Timing performance of pixellated CdZnTe detectors

Recently introduced nuclear medicine cameras in which Positron Emission Tomography (PET) and the traditional Single Photon Emission Computerized Tomography (SPECT), are combined opened new horizon for the nuclear medicine field These systems applying NaI(Tl) scintillation detectors we very well tested and mailable for some time in the medical imaging field However the traditional NaI(Tl) cameras, optimized for low energy radiation imaging, suffer some severe limitations. The relatively low density (3.67 g/cm2) of NaI(Tl) limits the sensitivity. By incarcerating the NaI(Tl) thickness, the spatial resolution decreases. The long decay time (230 nsec) of the light emitted in NaI(Tl) restricts the the use of coincidence technique , as well as the count rate. In recent years CdZn Te (CZT) detectors are studied for the purpose of SPECT nuclear medical radiation imaging in the form of pixellated and microstrip detectors. CZT detector can served as a good candidate for replacing NaI(Tl) for PET and SPECT imaging due to their relatively high stopping power (density = (6.0 g/im3, high Z(48, 30, 52)) and their high count rate capability. Unfortunately there are several difficulties in PET application due to the difficulty in manufacturing thick crystals, registration of the full energy deposited in several pixels and their timing capabilities. The latter is due to large ballistic signal variation induced. This variations caused by the pulse shape, which is composed of two main components, the electron and the hole. The electrons travel about ten times faster than the holes. A photon absorbed clear the cathode plane will cause a large and fast signal induced by the electron and a small and slow signal induced due to the holes. Photons absorbed near the anode plane will induce the opposite signals. The distribution of photon absorption depth in the crystal causes signal splits over a number of pixels, due to the well-known 'small pixel effect'. The different pulses slope, together with the electronic noise, cause serious difficulties in implementing a coincidence circuit. The present work examines in a direct way, the tinting limitations, in measuring coincidence events in CZT pixellated detectors