 21, 82100 Benevento, Italy ABSTRACT Preliminary results are presented on the activity carried out by our research group on possible application of Fiber Optic Sensor (FOS) techniques to monitor high-energy physics (HEP) detectors. Assuming that Fiber Optic radiation hardness has been deeply studied for other field of application, we have applied the FOS technology to the HEP research domain. In present paper we give the experimental evidences of the solid possibility to use such a class of sensors also in HEP detector very complex environmental side conditions. In particular we present first results of FOS measurements in the Compact Muon Solenoid (CMS) experiment set up at the CERN, where we have monitored temperatures and strains in different locations by using Bragg Grating sensors during the detector operation with the Large Hadron Collider (LHC). On specific request of HEP detector experts we have also started the development of a new class of Relative Humidity (RH) sensor based on Fiber Optic technology, able to perform the monitoring of RH at low temperature, aiming to probe the dew point value with few degree precision. Preliminary results are very encouraging, letting us consider the use of FOS technique as a robust and effective solution for monitoring requirements in HEP detectors for physical and environmental parameters. Keywords: High Energy Physics, Fiber Optic Sensors, FBG Temperature Sensors, FBG Strain Sensors, Humidity Sensors, CMS, CERN.
1. THE CERN AND THE LHC CERN, the European Organization for Nuclear Research [1], is the world's leading laboratory for particle physics and is one of the world’s largest and outstanding centers for scientific research. Its business is fundamental physics, finding out what the universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter: the fundamental particles. By studying what happens when these particles collide, physicists learn about the basic laws of Nature. The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies (few 1012 eV) before they are made to collide with each other or with stationary targets. Large size instruments called “detectors” observe and record the results of these collisions. Founded in 1954, the CERN Laboratory sits across the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 20 Member States. Present major research activity at CERN is linked to the recording of events generated by proton-proton collisions in the new accelerator called Large Hadron Collider (LHC). The LHC, the world’s largest and most powerful particle accelerator is the latest addition to CERN’s accelerator complex. It mainly consists of a 27 km ring of superconducting magnets with a number of accelerating structures to boost the energy of the particles along the way. The LHC is designed to collide two counter rotating beams of protons or heavy ions. Proton-proton collisions are foreseen at an energy of 7 TeV per beam. The beams move around the LHC ring inside a continuous vacuum guided by magnets.
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The magnets are superconducting and are cooled by a huge cryogenics system. The cables conduct current without resistance in their superconducting state. The beams will be stored at high energy for hours. During this time collisions take place inside the four main LHC experiments (ALICE, ATLAS, CMS, LHCb). Figure 1 shows a schematic of the LHC accelerator and experiments.
Fig.1 Schematic of the LHC accelerator Inside the accelerator, two beams of particles travel at close to the speed of light with very high energies (up to 7 TeV for each beam) before colliding with one another. The beams travel in opposite directions in separate beam pipes – two tubes kept at ultrahigh vacuum. They are guided around the accelerator ring by a strong magnetic field (of around 8.4 Tesla at a current of around 11700 A), achieved using superconducting electromagnets. These are built from coils of special electric cable that operates in a superconducting state, efficiently conducting electricity without resistance or loss of energy. This requires chilling the magnets to about -271°C . For this reason, much of the accelerator is connected to a distribution system of liquid helium, which cools the magnets, as well as to other supply services. Thousands of magnets of different varieties and sizes are used to direct the beams around the accelerator. These include 1232 dipole magnets of 15 m length which are used to bend the beams, and 392 quadrupole magnets, each 5–7 m long, to focus the beams. Just prior to collision, another type of magnet is used to put the particles closer together to increase the chances of collisions. First collisions of two proton beams at 2.2 TeV were recorder in last November 2009. After a technical stop of 3 months LHC has restarted the collision program, reaching in last March 2010 the colliding energy of 3.5+3.5 TeV in centre of mass. In 2010-2012 CERN's plan is to run continuously for a period of 18–24 months, with a short technical stop at the end of 2010. Experiments will run throughout this time, with researchers expecting to accumulate 1fb-1 (one "inverse femtobarn") of data - roughly 10 trillion proton–proton collisions at 7TeV energy - in order to collect enough statistic to evaluate possible interesting events with new elementary particles never seen up to now.
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2. THE CMS EXPERIMENT The Compact Muon Solenoid (CMS) [2] is one of the four experiments installed in the LHC accelerator. CMS detector is designed to see a wide range of particles and phenomena produced in high-energy collisions in the LHC.
