Direct conversion semiconductor detectors for radiation imaging

The detection and conversion of high-energy photons (such as x-ray, gamma-ray, and 511 keV annihilation photons) to electric charges can be divided into two classes: direct conversion and indirect conversion (Figure 1.1). Direct conversion detectors contain a photoconductive material, which converts high-energy photons directly into electrical charges. In contrast, indirect conversion detectors have a scintillation layer that converts the high-energy photons into optical photons. These optical photons are then converted into electrical charges using photodetectors, such as silicon photomultipliers (SiPMs). The indirect conversion detection is the most dominant technology for high-energy photon imaging due to the maturity of scintillation material and optical detection technology. Direct conversion semiconductors, especially compound semiconductors, have experienced slower development due to challenges in growth of high-purity, stable, and uniform crystals. However, there has been an ongoing interest in direct conversion for higher spatial resolution applications [1,2]. In this chapter, we discuss the key properties of photoconductive materials for high-performance direct conversion detectors for medical imaging applications. Two commercially available direct conversion photoconductors, amorphous selenium (a-Se) for large area x-ray imaging and cadmium zinc telluride (CZT) for high-resolution positron emission tomography (PET), are presented.