Introduction to CdTe/CZT Physics and Applications

There have been a large number of technological advances in cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe or CZT) materials recently, and a new generation of advanced applications using these materials are poised to enter the commercial market. The main application area in the field of X-ray detection is provided by direct-conversion-detector-based CdTe and CdZnTe sensors. Direct conversion quantum counting detectors improve performance in several different ways. First, the electronic noise in quantum counting can be lowered because of the thresholding effect. Second, quantum counting can provide better energy weighting. Rather than increasing the weights of higher-energy photons because of the integrating effect, it is possible to increase the weights of lower-energy photons with better contrast. Third, the fill fraction of these detectors can be higher. In some cases, the required X-ray dosage can be decreased by up to 80% compared to energy-integrating systems. Photon-counting-detector-based X-ray computed tomography (CT) has the potential to not only improve current medical images but also enable new clinical applications. High atomic number (high-Z) materials like CdTe and CZT offer the best possibility for the implementation of direct conversion detectors and are the subject of this book. We discuss selection of materials, detector operation physics and technology and readout integrated circuits required to detect signals processes by CdTe and CdZnTe sensors. We contrast these emerging technologies with more established ones based on scintillator materials. CdTe and CdZnTe materials are extensively used in medical imaging, industrial testing, and security applications, with medical imaging being the largest application. X-ray computed tomography (CT) is not the only application of CdTe and CdZnTe technologies as they are also used in other branches of medicine, for example, in nuclear medicine. Nuclear medicine tests are different from most imaging techniques in that the main function of the diagnostic test is to show the physiological function of the system being investigated, unlike traditional anatomical imaging such as magnetic resonance imaging (MRI) or CT. Nuclear medicine imaging studies are typically more organ or tissue specific (e.g., lung scan, heart scan, bone scan, brain scan, etc.) than those on traditional radiology imaging, which instead focus on a specific section of the body (e.g., chest X-ray, abdomen/pelvis CT scan, head CT scan, etc.). CdTe and CdZnTe (CZT) are not universally known compound semiconductor materials used in X-ray and gamma-ray detection applications. The purpose of the chapter is to introduce these interesting materials. We will start this chapter with a review of the basic material properties, followed by a discussion on radiation detection principal operation. The chapter will end with a description of radiation detector readout electronics and application examples.