Fig.2 CMS detector schematic
Fig.3 CMS detector picture Different layers of detector stop and measure the different particles, and use this key data to build up a picture of events at the heart of the collision. CMS Experiment is a very complex and large detector made of a large superconductive
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magnetic solenoid (which is able to produce a magnetic field up to 4T) and several particles sub-detectors: silicon inner trackers, electromagnetic and hadron colorimeters, muon detectors. As described in the schematic shown in fig. 4, detectors consist of layers of material that exploit the different properties of particles to catch and measure the energy and momentum of each one.
Fig.4 CMS detector transverse slice and possible detection of different particles CMS was designed around getting the best possible scientific results, and therefore to look for the most efficient ways of finding evidence for new physical theories. This put certain requirements on the design. CMS needed: a high performance system to detect and measure muons, a high resolution method to detect and measure electrons and photons (an electromagnetic calorimeter), a high quality central tracking system to give accurate momentum measurements, a “hermetic” hadron calorimeter, designed to entirely surround the collision and prevent particles from escaping. With these priorities in mind, the first essential item in the design of CMS detector was to provide a very strong magnetic field. The higher a charged particle’s momentum, the less its path is curved in the magnetic field, so when we know its path we can measure its momentum. A strong magnet was therefore needed to allow an accurate measurement even the very high momentum particles, such as muons. A large magnet also allowed for a number of layers of muon detectors within the magnetic field, so momentum could be measured both inside the coil (by the tracking devices) and outside of the coil (by the muon chambers). The magnet is the “Solenoid” in Compact Muon Solenoid. The solenoid is a coil of superconducting wire that creates a 4T magnetic field; in CMS the solenoid has an overall length of 13m and a diameter of 7m. It is the largest magnet of its type ever constructed and allows the tracker and calorimeter detectors to be placed inside the coil, resulting in a detector that is, overall, “compact”, compared to detectors of similar weight. Physicists will use CMS data to answer questions such as: what is the Universe really made of and what forces act within it? (search of components of so called “dark matter and dark energy”)? And what gives everything substance? (search of so called Higgs boson”). CMS will also measure the properties of previously discovered particles with unprecedented precision, and be on the lookout for completely new, unpredicted phenomena.
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The entire CMS detector requires to be monitored in all its complex functions and regions. In order to do that few thousands of classical sensors are installed to monitor: temperature, relative humidity, current, gas , magnetic field, positions, displacements etc . Since July 2009 we have installed for the very first time in CMS detector 42 Fiber Optic Sensors (FOS). If this technique will give the expected results we are planning to install additional FOS in next maintenance periods in different regions and to monitor different parameters. In present document we describe the installation and very first results of FOS technique applied in CMS detector. The synergy of technological complexities in high energy physics detectors is such that the FOS technique appears to be a very interesting solution to match the demanded performance both in term of resolution and compactness. First results both for temperature and strain measurements in CMS are presented. The flexibility of FOS technique to sense alternative parameters has triggered R&D studies for FOS applications in relative humidity. Very preliminary results in these fields are presented too.
3. FIBER OPTIC SENSORS IN HIGH-ENERGY PHYSIC APPLICATIONS Nuclear radiation effects on optical materials and photonic devices have been studied since several decades. Today many applications associated with presence of highly energetic radiation can benefit from the enhanced functionalities offered by photonic technologies and especially by fiber optic devices for communication and sensing. Examples of these application fields include space, healthcare, civil nuclear industry and high energy physics experiments. Electronic and photonic components are well known to suffer from exposure to nuclear radiation. The radiation interacts with the materials and alters their characteristics. This most often modifies the performance and affects the reliability of the device. Resulting device failures and system malfunctions may have dramatic consequences on safety and carry significant financial repercussions. Modern fiber fabrication technologies now allow obtaining fibers with low to moderate levels of RIA, depending on the wavelength range. Actually it is important to observe that SMF28 optical fibers have been selected in CMS experiment for data transfer and thus the issues related to their radiation hardness have been already addressed. For this reason we selected SMF28 optical fiber also for our FOS samples. This aspect together with the undeniable and well-known advantages of fiber optic technology and the increased availability of compact, efficient and lower cost devices promoted renewed interest in the applications of optical fiber based systems in radiation environments. Fiber Bragg Grating (FBG) represent a versatile photonic component that can be applied in both optical communication and sensing systems. The response of different types FBGs to various kinds and intensities of highly energetic radiation has therefore been the subject of many studies in the last decade. A very exhaustive review report can be found in [3]. Whereas the basic grating characteristics such as peak wavelength, spectral width and amplitude have all been shown to change under radiation, the magnitude of these changes is very dependent on the grating type and on the grating fabrication. Solutions to mitigate the radiation sensitivity of FBGs have been proposed. Further progress is however still necessary before the widespread use of fiber Bragg gratings in high radiation environments can become a reality. In the LHC detectors the absorbed dose in the most critical region (silicon tracker detector whose inner region called pixel detector is placed only few cm far from collision point) is at level of 10 6 Gy for integrated luminosity of 5x105 pb-1, meaning 10 years of LHC operation including the ramp up phases, or 107 seconds of LHC integrated operation at peak luminosity[4]. This dose value means about 0.36 kGy/h in the maximum luminosity period, in the most critical region of LHC detectors (pixel detector). This value is lower at least by 2 orders of magnitude with respect to the reference doses to be considered for FOS application in ITER or nuclear central applications widely studied in. Of course the absorbed dose in different regions of HEP detector, farer from the collision point is further reduced by orders of magnitude. However we are planning in next months specific radiation hardness campaigns of FOS to ionizing and non-ionizing energy losses.
