Performance and limitations of high granularity single photon processing X-ray imaging detectors

Progress in CMOS technology and in fine pitch bump bonding has made possible the development of single photon counting detectors for X-ray imaging with pixel pitches on the order of 50 µm giving a spatial resolution which is comparable to conventional CCD and flat panel detectors. This thesis studies the interaction of X-ray photons in the energy range of 5 keV to 70 keV with various sensor materials as well as the response of the Medipix2 readout system to both monochromatic and wide spectrum X-ray sources. Single photon processing offers the potential for spectroscopic imaging. However, this thesis demonstrates using simulations and measurements that the charge deposition and collection within the semiconductor sensor impose fundamental limits on the achievable energy resolution. In particular the discussion of charge during collection in the sensor and the generation of fluorescence photons in heavier sensor materials contribute to the appearance of a low energy tail on the detected spectrum of an incoming beam. On the other hand, with the application of appropriate flat-field correction maps, the systemem operates to the Poisson limit. As the electronic noise in a pixel channel is around 100e rms and typical energy thresholds are over 1000e excellent quality images of low contrast objects have been produced using both intense and weak X-ray sources. The Modulation Transfer Function has been measured to be 9.1 lp/mm corresponding to the pixel aperture function. The Detective Quantum Efficiency is only limited by the sensor material used and does not change as a function of dose, highlighting the enormous potential for low dose imaging with such a system. The Medipix2 system has an upper threshold in each pixel too which allows an energy window of down to 1.4 keV FWHM to be selected and scanned in the range of 4 keV to 100 keV. First images using the energy threshold window demonstrate the potential of the technique to extract a spectral image when only a broad band source of X-rays is available.