Cadmium Zinc Telluride (CZT) semiconductor detectors have recently attracted extensive research interests in the development of novel PET imaging systems. Due to direct charge carrier generation/collection rather than indirect process of photon emission as in scintillator based detectors, it is able to demonstrate significantly improved energy resolution. In addition, by incorporating the depth of interaction (DOI) capability, a CZT detector is able to perform 3D positioning of high spatial resolution [1-3]. With high voltage applied between anode and cathode, electron-hole pairs excited by gamma ray photons would migrate within CZT bulk. By deploying single polarity sensing techniques  and fine electrodes (also known as small pixel effect) , both pixelated and cross-strip detector configurations can be used to improve charge collection and energy resolution. In a CZT detector, the spatial resolution is primarily dependent on the pitch of electrodes. However, as the pitch decreases, a number of challenges arise, such as the large number of readout channels, leakage current, multiple interactions, and charge sharing across adjacent electrodes. The major motivation of our study is to answer the question to what extent one can achieve sub-pitch spatial resolution (i.e., less than the pitch of electrodes) in CZT detectors using the following two techniques: charge sharing effect and transient charge analysis. charge sharing occurs when the interactions locate near the electrode edge or within the gap, and electrons are collected by at least two adjacent electrodes. As a result, it becomes more noticeable as the sizes of electrodes and/or the gap are reduced. If not accounted for properly, it degrades the detector performances with respect to spatial resolution, energy resolution, as well as counting efficiency of 511 keV photons.
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