By Christine Turner,2014-01-20 02:41
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    V. Gautard, O. Cloué, CEA-Saclay, DAPNIA, France

    Abstract measurement using triplets of precision drift chambers ATLAS is a particle detector that will be built at CERN with a mean inter chamber distance of 5 meters. The in Geneva on the LHC accelerator. In the barrel, it is target degree of accuracy for the precision chamber constituted of 600 chambers of few square meters, alignment is such that the alignment contribution to the amongst other things. The relative position of a chamber final sagitta measurement error stays below the intrinsic within a triplet must be known with a spatial resolution of chamber measurement error which contributes at a level 30?m. To fulfil these requirements, different alignment of 50 ?m. systems have been designed. The PRAXIAL sensor that To fulfil this global precision one of the key element of we have developed at Saclay is one of them and its goal is the alignment system is the PRAXIAL system. Its to measure the relative position of a chamber with respect measuring accuracy should be 30?m in translation and to the neighbouring chambers. 50?rad in rotation. The initial positioning of the chambers In order to reach the required precision, each and the possible chamber displacements during the life of PRAXIAL sensor must be individually calibrated. Since a the experiment drives the measuring range of each chamber must be equipped with four (half) sensors, the PRAXIAL sensor: ?5mm and ?5mrad. total number of PRAXIAL to be calibrated will be 1250.

    After a short introduction on the experiment, the second 2 PRAXIAL ALIGNMENT part of this paper is devoted to the PRAXIAL alignment.

    The last part is related to the calibration bench: the We will now detail the PRAXIAL sensor. First we will hardware part and its associated software. explain the RASNIK optical system, which is the basic

    sensor for the PRAXIAL sensor. 1 ATLAS, THE MUON DETECTOR AND 2.1 The Rasnik sensor

    THE ALIGNMENT PRINCIPLE This sensor has been developed by the NIKHEF

    institute in Amsterdam [5]. It is called RASNIK for

    Relative Alignment System from NIKhef. It measures the 1 to 2 m relative position between three elements: a coded mask lightened by a set of infrared LED, seen by a camera through a lens (see Fig. 2). 20 m 2 to 6 m This optical system is able to measure four coordinates: i) and ii) the 2D transverse position with a resolution of ~2?m, iii) the mask magnification on the camera with a -4resolution below 10 and iv) the angle between the mask line and the pixels line of the camera with a resolution of 45 m


     Figure 1: The ATLAS detector and a muon chamber

    The ATLAS experiment, see Fig. 1, is a detector that will be installed on the LHC accelerator [1, 2, 3] at CERN [4]. The LHC will provide proton-proton interactions with 12a centre of mass energy of 14.10 eV. One of the physics

     goals of the experiment is to detect the Higgs particle. coded Despite the fact that its existence is crucial for the particle (front view) physics Standard Model, it has not yet been observed. 0 The higgs particle may decay through two Z particles

     each decaying into two leptons: e.g. muons or electrons. Figure 2: The RASNIK sensor.

     Thus the muon channel is of particular importance. The

    NIKHEF has also developed a readout electronic momentum measurement in the ATLAS muon

    system together with an image analysed software called spectrometer aims at a precision of the order of 10% for

    ICARAS. muons of momentum 1 TeV. It proceeds from a sagitta

    ICARAS drives a multiplexer in order to operate the each position we record and reconstruct the RASNIK infrared LEDs and the camera, through a RS232 device. images. Using this set of coordinates and knowing each An image of the coded mask as seen by the camera is element position thanks to mechanical displacement digitised through a frame-grabber card. Finally, the four probes we can calculate the transfer function of the sensor. reconstructed coordinates are stored in a file. 3.2 Hardware 2.2 The praxial sensor The mechanics of the calibration bench is very

    important. It is composed of four distinct components (see The PRAXIAL sensor that has been developed at

    Fig. 3, top part and Fig. 4): Saclay is composed by two crossed RASNIKs (see Fig. 3,

    bottom part). The optical components are mounted on two 1. Mobile support: it is made up six displacement stages

    (three translations and three rotations). mechanical elements each installed on two neighbouring

    chambers. 2. Static support: it has precise support in order to

    The principle of the sensor is to take the four position the static element in a reproducible manner. coordinates of each RASNIK in order to calculate the six 3. Mechanical interface: it is an interface in-between parameters, three translations and three rotations, the mobile element and the stages. This piece has to describing the relative position of one element with have geometry precisely known (<10?m). respect to the other one. 4. Support of the mechanical probes: it has eight As it is impossible to mount all optical components on adjusted holes in order to fix the eight mechanical the platforms with the required accuracy, we have to probes on it. It is mechanically linked to the static perform a calibration of our PRAXIAL sensors (see support. The probes measure the mobile platform section 3). position with respect to the static one relatively to the first movement. We used eight probes for redundancy (only six are needed). Each probes has a translation resolution of 1?m. The bench stays on a concrete block to ensure stability. The required accuracy imposes to work with a stable temperature close to the operating one. The operating temperature is 20?C ? 0.5?C. In order to monitor the thermal effect, we have installed five PT100 probes.

