Timing performance of pixelated CdZnTe detectors

In comparison with most medical imaging techniques, Nuclear Medical imaging has traditionally been plagued by inferior image quality as a result of the limitations imposed by mechanical collimation, which is necessary to form an image using single-photon emitters. To a large extent, this basic limitation may be overcome with the use of positron-emitting radiotracers to detect the two photons that are emitted following positron annihilation. Although CdZnTe semiconductor detectors are renowned for their increased count-rate capabilities and improved energy and spatial resolution, they have some limitations regarding timing and charge collection efficiency. The CdZnTe rise-time has two components: electrons, and hole-induced charge collection. Since the holes move much slower than the electrons, and because there is an equal absorption probability at all depths and energies (in the region of 511 keV), the different slopes between the rise time and the timing signal, vary dramatically. These slopes are mostly influenced by the ‘small pixel effect’. This effect is relative to the amount of charge collected and therefore, a different pulse shape slope will be obtained for different charges collected. The current study directly examines the timing limitations of measuring coincidence events in CdZnTe-pixelated detectors. The results show that it is possible to use these detectors for positron emission applications.