Straw Performance Studies and Quality Assurance for the ATLAS Transition Radiation Tracker

The Transition Radiation Tracker (TRT) of the ATLAS experiment at the LHC is part of the Inner Detector. It is designed as a robust and powerful gaseous detector that provides tracking through individual drift-tubes (straws) as well as particle identification via transition radiation (TR) detection. The straw tubes are operated with Xe-CO2-O2 70/27/3, a gas that combines the advantages of efficient TR absorption, a short electron drift time and minimum ageing effects. The modules of the barrel part of the TRT were built in the United States while the end-cap wheels are assembled at two Russian institutes. Acceptance tests of barrel modules and end-cap wheels are performed at CERN before assembly and integration with the Semiconductor Tracker (SCT) and the Pixel Detector. This thesis first describes simulations the TRT straw tube. The argon-based acceptance gas mixture as well as two xenon-based operating gases are examined for its properties. Drift velocities and Townsend coefficients are computed with the help of the program Magboltz and used to study electron drift and multiplication in the straw using the software Garfield. The inclusion of Penning transfers in the avalanche process leads to remarkable agreements with experimental data. A high level of cleanliness in the TRT s acceptance test gas system is indispensable. To monitor gas purity, a small straw tube detector has been constructed and extensively used to study the ageing behaviour of the straw tube in Ar-CO2. A variety of ageing tests are presented and discussed. Acceptance tests for the TRT survey dimensions, wire tension, gas-tightness, high-voltage stability and gas gain uniformity along each individual straw. The thesis gives details on acceptance criteria and measurement methods in the case of the end-cap wheels. Special focus is put on wire tension and straw straightness. The effect of geometrically deformed straws on gas gain and energy resolution is examined in an experimental setup and compared to simulation studies. An overview of the most important results from the end-cap wheels tested up to this point is presented.CERNissä Geneven lähistöllä, Sveitsin ja Ranskan rajalla, tuhannet tutkijat ja insinöörit valmistautuvat kokeisiin, joilla etsitään vastauksia fysiikan peruskysymyksiin: mistä alkeishiukkasten massa tai antimaterian häviäminen aiheutuu. Luonnon ymmärtämisen kannalta keskeisiin kysymyksiin toivotaan vastauksia CERNiin rakennettavan 27 km pitkän LHC-törmäyttimen avulla. LHC-kiihdyttimen kokeissa, satakunta metriä maan alla, lähes valon nopeudella rengasta kiertävät protoniryppäät kohtaavat toisensa ja aiheuttavat perushiukkasten, kvarkkien ja gluonien, välisiä "nokkakolareita", joissa energia tiivistyy äärimmilleen - simuloiden maailmankaikkeuden syntymähetkeä. Hiukkasfysiikan laboratorion mikroskooppiseen mittakaavaan luodut energiatihentymät purkautuvat energiana ja hiukkasryöppyinä törmäyskohtiin rakennettavien koeasemien mitattaviksi. Massiiviset LHC-törmäyttimen koeasemat rekonstruoivat energiapurkauksista syntyneet hiukkasjäljet ja -energiat, identifioivat erityyppisiä törmäystuotteita ja tallentavat ne maailmanlaajuisten tutkijakollaboraatioiden tutkittaviksi. LHC-törmäyttimen viisi koeasemaa koostuvat suuresta määrästä erityyppisiä hiukkasilmaisimia, säteilyantureita, joilla on omat tarkoin määrätyt tehtävänsä laboratoriossa aiheutettujen "alkuräjähdysten" mittaamisessa. Osa tuhansia tonneja painavan koeaseman hiukkasantureista on asemoitava hiuksen paksuutta vastaavalla tarkkuudella ja niiden on toimittava kymmenen vuotta supertörmäysten aiheuttamassa säteily-ympäristössä. Koeasemien sisälle sijoitettuja antureita ei juuri päästä huoltamaan. Koeasemien ilmaisimien tarkkuuden ja luotettavuuden varmistamiseksi anturit valmistetaan ja testataan ensiksi puhdastilassa. Tämän jälkeen anturit läpikäyvät monipuolisen testiohjelman, joka jatkuu asennusvaiheen loppuun saakka. Tuhansien erillisten antureiden kokonaisuus on lopulta osa LHC-törmäyttimen koeasemaa, jonka rakentamiseen osallistuu suuri joukko fysiikan ja tekniikan asiantuntijoita eri maista. Tämän väitöskirjan tekijä on keskeisesti osallistunut LHC-ATLAS -koeaseman ns. transitiosäteilyyn perustuvan ilmaisinjärjestelmän (Transition Radiation Tracker - TRT) toteuttamiseen ja erityisesti sen toimivuuden testaamiseen. CERN tunnetaan internet-teknologian pioneerina, joka loi World Wide Webin. Nyt CERNissä keskitytään www:n seuraavaan vaiheeseen, GRID-hankkeeseen, jolla LHC-törmäyttimen tuottama valtava tietomäärä jaetaan kansainvälisten tutkijakollaboraatioiden analysoitavaksi

