A photon counting pixel detector for X-ray imaging
Hybrid semiconductor pixel detector technology is presented in this thesis as an alternative to current imaging systems in medical imaging and synchrotron radiation applications. The technology has been developed from research performed in High Energy Physics, in particular, for the ATLAS experiment at the LHC, planned for 2005. This thesis describes work done by the author on behalf of the MEDIPIX project, a collaboration between 13 international institutions for the development of hybrid pixel detectors for non-HEP applications. Chapter 1 describes the motivation for these detectors, the origin of the technology, and the current state of the art in imaging devices. A description of the requirements of medical imaging on X-ray sensors is described, and the properties of film and CCDs are discussed. The work of the RD19 collaboration is introduced to show the evolution of these devices. Chapter 2 presents the basic semiconductor theory required to understand the operation of these detectors, and a section on image theory introduces the fundamental parameters which are necessary to define the quality of an imaging device. Chapter 3 presents measurements made by the author on a photon counting detector (PCD1) comprising a PCC1 (MEDIPIX1) readout chip bumpbonded to silicon and gallium arsenide pixel detectors. Tests on the seperate readout chip and the bump-bonded assembly are shown with comparisons between the performance of the two materials. Measurements of signal-tonoise ratio, detection efficiency and noise performance are presented, along with an MTF measurement made by the Freiburg group. The X-ray tube energy spectrum was calibrated by REGAM. The performance of the PCD in a powder diffraction experiment using a synchrotron radiation source is described in chapter 4. This chapter reports the first use of a true 2-D hybrid pixel detector in a synchrotron application, and a comparison with the existing scintillator based technology is made. The measurements made by the author have been presented at the 1st International Workshop on Radiation Imaging Detectors at Sundsvall, Sweden, June 1999. The PCD1 operates in single photon counting mode, which attempts to overcome the limitations of charge integrating devices such as CCDs. The pros and cons of the two detection methods are discussed in chapter 5, and a comparison was made of the PCD1 performance with the performance of a commercial dental X-ray sensor. The two detectors are compared in terms of contrast and signal-to-noise ratio for identical X-ray fluences. The results were presented at the 2nd International Workshop on Radiation Imaging Detectors, Freiburg, Germany, 2nd-6th July 2000. The author was involved in the conversion of the LabWindows MRS software to a LabView platform, which was presented in an MSc- thesis in the University of Glasgow by F. Doherty. All image processing, data manipulation and analysis code was written by the author.