Identification and Reconstruction of Single-Pixel Incomplete Charge Collection Events

Incomplete charge collection in pixelated CdZnTe detectors with a common anode steering grid has been studied using an advanced digital ASIC readout system, which is capable of sampling and digitizing the preamplifier output signals of an array of 11×11 pixelated anodes as well as the cathode and grid electrodes. Due to insufficient steering, only a fraction of the electron cloud may be collected by anode pixels for some events. These events are called incomplete charge collection events, and they tend to take place on edge pixels. Since the current single-pixel event reconstruction algorithm is based on the anode signal amplitude and the cathode-to-anode signal amplitude ratio (C/A), these events will be reconstructed to the wrong depth and energy bins. An algorithm has been developed to reconstruct these special events based on the system response function. Experiments were performed on a pixelated 20×20×15-mm³ CdZnTe detector with a common grid between pixels manufactured by Redlen Technologies. With this algorithm, both the interaction depth and energy deposition can be reconstructed correctly in the absence of a reliable anode amplitude. This reconstruction algorithm improves the single-pixel photopeak counts by close to 10%. The electron cloud size for 662 keV was estimated with a square-shaped model based on the improvement of photopeak efficiency and showed consistent results with Geant4 simulations.