Study and optimization of RPCs for high rate applications

Due to the low cost, good time resolution and the properties of RPCs with respect to electronics damage protection, they are chosen for many large experiments. These detectors are reliable and stable in their operation with counting rates up to kHz/cm2. The aim of this work is to understand the fundamental rate limits of RPCs in order to find an efficient way for their optimization and hence, extend their applications. Several types of materials have been used and operational parameters have been optimized in this work comprising simulations and experiment. High efficiency, excellent position resolution, low noise and high rate capability is demonstrated. These type of RPCs open new avenues in several applications, for example in crystallography, biology and medicine

Progress in simulations of micropattern gas avalanche detectors

Helpful for a better understanding of the intrinsic processes in the various gas avalanche detectors are simulations, involving three- dimensional Finite Element Method (FEM) field map computations in order to describe the more and more complex geometries. Drift, multiplication and attachment procedures are simulated using Monte Carlo techniques. Recent results show a remarkable agreement with gain and energy resolution measurements thanks to the refined computations of gas transport properties and improved avalanching models. As examples the influence of wire eccentricity on gas gain and energy resolution in the ATLAS TRT straws is shown as well as performed studies of the Micromegas detector. 8 Refs

A triple GEM detector for LHCb

99-024 The full size (23 cm $\times$ 25 cm) prototype of a triple GEM detector worked well in a hadronic beam. Extreme high gains ($>$ 20'000) with almost no sparking could be reached. But it is possible that a single GEM spark can lead to a short in a GEM foil and destroy the detector. It seems, that a carefull conditioning of the detector during strong irradiation, results in a much more stable operation. Fast signals from Fe$^{55}$ and MIP have been recorded, in the latter the clustering of the primary ionisation charge has been observed. The influence of the shaping time of a simple gaussian filter on the detector signals was studied. If the pulse time is defined by the pulse maximum a longer shaping time improves the time resolution. A longer shaping reduces the observed cluster width as well. The optimal electronic shaping time seems to be of order 15 ns for such a simple filter algorithm

Studies of wire offset effects on gas gain in the ATLAS TRT straw chamber

Results of the measurements and simulation (both Analytical and Monte Carlo) ofthe amplification process in straw tubes in the ATLAS TransitionRadiation Tracker are presented. Main attention was paied to studies of the evolution of the signal amplitude spectrum for point ionisation as a function of the wireoffset in the TRT standard gas mixture. Good argreement of the experimental data and simulation points to a big step in understanding the intrinsic behaviour of the detector

Gas flow uniformity studies for the end-cap ATLAS TRT

This note presents the results of the gas uniformity measurements for the first 4-plane module, complemented by similar studies done on the 8-plane wheel prototype. The gas flow at different straw positions with respect to the inlets was studied using one, two or five inlets to supply active gas at different gas flows. The gas distribution proved to be sufficiently uniform for the 8-plane wheel with the use of two inlets, two outlets and four internally connecting tubes at a gas flow of 27~l/h

Aging studies for the ATLAS Transition Radiation Tracker (TRT)

A summary of the aging and material validation studies carried out for the ATLAS Transition Radiation Tracker (TRT) is presented. Particular emphasis is put on the different phenomena observed in straw tubes operating with the chosen Xe/CF//4/CO//2 mixture. The most serious effects observed are silicon deposition on the anode wire and damage of the anode wire gold plating. Etching phenomena and active radical effects are also discussed. With a careful choice of all materials and components, and with good control of the water contamination in the active gas, the ATLAS TRT will operate reliably for 10 years at the LHC design luminosity. To demonstrate this fully, more work is still needed on the gas system purification elements, in particular to understand their interplay with the active species containing fluorine created in the avalanche process under irradiation