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Signal Formation Processes in Micromegas Detectors and Quality Control for Large Size Detector Construction for the ATLAS New Small Wheel

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Signal Formation Processes in Micromegas Detectors and Quality Control for Large Size Detector Construction for the ATLAS New Small Wheel Signal Formation Processes in Micromegas Detectors and Quality Control for large size Detector Construction for the ATLAS New Small Wheel Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Julius-Maximilians-Universität Würzburg vorgelegt von Fabian Kuger aus Schweinfurt Würzburg 2017 Eingereicht am 12. April 2017 bei der Fakultät für Physik und Astronomie 1. Gutachter: Prof. Dr. Thomas Trefzger 2. Gutachter: Prof. Dr. Otmar Biebel 3. Gutachter: Prof. Dr. Gregor Herten der Dissertation Vorsitzende(r): Prof. Dr. Karl Mannheim 1. Prüfer: Prof. Dr. Thomas Trefzger 2. Prüfer: Prof. Dr. Otmar Biebel 3. Prüfer: Prof. Dr. Werner Porod im Promotionskolloquium Tag des Promotionskolloquiums: 31. Juli 2017 Doktorurkunde ausgehändigt am: .................... Abstract The Micromegas technology is one of the most successful modern gaseous detector concepts and widely utilized in nuclear and particle physics experiments. Twenty years of R & D rendered the technology sufficiently mature to be selected as precision tracking detector for the New Small Wheel (NSW) upgrade of the ATLAS Muon spectrometer. This will be the first large scale application of Micromegas in one of the major LHC experiments. However, many of the fundamental microscopic processes in these gaseous detectors are still not fully understood and studies on several detector aspects, like the micromesh geometry, have never been addressed systematically. The studies on signal formation in Micromegas, presented in the first part of this thesis, focuses on the microscopic signal electron loss mechanisms and the amplification processes in electron gas interaction. Based on a detailed model of detector parameter dependencies, these processes are scrutinized in an iterating comparison between exper- imental results, theory prediction of the macroscopic observables and process simulation on the microscopic level. Utilizing the specialized detectors developed in the scope of this thesis as well as refined simulation algorithms, an unprecedented level of accuracy in the description of the microscopic processes is reached, deepening the understanding of the fundamental process in gaseous detectors. The second part is dedicated to the challenges arising with the large scale Micro- megas production for the ATLAS NSW. A selection of technological choices, partially influenced or determined by the herein presented studies, are discussed alongside a final report on two production related tasks addressing the detectors’ core components: For the industrial production of resistive anode PCBs a detailed quality control (QC) and quality assurance (QA) scheme as well as the therefore required testing tools have been developed. In parallel the study on micromesh parameter optimization and production feasibility resulted in the selection of the proposed mesh by the NSW community and its full scale industrial manufacturing. The successful completion of both tasks were im- portant milestones towards the construction of large size Micromegas detectors clearing the path for NSW series production. Kurzdarstellung Die Micromegas Technologie zählt zu den erfolgreichsten Konzepten moderner Gas- detektoren und findet Anwendung in zahlreichen Experimenten der Kern- und Teil- chenphysik. Nach zwanzig Jahren Weiterentwicklung wurde die Micromegas Technologie für hinreichend ausgereift befunden, um als Präzisionsspurdetektor in den New Small Wheels (NSW) des ATLAS Myon Spektrometers verwendet zu werden. Dies stellt den ersten großflächigen Einsatz der Micromegas Technologie in einem LHC Experiment dar. Dennoch blieben einige der grundlegenden Prozesse in Gasdetektoren nach wie vor unzureichend verstanden und ausgewählte Detektoraspekte, wie die Geometrie der Mikrogitter, wurden bisher kaum systematisch untersucht. Die im ersten Teil dieser Doktorarbeit präsentierten Studien zu Signalenstehungspro- zessen in Micromegas richten sich daher auf die mikroskopischen Mechanismen zum Elek- tronenverlust und die Verstärkungsprozesse in Elektron-Gas-Wechselwirkungen. Diese Prozesse werden auf Basis eines Modells ihrer Abhängigkeiten von den Detektorpara- metern untersucht, wobei stets der Vergleich zwischen experimentell gemessenen Daten, theoretischen Vorhersagen dieser makroskopischen Größen und der Simulation von Pro- zessen auf mikroskopischer Ebene gezogen wird. In Verbindung mit den im Rahmen die- ser Arbeit entwickelten Detektoren und verbesserten Simulationsalgorithmen lieferten diese iterativen Vergleichsstudien ein vertieftes Verständnis der fundamentalen Prozesse in gasgefüllten Detektoren. Der zweite Teil widmet sich den mit der