High resolution x-ray and γ-ray imaging using a scintillator-coupled electron-multiplying CCD

Over the last decade the rapid advancements in CCD technology have lead to significant developments in the field of low-light-level, Electron-Multiplying CCDs (EM-CCDs). The addition of a gain register before output allows signal electrons to be multiplied without increasing the external noise. This low effective readout noise, which can be reduced to the sub-electron level, allows very small signal levels to be detected. Caesium iodide is one of the most popular scintillation materials due to its many desirable properties. Approximately 60 photons are produced per keV of incident X-ray or γ-ray with wavelengths peaking at 550 nm (dependent on doping), matching the peak in the quantum efficiency of the back-illuminated CCD97 of over 90%. Using a scintillator coupled to an EMCCD it is possible to resolve individual interactions inside the scintillator. Multiple frames can be taken in quick succession with hundreds of interactions per frame. These interactions can be analysed individually using sub-pixel centroiding and the data compiled to create an image of a much higher resolution than that achieved with a single integrated frame. The interaction mechanism inside the scintillator is discussed with relation to the spatial and spectral resolution of the camera system. Analysis of individual events opens up the possibility of energy discrimination through the profiling of each interaction