Modelling Polarization Effects in CdZnTe Sensor at Low Bias

Semi-insulating CdTe and CdZnTe crystals fabricated into pixelated sensors and integrated into radiation detection modules have demonstrated great ability to operate under the rapidly changing X-ray irradiation environments. Such challenging conditions are required by all photon-counting-based applications including medical CT, airport scanners and non-destructive testing. High-flux sensors typically require operating bias of the order of 1000 V inducing in 2 mm thick detector the electric field 5 kV/cm or locally even higher in polarized detector. This high electric field might cause some long-term reliability concerns so it would be desirable to reduce its value. In addition, the requirement of generating high-voltage causes limitations for portable scanning equipment. It is therefore important to study what minimum electric field is required for proper counting operation.In this presentation, we investigate the possibility to pursue the detector at the high-flux up to 80 Mcps/mm ² X-ray irradiation at a low electric field satisfactory for maintaining good counting operation. We numerically simulate the high-flux-induced polarization in commercial 2 mm thick pixelated CdZnTe detector with 330 μm pixel pitch used in spectral Computed Tomography applications and define defect model that is consistent with the electric field profile visualized by Pockels effect. We find the low bias 300 V as acceptable for utilization the detector for photon counting and spectral sensing applications. We calculate collected charge and construct X-ray spectrum considering the processing of the signal by allied electronics. We identify a strong ballistic deficit arising from the extended charge collection time as the main reason for the spectrum distortion. Finally, we suggest possible optimization of the setup to improve the charge collection efficiency and present optimized spectrum.