Nowadays, X-ray imaging methods have found their way into manifold applications in everyday life and scientific fields. Besides classical X-ray detector systems, semiconductor pixel detectors have gained increasing attention during the last years. In so-called hybrid detectors, a pixelated readout electronics, which processes the charge signal created in the sensor and provides analyzable data, is connected to a suitable sensor material via small solder bumps. Thus, besides the ongoing development of the readout electronics itself (e.g. in the framework of the Medipix-collaborations based at CERN), the growth and characterization of suitable semiconductor sensor materials is one of the main research activities in this field. Silicon, despite showing an excellent material homogeneity and being available in large wafers at low cost, cannot be used efficiently for photon energies above 20 keV due to its low absorption coefficient. Hence, especially for medical applications and material testing, which take place at higher X-ray energies, there is a need for alternative sensor materials to ensure high absorption efficiencies. For this purpose, so-called high-Z materials like germanium (Ge), cadmium telluride (CdTe) and gallium arsenide (GaAs) are promising candidates. While Ge needs cooling and CdTe and semi-insulating GaAs have been in the center of attention since many years, there is considerably less work on chromium compensated, high resistivity GaAs, which was introduced by a group at the Tomsk State University already in 2001 and shows very promising material properties.
The aim of this work was thus to provide a detailed characterization of this material as sensor for pixelated X-ray detectors based on photon counting readout electronics of the Medipix chip family.
It could be shown that from the electron charge transport parameters, high resistivity (HR) GaAs is very well suited to be applied in semiconductor pixel detectors. The presence of intrinsic material inhomogeneities mainly caused by dislocations typical of GaAs grown from the melt was revealed by synchrotron white beam topography and was found to result in local X-ray sensitivity variations. However, it could be shown that these structures remain stable in time even under high flux conditions and could be easily corrected for by a simple flatfield correction, leading to a very good image quality with high signal-to-noise ratios. Further, the limiting factor for the detector performance was rather found to be the readout electronics than the sensor material. Count rate variations of the detector assemblies during long term measurements could be attributed to variations in the supply voltage of the readout chips. In addition, measurements performed at the ANKA synchrotron at KIT showed that the high flux behavior of the sensor material was limited by the pulse processing dead time of the pixel electronics, although the performance at high fluxes could be improved by optimized chip settings. By doing so, fluxes > 10^10 (s*mm²)^-1 could be tolerated over a long period, thus enabling these detectors to be used at synchrotron beamlines. The spatial resolution of the detectors, which was determined by MTF measurements, as well as the spectroscopic performance was found to be limited by the small pixel size of the Medipix chips (55×55 µm²) in combination with a sensor thickness of 500 µm, causing them to suffer from charge sharing and from the presence of characteristic X-rays. In this regard it could be shown that the charge summing mode implemented in the most recently developed Medipix chip can overcome many of these limitations and considerably improves the spectroscopic performance as well as the quantum efficiency. The ability to discriminate contrast agents in CT measurements makes these detectors suitable for future applications in imaging methods like spectroscopic X-ray imaging/CT in medical diagnostics and non-destructive material testing.