    Finally, to have an absolute calibration, we have built a mechanical support with a known geometry with an

    accuracy of 5?m. This system is called ZEROPRAX and is used to define a common mechanical frame for both

    PRAXIAL elements.

     ------- mobile static ------

     Mobile element Static element Figure 3: The Praxial system (bottom) and the Praxial sensor installed on the Caliprax calibration bench (top).

     3 CALIPRAX BENCH As we have to calibrate 1250 PRAXIAL sensors, we have built a PC controlled calibration bench. It has been Figure 4: Overview of the calibration bench. installed in an air-conditioned room to avoid thermal

    3.3 Software variation during calibration.

    For cost consideration, we decided to use a computer 3.1 Principle equipped with PCI and ISA cards. The principle of the calibration is to scan all the active Two C++ applications have been written to control the working space of the sensor (?5mm and ?5mrad) and to bench: i) a set-up program and ii) a main program. We determine the transfer function of the sensor. To do that, developed and wrote both programs in our institute. In we fixed one of the two elements on a static support and addition we use ICARAS. Thus the three software the other one on a mobile support. Then we move the programs are: mobile element approximately one hundred times. For 1. the set-up program allows:

    - the configuration of the mechanical parameters all necessary information during all steps of the

    of the bench: e.g. the dimensions of the calibration. In particular, each element must be labelled

    mechanical interface, with a unique bar code. - the definition of the electronic parameters of the

    measurement channels, the gain, the conversion 4 CONCLUSION coefficient and the alarm thresholds.

    In this paper we have presented the software used to 2. ICARAS: this program performs the acquisition and control the calibration bench of the ATLAS PRAXIAL reconstruction of both RASNIKsensors. It is

    sensors. triggered by the main program.

    The bench now works correctly but still has to be 3. the main program: it controls all the calibration

    improved in particular on what concerns the record operations. It is the main software part of the bench

    procedure and the documentation. The resolution obtained since each function can be executed and seen

    on few PRAXIAL prototypes already fulfils the ATLAS separately in a dedicated window.

     specifications: 30?m in translation and 150?rad in

    Let us now detail the six different calibration operations rotation. needed to calibrate a PRAXIAL sensor: This bench is planed for two and a half years of

    operation and will start mid of 2003. 1. Initialisation: it consists in:

    - verifying the electronics of the sensor to be

    calibrated, reading and verifying the bar code 5 REFERENCES using the database,

    - measuring hall temperature. [1] 2. Starting point search: the first position is adjust in order to find the reference position as given by the [2] G. Cohen-Tannoudji, ? Les enjeux du LHC, le ZEROPRAX mechanic. modèle standard ? (in French). Scintillations, journal 3. Data acquisition: up to now the exact number of du DAPNIA nº23, may 95 and nº24, june 1995 movements necessary to obtain the correct calibration is not fixed. Nevertheless, a Monte-Carlo simulation [3] “ATLAS Technical Proposal”, CERN/LHCC/94-43, shows that hundred movements is a maximum. In 15 December 1994; order to synchronise the following actions a “task “ATLAS Muon Spectrometer Technical Design sequencer” is realised. Report”, CERN/LHCC/97-22, 31 May 1997; 3.1. Motion of the mobile platform: The software controls the motorised stages through a RS232

    protocol following a list of pre-defined [4]


    [5] H. van der Graaf et al., “Rasnik, an alignment 3.2. Alarm temperature: this task consists in reading

    system for the ATLAS MDT barrel muon five probes to secure the process.

    chambers”. Technical system description. 3.3. Reconstruction of the mobile platform position:

    Publication of NIKHEF institute: ET38110, april This module calculates the position of the

    2000. See also: mobile support with respect to the ZEROPRAX position reading the mechanical probes through [6] V.Gautard, J.-P.Schuller, ? Résolution d'un système a RS485 card and using a DLL calculation. physique à 26 paramètres ? (in French), Proceeding 3.4. RASNIK acquisition: it consists in triggering the of the CANUM conference (2002), Anglet, France, ICARAS program to acquire RASNIK data. Eds. SMAI 301, may 2002. 3.5. Storage: record all results. 4. Analysis: It determines the transfer function. Two [7] E.Franchomme, ? Développement d’une interface methods can be used: a linearisation method or a permettant de gérer l’étalonnage des systèmes geometrical method [6]. d’alignement dans l’expérience ATLAS-muon ? (in 5. Final validation: This operation validates or refuses French). Rapport de stage, septembre 2002. the analysis just realised. For this, the program starts

     again some known displacements and computes the

    theoretical position using the previously calculated Acknowledgements: transfer function. The error between these known We would like to thanks our Saclay colleagues for fruitful positions and the computed positions must not discussions: J.-Ch.Barrière, B.Duboué, M.Fontaine, exceed the required accuracy. E.Franchomme, C.Guyot, P.Perrin, P.Ponsot, Y.Reinert, 6. Storage: The results are stored in the ATLAS J.-P.Schuller, P.Schune. experiment database [7].

Finally, to calibrate these 1250 PRAXIAL sensors a

    quality control method has been established. We record

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