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A Gas Monitoring Chamber for ATLAS Mdts

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A Gas Monitoring Chamber for ATLAS Mdts A Gas Monitoring Chamber for ATLAS MDTs Dissertation zur Erlangung der Doktorwürde Vorgelegt von Song Xie November 2011 Fakultät für Mathematik und Physik Albert-Ludwigs-Universität Freiburg Dekan: Prof. Dr. Kay Königsmann Referent: Prof. Dr. Ulrich Landgraf Koreferent: Prof. Dr. Karl Jakobs Tag der Verkündigung des Prüfungsergebnisses: 29. November 2011 Abstract As one of the four experiments at the Large Hadron Collider (LHC), the ATLAS experiment features the largest muon tracker in terms of volume. The main detecting elements of the muon tracker are Monitored Drift Tubes (MDTs) which measure the drift times of the electrons created by a traversing muon ionizing the operating gas. The trajectory of the muon is reconstructed from the drift times according to the so-called space-to-drift time relation (rt-relation). The design goal of the performance of the muon tracker is a stand-alone transverse momentum resolution of approximately 10 % for tracks of 1 TeV particles. This requires the MDT detector to have a spacial resolution better than 50 µm within a volume of a length of 40 m and a diameter of 25 m. The ATLAS MDTs use Ar:CO2 93:7 plus a few hundred ppm water vapour as the operating gas. The types of the gas components and their ratios should be kept very steady in order to have a stable rt-relation. In this thesis, the influences of the fluctuations of the mixture ratio on the elec- tron velocity are discussed. Gas Monitoring Chambers (GMCs) have been designed and implemented to monitor the gas of the ATLAS MDTs by measuring the electron drift velocity in the operating gas as a function of the reduced electric field (v-r.e.f relation). The performance of the GMC has been tested. It has a better resolution than that of the commercial gas mixture we can get. With the GMC even a small change of the mixture ratio can be detected in a short time. Additionally, an empirical method based on artificial neural network with multi- layer perceptron has been developed to analyse the mixture ratio from a measured v-r.e.f relation. With this method a fall in water vapour content in the ATLAS MDT gas in the end of 2010 can be seen. 4 Contents 1 Introduction 1 1.1 The standard model . .1 1.2 The Large Hadron Collider . .3 1.3 ATLAS . .4 1.4 ATLAS physics . 14 2 Monitored Drift Tubes 19 2.1 Layout of the drift tube . 19 2.2 Work principle . 20 2.3 Hit rate of MDT . 29 3 Design of Gas Monitor Chamber 33 3.1 Principle . 33 3.2 Drift of electrons in gas . 34 3.2.1 Drift velocity . 34 3.2.2 Monte Carlo simulations . 35 3.2.3 Diffusion . 41 3.2.4 Attachment effect . 41 3.3 Design of the GMC . 45 3.3.1 Electron source . 46 3.3.2 Electron drift volume . 52 3.3.3 Drift time measurement . 57 3.3.4 GMC setup . 60 3.4 Peripheral Device . 61 3.4.1 Gas regulation . 61 3.4.2 DC powers . 63 3.4.3 Control and monitoring . 64 4 Data Analysis 67 4.1 Signals . 67 4.1.1 Drift time of the electron clusters . 68 4.1.2 Current signal . 68 4.1.3 Voltage signal . 70 4.1.4 Measured signal . 74 4.2 Time determination . 74 i ii CONTENTS 4.2.1 Fixed threshold . 75 4.2.2 Fit rising edge with error function . 75 4.2.3 Linear fit of rising edge . 76 4.2.4 Comparison of time determination algorithms . 80 4.3 Velocity . 82 4.3.1 Pulse recognition . 82 4.3.2 Parameters and characteristic variables . 83 4.3.3 Arrival time measurement . 84 4.4 Electron velocity as a function of reduced electric field . 84 4.4.1 Measurement . 84 4.4.2 Error of reduced electric field . 85 4.4.3 Corrections of electric field and temperature . 86 4.5 Sensitivity . 88 5 Software 91 5.1 Runtime software . 91 5.1.1 Scope, function and considerations . 91 5.1.2 Design . 94 5.1.3 Development tools . 104 5.2 Tool kit . 104 6 Running Status 107 6.1 Installation in ATLAS . 107 6.2 Integrated into Detector Control System (DCS) . 107 6.3 Analysis of measurements . 109 6.3.1 Define the indicator . 109 6.3.2 Regression analysis of gas proportions . 110 6.3.3 Results of measurements in ATLAS . 