4. TEMPERATURE MONITORING WITH FOS AT CMS The temperature monitoring in the HEP experiment is a very relevant task to be covered in order to control the correct behavior of the electronics in different sub-detectors. Precise T monitoring will also allow de-convoluting T effects on possible change of sub-detector performances. Today there are few thousands temperature sensors located in several regions of the CMS detector. Their main scope is to get information about the working condition of specific subsystems and their components. The monitoring becomes crucial for all the equipments that are temperature dependent and/or have to work under particular thermal conditions. Up to now, mostly classical commercial PT100 sensors are used to perform this task. Obviously, each PT100 requires at least two wires to be connected to a single channel of a proper data acquisition system. This means that cabling issues have to be seriously addressed when the sensors number increases, as the case of CMS experiment. Moreover the operation and noise of standard electronic sensors is very often dependent
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from the electromagnetic side condition stability. The presence of intense magnetic field and the large amount of additional signal, LV and HV cables can often procure induced noise and instabilities. On top of that, radiation hardness of electronic sensors is often a limit in the detector region they can be installed. It is easy to recognize that the use of Fiber Bragg Grating technology, enabling the interrogation of many tens of sensors along a single optical cable, is the strongly reducing by 1-2 order of magnitude the amount of fiber to be installed for a specific amount of sensors, and consequently the levels of complexity and installation issues. In addition the intrinsic independence of performance vs. electromagnetic side conditions and the total absence of induced noise is a relevant advantage for their use in HEP detectors. In the last July 2009, we have installed and successfully tested 20 FBG thermal sensors in CMS. This first installation has covered the temperature monitoring of the two end-flanges of the tracker volume on both positive and negative sides of the CMS detector. Two arrays composed of 10 FBGs each, packaged with ceramic materials were installed on the most interesting positions. The end-flange surface separates the CMS inner tracker from the external zone. This panel is meant to secure the stability of the tracker T and relative humidity versus possible variations induced by the external side conditions. Specific thermal shielding is installed in order to compensate for possible changes of T induced by heating cables or other thermal sources. Several electronic sensors (for both T and RH measurements) are installed in the internal area of the CMS inner tracker. FOS were installed in the external surface of the end-flange in order to cross check the efficiency of the thermal shielding system and to additionally monitor the thermal fluctuation along the tracker cable trays and cooling circuits. As readout system a commercial Micron Optics [5] FBG interrogator has been used. All the data taking system has been fully integrated in the CMS central Detector Control System (DCS) standards. Due to the spectral characteristic of the single FBG and the used readout system, the expected temperature resolution is in the range of +- 0.2°C. Fig. 5 reports some details of the sensors installation in the bulkhead and along the cable tray.
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b) Fig.5 Pictures of CMS end-flange surface and tracker cable tray where 10 FBG sensors were installed as temperature sensors. 5 sensors were installed on the bulk head surface (Fig. 5a) and 5 along a cable tray (Fig. 5b). Red circles evidence some of the sensor locations.
Figure 6 reports some preliminary results obtained during some cooling cycling of the end-flange subsystems, in the CMS negative side, in a period of time of 6 days.
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Figure 6. Plots of the temperature variation of the inner part of the CMS obtained by means of the installed FBG arrays. On the top the 5 sensors on the separation plate, on the bottom the 5 along the PY1 line.
