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Impact of Humidity on Deformations Offr-4 Based Detector

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Impact of Humidity on Deformations Offr-4 Based Detector Impact of Humidity on Deformations of FR-4 Based Detector Elements in the ATLAS New Small Wheel Yanwen Hong Supervisor: Prof. Dr. Ulrich Landgraf Dr. Stephanie Zimmermann Fakultät für Mathematik und Physik Albert-Ludwigs-Universität Freiburg This dissertation is submitted for the degree of Master of Science Abteilung Prof. Dr Gregor Herten July 2019 Abstract The discovery of the Higgs boson in the ATLAS and CMS experiments showed the necessity of high luminosity in discovering new physics. The LHC is undergoing upgrades of the collider, accelerator chain and detectors for the High-Luminosity LHC project in several phases. The inner endcap of the muon spectrometer in the ATLAS detector will be replaced by the New Small Wheel. The two technologies used in the NSW are sTGC and MicroMegas, which can achieve a high spatial resolution in muon tracking and better angular resolution in trigger performance by implementing strip-structured detector modules. FR-4 grade PCBs are one of the crucial components of the detectors. Mechanical deformations caused by a humid environment were observed during the single layer detector element machining and detector module assembly processes. These deformations will lead to strips misalignment and furthermore have a direct impact on resolution of the detectors. In order to evaluate the impact of these deformations on the performance of the detector, a designed humidity study is conducted in this thesis. The maximum and residual strip misalignment of three detector elements at an extremely humid environment and after the air-drying process is determined by measurement data. Considering the machining and manufacturing process, the values of mechanical tolerances of the detector modules are compared to the maximum and residual strip misalignment. Contents 1 Introduction 1 2 New Small Wheel Project in Phase-I Upgrade of the ATLAS Experiment 3 2.1 Large Hadron Collider and the ATLAS Detector . 4 2.1.1 The Muon Spectrometer . 6 2.1.2 Trigger System . 7 2.2 The New Small Wheel Upgrade Project . 8 2.2.1 Motivation of the NSW . 9 2.2.2 Detector Technologies of the NSW . 10 2.3 Operating Environment and Construction of the Investigated Elements . 13 3 Experimental Setup 17 3.1 The Humidification Chamber . 17 3.2 The Gas Input System . 19 3.3 The Thermo-Hygrometer . 20 3.4 The Mechanical Coordinate Measurement Machine . 22 3.5 Choice of Reference Spheres’ Positions and Measurement Coordinate System 24 4 Deformations of Production Line Detector Elements 29 4.1 Introduction of the Experiment . 29 4.2 Results of sTGC Strip Cathode Board S1 Module . 30 4.3 Results of MM Readout Panel with Stereo Strips SM2 Module . 43 4.4 Results of MM Readout Anode Boards with Eta Strips SE6, SE7, SE8 Modules 53 5 Summary 63 Appendix A Figures of the Results 65 Appendix B Technical Drawing and Data Sheets 69 Contents References 75 List of Figures 77 List of Tables 81 vi Chapter 1 Introduction The High-Luminosity Large Hadron Collider (HL-LHC) project aims to reach a peak in- stantaneous luminosity of 7.5 1034cm 2s 1 [1] to increase the potential of discovering · − − new physics, with operation beginning at 2026. To cope with this, the LHC is undergoing extensive upgrades of the collider, accelerator chain and detectors in order to achieve data collection with high statistics in a short timescale. The first phase of the upgrade is planned during the Long Shutdown (LS2) in 2019-2020. One of the multipurpose detectors, the ATLAS is now undergoing upgrades of all sub-systems for a new trigger and data acquisition architecture. For the muon spectrometer, besides the power system and electronics, the innermost endcap Small Wheel (SW) will be replaced by New Small Wheel (NSW). The NSW will be able to reduce the fake trigger rate and perform a precision track measurement of muons at a high radiation background. This leads to great improvements in the investiga- tion of the leptonic signatures of the Higgs Boson, as well as for super-symmetric particles (SUSY), and in the search for heavy and beyond standard model (BSM) particles. The NSW utilises two detector technologies: small-strip Thin Gap Chamber (sTGC) for a precise trigger signal and Micro Mesh Gaseous Detector (MicroMegas, MM) to obtain a high spatial resolution in the muon tracks. These demanding goals are achieved by a complex construction of the detectors. Readout boards, which are one of the crucial parts of detector layers, are based on FR-4 material with copper readout strips. FR-4 is a grade designation for glass fabric reinforced epoxy laminate material. FR stands for flame retardant, which is commonly used in Printed Circuit Board (PCB) production [2]. The readout boards in the NSW project are multi-layer materials with regular FR-4 PCBs produced in companies. Nu- merous studies have been done for the