113 6.3.4 Comparison between indicator I1 and regression analysis . 120 7 Summary 125 A Gas Control Unit 127 B Graphic user interface of software 133 B.1 Runtime software . 133 B.1.1 Monitoring and control panels . 133 B.1.2 Configuration panels . 133 B.2 Tool kits . 144 C Database 149 List of Figures 1.1 CERN accelerator complex . .4 1.2 Cut-view of the ATLAS detector . .6 1.3 ATLAS inner detector . .6 1.4 Cut-away view of the ATLAS calorimeters . .8 1.5 ATLAS muon spectrometer . .9 1.6 Schematic view of the triggering principle . 10 1.7 Schematic drawing of a rectangular MDT chamber constructed from multi-layers of three monolayers each, for installation in the barrel spectrometer . 11 1.8 Schematic drawing of Cathode Strip Chamber . 12 1.9 Schematic view of the Thin Gap Chamber . 13 1.10 Most important Higgs-production processes in LHC . 14 1.11 Expected (dashed) and observed (solid) cross section limits for the individual search channels as functions of the Higgs boson mass . 16 1.12 Combined upper limit on the Standard Model Higgs boson production cross section divided by the Standard Model expectation as a function of mH.................................... 16 1.13 Expected and observed exclusion limits based on CLs in the mA- tan β plane of the MSSM derived from the combination of the analyses for the eµ, lτhad and τhadτhad final states . 17 1.14 Observed and expected 95% CL exclusion limits in the mg~-mt~1 plane 18 2.1 Exploded view of a drift tube of the ATLAS MDT chambers . 20 2.2 Stopping power of positive muon in copper as a function of momentum 22 2.3 Electron cluster size distribution of a 100 GeV muon in Ar:CO2 93:7 at absolute 3 bar . 22 2.4 MDT signal generated by a muon . 23 2.5 MDT signal generated by photon . 24 2.6 rt-relations for different gas mixtures at 3 bar . 25 2.7 Maximum drift time as a function of the proportion of argon (left) and water(right). 26 2.8 Gas gain for Ar:CO2 93:7 . 27 2.9 Fluctuation of the gas gain is described by Polya distribution . 27 2.10 Ion velocity in argon . 28 2.11 Simulation results of expected fluxes of photons and neutrons in ATLAS 30 iii iv LIST OF FIGURES 2.12 Estimation of total MDT count rates at the luminosity of 1034cm−2s−2 30 3.1 Monte Carlo simulations of electron velocity as a function of reduced electric field in several gas mixtures . 36 3.2 Monte Carlo simulations of electron velocity as a function of reduced electric field in several gas mixtures for reduced electric field up to 1400 V·cm−1·bar−1 ............................ 37 3.3 Monte Carlo simulations of velocity variations caused by changing the fraction of CO2 .............................. 38 3.4 Monte Carlo simulations of velocity variations caused by changing the fraction of H2O.............................. 39 3.5 Monte Carlo simulations of velocity variations caused by changing the fraction of air . 40 3.6 Monte Carlo simulations of longitudinal diffusion . 42 3.7 Monte Carlo simulations of transversal diffusion . 43 3.8 Monte Carlo simulations of attachment . 44 3.9 Conceptual design of gas monitoring chamber . 45 3.10 Band model of an aluminium photocathode with an oxidized surface . 48 3.11 Test of relation between incident angle and electron yield of the alu- minium photocathode . 50 3.12 Surface treatment for photocathodes . 50 3.13 Top view of assembly of laser and optics . 51 3.14 Wireboard . 53 3.15 Assembly of wire-boards . 55 3.16 Device for high voltage connection . 56 3.17 Scheme of signal tube . 57 3.18 End-plug of signal wire . 58 3.19 3-D model of gas monitoring chamber . 60 3.20 Front-end electronics box and high-voltage box . 61 3.21 Set up of 19-inch rack . 62 3.22 Gas regulation . 63 3.23 Control and monitoring of devices . 64 3.24 Pulse generator for the laser . 65 4.1 Trajectories of electrons from the L-photocathode to the signal wire . 68 4.2 Monte Carlo simulations of drift time of electrons in gas monitoring chamber. 69 4.3 Calculated current signals on signal wire . 70 4.4 Readout electronics . 71 4.5 Simplified model of readout electronics . 71 4.6 Voltage signal on the input of pre-amplifier . 72 4.7.
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