X-Ray Detectors

Abstract

The basic principle of operation of an x-ray detector is described through the Shockley-Ramo theorem, and the ionization energy, i.e., electron and hole pair creation energy, is introduced and used in formulating the responsivity of the detector. Typical detector materials and structures are also described. The spectroscopic detector operation is explained and its resolution is discussed. One of the most extensive modern applications of semiconductor detectors is in medical x-ray imaging. Flat panel x-ray imagers (FPXIs) are described in detail due to their extensive use in imaging. In direct conversion (DC) FPXIs, the absorbed x-ray photons are directly converted to charges in the pixel’s photoconductor. The applied field then drifts them to the collecting electrodes of the pixel. These FPXIs use either a thin-film transistor (TFT) active matrix array (AMA) on a glass substrate, based on a-Si:H technology or, in a smaller area, CMOS (complementary metal oxide semiconductor), CCD (charge-coupled device), or ASIC (application-specific integrated circuit) silicon chips as the sensor to read the image. The image is a charge distribution on the pixels of the TFT-AMA/CMOS. The TFT-AMA and CMOS are currently the most popular with larger size detectors based on the TFT-AMA sensor. A semiconductor, such as a-Se, HgI2, and CdZnTe, is used with these readout technologies as the photoconductive medium. The current competition between TFT-AMA and CMOS sensors discussed and the advantages and disadvantages of various semiconductors are highlighted. Present DC FPXI principles are reviewed, important physical phenomena are underlined, and the sensitivity, responsivity, linearity, signal-to-noise ratio, noise sources, and resolution, in terms of modulation transfer function, detective quantum efficiency, and lag and ghosting properties of FPXIs, are examined and explained with examples and extensive references where details can be found.