The comparisons between T data recorded by FBG sensors and by classical PT100 thermocouples demonstrated an excellent agreement.
5. STRAIN MONITORING AT CMS In HEP detectors the capability to have a structural monitoring of the position and deformations of mechanical main structure is a relevant issue in order to secure the stability in time of the correct alignment of different sub-detectors and to compensate/correct possible deformations and/or displacements induced by the magnetic forces and aging effects. Given its mechanical modular structure, the large amount of iron structures (forming the magnet yoke) and the very intensive magnetic field, the CMS experiment is strongly requiring a very precise structural monitoring. Due to the very large dimension and complexity of the detector, and also due to the large forces induced on the metallic part of the detector by the intense magnetic field, capabilities to sense displacements, stresses and strains are strongly required. It is well known the high level of performances of FBG sensors to monitor the strain variations. We have successfully installed FOS strain sensors to monitor the very forward part of the detector in the negative side of the experimental area. In this region of the detector the mechanical structure is composed by 4 metal supports called “RAISERS” which are piled up to reach the correct high where the beam pipe is placed and support there the platform where the Very Forward Hadronic Calorimeter (called HF) is placed together with an additional CSM sub-detector called CASTOR, additional calorimeter very close to the beam pipe. Both these calorimetric detector (HF and CASTOR) are placed around and very close (few mm) to the beam-pipe of the LHC and collect the particles produced by the proton-proton collision in a limited angle of the very forward region. Any unexpected motion in this region could cause dramatic effects of the stability of the LHC beam pipe (kept under vacuum). The pile of the 4 RAISERS constitutes a large size metallic scaffold (the 4 yellow structures in the picture in fig.7). All this metallic structure is mechanically independent from the main part of CMS experiment, and is attracted toward the CMS central region, when the 4T magnetic field produced by CMS solenoid is powered ON. The consequent induced movement on HF and CASTOR could produce damages to the pipe, so it is crucial to have full control of the RAISER pile deformations and/or displacements. On the 4 corners of the RAISER pile there are 4 metal beams to secure the relative locking of the 4 raisers. We have installed 10 strain FBGs plus 2 for the temperature compensations on these 4 metal reinforcement beams. Moreover we have also installed additional 4 strain + 1 temperature FBG sensors, under
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the metallic platform (the green suspended platform in the picture) on which HF and CASTOR are placed. Additional 4 strain sensors and 1 temperature sensor have been installed in the CASTOR detector support.
Fig. 7 Very Forward region in negative side of CMS detector. There are visible: 4 raisers (yellow structures), metal platform (green structure on last raiser), CASTOR metallic support (central orange structure on the green platform) and LHC beam pipe (yellow central pipe).
Only as exemplification, the plots in fig. 8 report the strain measurements acquired during one of the turn-ON phase of the magnetic field up to 3.8 T. Strain data clearly show that on front part of the RAISER (the non-interaction-point side) the reinforcement metal beams are extending their dimensions, while the rear part (interaction-point side) are compressing their dimensions. These measurements demonstrated that the magnetic field forces produce an overall deformation of the whole RAISER pile, procuring a tilt without displacement of the support metal platform holding HF and CASTOR. This means that both sub-detectors on the platform remain limited within the safety distance region with respect to the beam pipe passing in their center.
