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Photon Counting X-Ray Detector Systems Thesis for the degree of Licentiate Sundsvall 2005 Photon Counting X-ray Detector Systems Börje Norlin Supervisors: Docent Christer Fröjdh Professor Hans-Erik Nilsson Electronics Design Division, in the Department of Information Technology and Media Mid Sweden University, SE-851 70 Sundsvall, Sweden ISSN 1652-8948 Mid Sweden University Licentiate Thesis 2 ISBN 91-85317-01-2 Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall framläggs till offentlig granskning för avläggande av licentiatexamen i elektronik fredagen den 11 februari 2005, klockan 10.15 i sal M102, Mittuniversitetet Sundsvall. Seminariet kommer att hållas på engelska. Photon Counting X-ray Detector Systems Börje Norlin © Börje Norlin, 2005 Electronics Design Division, in the Department of Information Technology and Media Mid Sweden University, SE-851 70 Sundsvall Sweden Telephone: +46 (0)60 148594 Printed by Kopieringen Mittuniversitetet, Sundsvall, Sweden, 2005 To my wife Monica! …my rose from Mid Sweden. ABSTRACT This licentiate thesis concerns the development and characterisation of X-ray imaging detector systems. “Colour” X-ray imaging opens up new perspectives within the fields of medical X-ray diagnosis and also in industrial X-ray quality control. The difference in absorption for different “colours” can be used to discern materials in the object. For instance, this information might be used to identify diseases such as brittle-bone disease. The “colour” of the X-rays can be identified if the detector system can process each X-ray photon individually. Such a detector system is called a “single photon processing” system or, less precise, a “photon counting system”. With modern technology it is possible to construct photon counting detector systems that can resolve details to a level of approximately 50 µm. However with such small pixels a problem will occur. In a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the picture achieved. For high photon energies the size of the charge cloud is comparable to 50 µm and might be distributed between several pixels in the picture. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “colour” information in the picture. The problem involving charge sharing which limits “colour” X-ray imaging is discussed in this thesis. Image quality, detector effectiveness and “colour correctness” are studied on pixellated detectors from the MEDIPIX collaboration. Characterisation measurements and simulations are compared to be able to understand the physical processes that take place in the detector. Simulations can show pointers for the future development of photon counting X-ray systems. Charge sharing can be suppressed by introducing 3D-detector structures or by developing readout systems which can correct the crosstalk between pixels. i SAMMANDRAG Denna licentiatavhandling berör utveckling och karaktärisering av röntgen- detektorer. ”Färgröntgen” öppnar nya perspektiv för medicinsk röntgendiagnostik och även för materialröntgen inom industrin. Skillnaden i absorption av olika ”färger” kan användas för att särskilja olika material i ett objekt. Färginformationen kan till exempel användas i sjukvården för att identifiera benskörhet. Färgen på röntgenfotonen kan identifieras om detektorsystemet kan detektera varje foton individuellt. Sådana detektorsystem kallas ”fotonräknande” system. Med modern teknik är det möjligt att konstruera fotonräknande detektorsystem som kan urskilja detaljer ner till en upplösning på circa 50 µm. Med så små pixlar kommer ett problem att uppstå. I en halvledardetektor ger varje absorberad foton upphov till ett laddningsmoln som bidrar till den erhållna bilden. För höga fotonenergier är storleken på laddningsmolnet jämförbar med 50 µm och molnet kan därför fördelas över flera pixlar i bilden. Laddningsdelning är ett centralt problem delvis på grund av att bildens upplösning försämras, men framför allt för att färginformationen i bilden förstörs. Problemet att laddningsdelning begränsar ”färgröntgen” diskuteras i denna avhandling. Bildkvalite, detektoreffektivitet och färgriktighet studeras för pixellerade detektorer från MEDIPIX-projekter. Karaktäriseringsmätningar och simuleringar jämförs för att ge förståelse för de fysikaliska processer som sker i detektorn. Simuleringar kan visa vägen för framtida utveckling av fotonräknande röntgensystem. Effekten av laddningsdelning kan begränsas genom att introducera detektorkonstruktioner i 3D-struktur eller genom att utveckla utläsningssystem som kan korrigera läckaget av signal mellan pixlarna. iii ACKNOWLEDGEMENTS As a child, my parents read physics books to me, and I looked at the fascinating pictures of the Bohr model and of the solar system. At the age of 11, I became