Konstruktion der großflächigen ATLAS NSW Micromegas Detektoren einhergehenden Herausforderungen und diskutiert Entscheidun- gen bezüglich ausgewählter Technologieoptionen, die teilweise substantiell durch diese Arbeit beeinflusst wurden. Darüber hinaus wird abschließend über zwei Tätigkeitsbe- reiche bezüglich der Produktion zentraler Detektorkomponenten berichtet: Für die in- dustrielle Fertigung der resistiven Anoden-PCBs wurde ein unfangreiches und verlässli- ches Qualitätssicherungs- und Qualitätskontroll-Schema sowie die hierzu notwendigen Messtechniken und -apparaturen entwickelt. Die parallel vorangetriebene Studie zur Optimierung der Parameter des Mikrogitters unter Berücksichtigung der produktions- bedingten Limitationen führte zu der Bestätigung der vorgeschlagenen Spezifikation durch die NSW Kollaboration und der industriellen Fertigung dieses Gewebes. Der er- folgreiche Abschluss beider Projekte waren essenzielle Meilensteine auf dem Weg zur Serienproduktion der NSW Micromegas Detektoren. Contents Introduction 1 I. Signal Formation in Micromegas Detectors 3 1. Signal Formation Processes in Gaseous Detectors 5 1.1. Operation Principles of Gaseous Detectors (GD) . 5 1.1.1. Fundamental Processes of Signal Formation . 5 1.1.2. Avalanche Growth and Gas Gain Operation Modes . 7 1.1.3. Detector Characteristics . 8 1.1.4. Parameters in a Gaseous Detector Setup . 9 1.2. From Gaseous Counting Devices to MPGDs . 11 1.2.1. Proportional Counters and (Monitored) Drift Tubes (MDT) - Radial Geometry Detectors . 11 1.2.2. Spark-, Streamer- and Resistive Plate Chambers (RPC) - Parallel Plate Devices . 12 1.2.3. Multi-Wire Proportional Chambers (MWPC) and other Wire Grid Applications . 14 1.2.4. Gaseous Electron Multiplier (GEM) . 15 1.2.5. Micromesh Gaseous Structures (Micromegas) . 17 1.2.6. Other Micro Pattern Gaseous Detectors (MPGD) . 18 1.3. Studies of Signal Formation Processes . 19 1.3.1. Factorized Approach on Signal Strength Description . 19 1.3.2. Dependency between Parameters, Processes and Characteristics . 21 1.3.3. Analytic Description, Microscopic Model and Experimental Results 23 2. Primary Ionization in Gases 25 2.1. Electromagnetic Interactions of Particles with Matter . 25 2.1.1. Energy Loss of Charged Hadrons and Muons . 26 2.1.2. Ionization Behavior of Electrons and Positrons . 28 2.1.3. Interaction of Photons with Matter . 30 2.2. Statistics of Exemplary Events in a Micromegas . 32 2.2.1. Ionization along a Minimum Ionization Particle’s Track . 33 2.2.2. Electron-Ion Clouds formed by X-Rays . 34 2.2.3. Single Electron Insertion . 36 i Contents 3. Electron Drift, Attachment Losses and Transparency of Micromeshes 39 3.1. Theory of Low Energy Electron-Gas Interaction . 39 3.1.1. Low Energy Electron-Gas Scattering Processes . 41 3.1.2. Transport Theory of Drift Velocities and Diffusion . 41 3.1.3. Attachment Losses in (contaminated) Gas-Mixtures . 44 3.1.4. Analytic Description of the Transparency of Wire Arrays and Micromeshes . 45 3.2. Simulation of Microscopic Electron Transport . 47 3.2.1. Attachment Losses during Electron Drift . 48 3.2.2. Electron Transition through Micromeshes and Transparency . 51 3.3. Experimental Assessment of Signal Electron Losses . 56 3.3.1. The Exchangeable Mesh Micromegas . 56 3.3.2. Experimental Setup and Data Analysis . 60 3.3.3. Experimental Results and Comparison to Simulation . 62 4. Electron Amplification and Avalanche Formation 69 4.1. Analytic Description of Electron Avalanches . 69 4.1.1. Direct and Indirect Ionization Processes - Penning Transfer . 69 4.1.2. Photon Induced Secondary Avalanches and Photon Quenching . 71 4.1.3. Fluctuation in Avalanche Formation and Polya Statistic . 72 4.2. Simulation of Electron Amplification . 74 4.2.1. Microscopic Avalanche Simulation in Garfield++ . 74 4.2.2. Penning Rate Determination from Simulation . 75 4.2.3. Simulation of Amplification Gap Size Variation . 76 4.3. Gain Dependence on the Micromegas Fine Geometry Parameters . 79 4.3.1. Qualitative Field Comparison with COMSOL Multiphysics® . 79 4.3.2. Influence of the Mesh Geometry . 81 4.3.3. Impact of the Inter-Pillar Distance . 81 4.3.4. Effect of the Anode Surface . 84 4.4. Statistical Extrapolation towards Large Avalanches . 86 4.4.1. Statistic of (sub-divided) Avalanche Growth . 86 4.4.2. Simulation Studies based on Avalanche Extrapolation . 88 4.5. Avalanche Statistics in Single Electron Response (SER) . 93 4.5.1. Gain Comparison between Experiment, Simulation and Analytic Model . 94 4.5.2. Assessment of Gain Fluctuation in Experiment and Simulation . 96 4.5.3. SER Spectra Comparison and Conclusive Results . 98 ii Contents II. Micromegas Detectors for the ATLAS New Small Wheel 101 5. The ATLAS Detector and its Upgrade Program 103 5.1. The ATLAS Detector . 104 5.1.1. The ATLAS Coordinate System . 104 5.1.2. The Inner Detector (ID) . 105 5.1.3. The Calorimeter System (Calo) . 106 5.1.4. The Muon Spectrometer (MS) . 107 5.1.5. The
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