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Fig. 8 Strain values recorded during magnetic field ramp up phase. Clearly visible the compression in the positive side (8.a) of RAISER pile and extension on the negative side (8.b)
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6. FOS HUMIDITY SENSORS FOR HEP DETECTORS AT CERN The high radiation levels to which silicon tracker detectors are exposed at the LHC demand their operation at temperatures below 0°C and consequently in a dry atmosphere to avoid condensation. For this reason, a constant and effective thermal and hygrometric control of the air filling the silicon tracker cold volumes is mandatory during steady state operations as well as during cool down processes. The technologies currently in use at CERN for Relative Humidity (RH) monitoring are mainly based on a thermoset polymer capacitive sensing element with on-chip integrated signal conditioning that needs three wires for each sensing point. This means that complex cablings are required to control the whole volume inside the tracker with enough RH sensors. In addition these devices are critically damaged when subject to high radiation doses, being not designed with radiation hardness characteristics. Also on that subject, optical fiber sensors provide many attractive characteristics among which immunity to electromagnetic interference, passive operation (intrinsically safe), water and corrosion resistance and, above all, ease of multiplexing with consequent reduction of cabling complexity, size and weight. In addition, as already mentioned, single-mode standard fibers (SMF-28) demonstrated good resistance to radiations. Recently our multidisciplinary research group has started to investigate the possibility to exploit fiber optic sensing technology at CERN also in this field. Several RH sensors were produced by depositing a tin oxide overlay onto the distal end of cleaved optical fibers by means of the Electrostatic spray pyrolysis technique [6-8]. As schematically represented in Figure 9, such a configuration constitutes a micrometer-sized Fabry-Perot (FP) interferometer in which any change of the refractive index of the SnO2 layer due to absorption of water molecules is translated in a change of the probe reflectance and hence it is detected as a change of the light reflected by the fiber tip. The key point of this configuration relies on some recent results that some of the authors of present article have achieved while studying the surprising sensitivity of SnO2-based optical fiber sensors for chemical detection [4-6]. In particular, it was demonstrated that significant sensitivity improvements of the standard FP configuration (involving almost flat overlays) could be obtained when structured morphologies of the sensitive coatings are considered, with the surfaces exhibiting microstructures with dimensions comparable or greater than the light wavelength [6]. Such structure is able to perturb the optical near field profile emerging from the fiber termination (see for example Figure 9.b and c), producing its local enhancement and focusing [9]. In this case indeed, the interaction of the field with the chemicals present in the surrounding environment occurs not in the volume of the layer but mainly on its surface by means of the evanescent part of the field, thus promoting a significant improvement of the fiber optic sensor performance. A robust and engineered optoelectronic interrogation unit has been designed and realized enabling the continuous monitoring of these variations [7]. The optical sensor output has been chosen as the ratio between the reflected light intensity and the one emitted by the source . In this way a direct measure of the probe reflectance is secured, not being influenced by even small changes of the source light.
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Figure 9 (a) Schematic representation of the fiber optic sensor based on micrometer-sized FP interferometer configuration; (b) typical AFM image of a tin oxide overlay deposited by the ESP technique onto the fiber termination and (c) corresponding optical near-field emerging from the fiber tip.
Figure 10 reports the typical results of RH monitoring carried out at CERN laboratories by a SnO 2-based fiber optic sensor at a temperature of 20°C and 0°C, respectively. For comparison, the response provided by a currently used commercial sensor (Honeywell HIH 4000 series) is also reported.
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Figure 10 RH variations detected by the realized fiber optic sensor and commercial capacitive sensor at a temperature of (a) 20°C and (b) 0°C, respectively.
A perfect agreement is clearly visible between the RH measurements provided by optical sensor and the commercial one, even at very low temperatures. The results here reported evidences that fiber optic technology has the strong potentiality to be used in the future as valid alternative to currently exploited polymer-based devices for RH monitoring in high energy applications at CERN.
7. CONCLUSIONS FOS technique looks very attractive for application in HEP detectors, due to the intrinsic characteristics of compactness, easy cabling and installation, absence of sensitivity to surrounding electromagnetic fields, radiation hardness. T and strain FOS have been successfully installed and operated in the CMS detector at CERN since few months. Their readout system has been fully integrated in the CMS central DCS standards. Preliminary results show very effective and reliable T measurements, fully in agreement with other standard electronic sensors already installed in the detector. The FOS strain measurements have proven a very effective technique to monitor possible deformations induced by the strong forces caused by magnetic field on the metallic structures of the detector. An R&D activity has started to verify the possibility to use FOS techniques for RH measurements in silicon trackers for HEP detectors. Preliminary results show a perfect agreement with commercial RH sensors.
8. ACKNOLEDGEMENTS Authors acknowledge A. Ball and P. Siegrist (CERN) for their continuous interest and support to the introduction of FOS technique in the framework of CMS detector. The technical support and very effective collaboration of M. Alidra, J. Antunes, S. DiVincenzo, D. Druzhkin, G. Faber and A. Gaddi (CERN) and T. Rodrigo (University of Cantabria) is also acknowledged for the installation of strain FOS and their data evaluation versus magnetic field cycles. F. Palmonari (INFN - Pisa) and A. Tsirou (CERN) are acknowledged for their support in the installation of T FOS in CMS bulk head region and data evaluation. F. Glege, M. Hansen (CERN), N. Beni and Z. Szillasi (Atomki) gave a relevant contribution for full integration of the readout system in the standards of CMS central DCS. R. Dumps (CERN) and L. Petrazzuoli (University of Naples) have given a crucial contribution in the laboratory setting up and measurement campaign of FOS RH prototypes at CERN.
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