active against nuclear power and decided to work in the field of radiation in the future. After six years of accident simulations for the nuclear industry I was more comfortable with nuclear power, since I found no design error of significance in the Swedish nuclear power plants. My friend Dr. Jan-Olov Andersson persuaded me to move north to Sundsvall and go back to the academic world, which has made it possible for me to explore other areas of radiation science. I want to express my gratitude to my supervisors for believing in my ability to accomplish scientific work. Christer Fröjdh has always showed a positive and helpful attitude, no matter how doubtful I have been towards my results. Hans-Erik Nilsson has always given rapid responses to my questions, often with advanced simulations. Thanks also to Lennart Bergström who employed me in the first case. I want to thank the members of the Medipix collaboration for allowing me to share their knowledge and equipment. I recall help and information from Xavier Llopart, Marino Maiorino, Giovanni Mettivier, Bettina Mikulec, Daniel Niederlöhner, Stanislav Pospíšil, Milan Siňor, Josef Uher and Daniel Vavřík. Special thanks to Heinz Graafsma, Cyril Ponchut, Guillaure Potdevin and Vedran Vonk at ESRF for measurement assistance. I want to thank all my colleagues and former colleagues at the Electronics Design Division at Mid Sweden University for making our division an inspiring and friendly environment. Ervin Dubarić and Kent Bertilsson introduced me to MCNP and Medici. The collaboration with Anatoliy Manuilskiy was extremely positve. Göran Thungström and Mattias O’Nils can always be trusted. Munir Abdalla, Suliman Abdalla, Krister Aldén, Henrik Andersson, Michael Andersson, Xavier Badel (KTH), Fanny Burman, Mats Hjelm, Jerzy Kirrander, Jan Lundgren, Claes Mattsson, Håkan Norell, Bengt Oelmann and Fiona Wait have helped me with this work in different ways. Special thanks to Benny Thörnberg who spent several hours helping me to repair the Sense-A-Ray CCD system. (Oh no, it is time again; it broke during the student project on scintillators.) I thank Ulrika Lindgren at the Department of Hospital Physics in Sundsvall Hospital and Lars Herrnsdorf at RTI Electronics AB for giving me some insight in X-ray imaging in reality. Mid-Sweden University, the KK-foundation and the European Commission are greatly acknowledged for their financial support. Finally I thank my family Monica, Johan and William for making my life complete. William is not able to talk yet, but Johan, at three years old, commented to me about my research with the words “Pappa, du förstår ingenting!” (Daddy, you don’t understand anything!) v TABLE OF CONTENTS ABSTRACT............................................................................................................. I SAMMANDRAG ................................................................................................. III ACKNOWLEDGEMENTS .................................................................................. V TABLE OF CONTENTS ...................................................................................VII LIST OF FIGURES ............................................................................................. IX LIST OF PAPERS ..................................................................................................1 1 INTRODUCTION...........................................................................................3 1.1 THESIS OUTLINE ........................................................................................3 1.2 THE DISCOVERY OF X-RAYS......................................................................3 1.3 APPLICATIONS FOR X-RAYS......................................................................4 1.4 NEGATIVE EFFECTS OF X-RAYS ................................................................4 1.4.1 X-ray dose and mammography .........................................................5 1.4.2 Increasing doses................................................................................6 1.5 SOME BASICS ON SEMICONDUCTOR X-RAY DETECTORS...........................6 1.5.1 Charge coupled devices ....................................................................7 1.5.2 Scintillator grids ...............................................................................7 1.5.3 Strip detectors ...................................................................................8 1.5.4 Pixel detectors...................................................................................8 2 THE MEDIPIX PROJECT..........................................................................11 2.1 SHORT DESCRIPTION OF MEDIPIX1.........................................................11
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