mechanical, electrical and physical properties of FR-4, for example flexural strength and moisture absorption. However, the deformations of the FR-4 based detector elements caused by humidity exposure have not been precisely studied 1 yet. Mechanical deformations were observed during the single layer detector machining and detector module assembly processes. These deformations misplace the readout strip pattern and furthermore have a direct impact on the spatial resolution of the detector. The aim of this thesis is to determine the shapes and dimensions of the deformations caused by a humid environment on the FR-4 based multi-layer detector elements in the NSW. To achieve this, a relatively gas-tight humidification chamber and a gas input system for ex- posing the materials in a controlled humid environment were developed. Several reference spheres are glued on the surface of the detector elements according to specific patterns, to trace the geometry information. A mechanical Coordinate Measurement Machine (CMM) is used to give precise position values of the reference spheres up to a micrometer range. The 3D surface geometry model of each detector element is reconstructed from the position values to study and analyze the deformations. The thesis is structured as follow. Chapter 2 gives a general introduction to the LHC, ATLAS detector, muon spectrometer and the two detector technologies of the NSW, as well as discusses the construction and operating environment of the three detector modules to be studied in this project. These are sTGC strip cathode board S1 module (FR-4 with copper strips and plane), MM readout anode boards with eta strips SE6, SE7 and SE8 modules (FR-4 with copper strips and Kapton foil), and MM readout panel with stereo strips SM2 module (FR-4 with copper strips and aluminum frame). Chapter 3 illustrates the experimental setup and explains the choice of measurement methods with the CMM machine. Chapter 4 describes experimental procedures and presents the results of observed deformations of the three elements, and gives reasonable evaluation of the maximal and residual deformations for further simulation and determination of the performance of the detectors. 2 Chapter 2 New Small Wheel Project in Phase-I Upgrade of the ATLAS Experiment The Large Hadron Collider (LHC) colliders protons or ions to near the speed of light at a centre-of-mass energy √s = 13 TeV and an instantaneous luminosity of 1 1034 cm 2 s 1 × − − (2016). The ATLAS (A Toroidal LHC ApparatuS) experiment is one of four detectors at the LHC to identify the particles emerging from these collisions. Different detector subsystems and magnet systems are arranged in layers around the interaction point, aiming for the reconstruction of the paths, momentum and energy of the particles. The high- resolution muon spectrometer is the largest subdetector system. It provides with stand- alone triggering and momentum measurement capabilities over a wide range of transverse momentum, pseudorapidity, and azimuthal angle [3]. In order to cooperate with the higher particle rates at the High-Luminosity LHC (HL-LHC), LHC is will undergo upgrade in several phases. Phase-I upgrade of LHC will take place during the second long shutdown (LS2) in 2019/2020. The first station of the ATLAS muon end-cap system (Small Wheel, SW) needs to be replaced. The New Small Wheel (NSW) will have to operate in a high background radiation region of up to 15 kHz/cm2 while reconstructing muon tracks with high precision as well as contributing for the Level-1 trigger [4]. The NSW uses two chamber technologies: small-strip Thin Gap Chambers (sTGC) are primarily devoted to the Level-1 trigger function, as well as to measure offline muon tracks with good precision; while MicroMegas detectors (MM) are primarily dedicated to precision tracking. 3 2.1 Large Hadron Collider and the ATLAS Detector 2.1 Large Hadron Collider and the ATLAS Detector In the LHC, two beams of up to 1011 protons per bunch travel inside 26.7 kilometers circum- ference beam pipe at close to the speed of light, guided by superconducting electromagnets. They collide 11 thousands times per second at designed centre-of-mass collision energy √s = 14 TeV and instantaneous luminosity 1 1034cm 2s 1. The four collision point are × − − equipped with detectors. They are ATLAS and CMS (Compact Muon Solenoid), as general purpose detectors, ALICE (A Large Ion Collider Experiment), and LHCb (Large Hadron Collider beauty). To cover the full solid angle, the ATLAS detector uses a cylindrical configuration around the collision point with a central barrel and end-caps on either side. It has a diameter of 25 m, a length of 44 m and weighs approximately 7000 t. The overall ATLAS detector layout is shown in Figure 2.1. Fig. 2.1 Illustration of the subsystems in the ATLAS detector. Figure taken from CERN Document Server. The coordinate system and nomenclature used to describe the particles of the experiment are illustrated in Figure 2.2.
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