Single Photon Detection with Semiconductor Pixel Arrays for Medical Imaging Applications

This thesis explores the functioning of a single photon counting pixel detector for X-ray imaging. It considers different applications for such a device, but focuses mainly on the field of medical imaging. The new detector comprises a CMOS read-out chip called PCC containing 4096 identical channels each of which counts X-ray hits. The conversion of the X-rays to electric charge takes place in a semiconductor sensor which is segmented into 4096 matching square diodes of side length 170 um, the 'pixels'. The photon counting concept is based on setting a threshold in energy above which a hit is registered. The immediate advantages are the elimination of background and the in principle unlimited dynamic range. Moreover, this approach allows the use of an electronic shutter for arbitrary measurement periods. As the device was intended for operation in the energy range of ~10-70 keV, gallium arsenide was selected as the preferred sensor material. The development of this detector followed on from about 10 years of research aimed at developing hybrid silicon pixel detectors for the future Large Hadron Collider at CERN. A general overview of the requirements for medical imaging is presented. Semiconductor detectors and in particular GaAs detectors are discussed. Charge deposition and charge collection simulations were carried out. Minimum threshold, threshold spread, noise and spatial resolution measurements are presented and finally some images are shown. The combination of the detector simulations and the measurements of the device give important information about the limits of single photon counting devices and indicators for future developments.