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Modelling Polarization Effects in a CdZnTe Sensor at Low Bias
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55JYp-l7CyFD9N3MbOvaoc8KA2xjr2h5TT9-nJXoXww
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Semi-insulating CdTe and CdZnTe crystals fabricated into pixelated sensors and integratedinto radiation detection modules have demonstrated a remarkable ability to operate under rapidlychanging X-ray irradiation environments. Such challenging conditions are required by all photoncounting-based applications, including medical computed tomography (CT), airport scanners, andnon-destructive testing (NDT). Although, maximum flux rates and operating conditions differ ineach case. In this paper, we investigated the possibility of using the detector under high-flux Xrayirradiation with a low electric field satisfactory for maintaining good counting operation. Wenumerically simulated electric field profiles visualized via Pockels effect measurement in a detectoraffected by high-flux polarization. Solving coupled drift–diffusion and Poisson’s equations, wedefined the defect model, consistently depicting polarization. Subsequently, we simulated the chargetransport and evaluated the collected charge, including the construction of an X-ray spectrum ona commercial 2-mm-thick pixelated CdZnTe detector with 330 \u0016m pixel pitch used in spectral CTapplications. We analyzed the effect of allied electronics on the quality of the spectrum and suggestedsetup optimization to improve the shape of the spectrum.

Procedure to extract critical bias is shown at the intersection of linear fit for bias < with the horizontal line at maximum CCE.
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Figure 6. Calculated space charge density from the numerical simulation. In the dark regime, there is no space charge in the sample. Subsequent measurements were performed using a standard X-ray tube with a 120 kVp
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The counts vs. bias were measured at a count rate of 20 Mcps/mm2, as shown for one representative pixel in Figure 7. The counts for U < Uc are approximated using a straight line, and the point at which maximum counts were achieved is considered to be Uc 265 V. This procedure was repeated for each pixel, and the distribution of Uc is shown in Figure 8. The decrease in measured counts at low bias is not mainly caused by carrier trapping but by the ballistic decit of low energy X-ray photons, which do not cross the threshold level, frequent pileups, and charge sharing between pixels , which is accentuated at the low electric eld near the pixelated anode. Figure 7. Measured counts in 2-mm-thick CZT sensor under X-ray conditions with a count rate of 20 Mcps/mm2 for a typical pixel. Procedure to extract critical bias Uc is shown at the intersection of linear t for bias U < Uc with the horizontal line at maximum CCE.
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Figure 8. Distribution of Uc values for all pixels. Sensors 2023, 23, x FOR PEER REVIEW 8 of 13
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