Study of CdTe and CdZnTe detectors for X-ray computed tomography

The application of CdTe and CdZnTe 2D-array detectors for medical X-ray Computed Tomography (XCT) is investigated. Different metallic electrodes have been deposited on High Pressure Bridgman Method CdZnTe and on Traveling Heater Method CdTe : Cl. These detectors are exposed to X-rays in the CT irradiation conditions (Emax=125 keV, Ē=70 keV, ΦX=4×1010 ph s−1 cm−2) and are characterized experimentally in current mode. High dynamic range and fast current decrease after the X-ray beam cut-off are required for the application. Detectors performances such as sensitivity and response speed are then studied. The performances achieved are encouraging: dynamic ranges higher than four decades and current decreases of about three decades in 4 ms after X-ray beam cut-off are obtained. Nevertheless, these detectors are still limited by a memory effect that makes them not yet suitable for XCT. Experimental results are explained by important charge trapping and de-trapping due to the presence of high-level densities. The trapped carrier space charges may also influence the injection from the electrodes. Our conclusions are confirmed by experiments at low temperature and by laser time-of-flight measurements. The information we get on the predominant involved levels are correlated with traditional level studies in order to identify them (energy depths, origins) and to overcome the actual CdTe and CdZnTe detectors limitations for XCT.