## Definition of Polarization in CZT Detectors
Polarization in CZT planar detectors refers to the time-dependent change in detector response when a bias voltage is continuously applied. It arises from the accumulation of trapped charges, particularly holes, at defect sites or near the electrodes. This trapped charge modifies the internal electric field distribution, altering charge collection efficiency, signal amplitude, and spectral characteristics over time. Polarization is especially significant in thick or high-flux detectors and can limit long-term operational stability.
## Observation of Spectroscopic Shifts
During long-term biased operation, polarization manifests as gradual shifts in the measured peak positions of gamma-ray spectra. Photon interactions occurring at the same energy may produce lower amplitude signals as the internal electric field is distorted by trapped charges. This leads to peak broadening, low-energy tailing, and overall degradation of energy resolution. Continuous monitoring of standard spectral lines is used to quantify the rate and extent of polarization.
## Current Drift and Leakage Variations
Polarization can also be characterized by changes in leakage current over time. As trapped charges accumulate, the effective electric field near the electrodes may increase or decrease local leakage paths, causing observable drift in dark current. Monitoring leakage current provides an indirect measure of polarization, correlating with spectral shifts and identifying the onset of performance degradation.
## Time-Dependent Charge Collection Efficiency
The gradual reduction in charge collection efficiency is a primary indicator of polarization. By analyzing the amplitude of the anode signal over time for monoenergetic photon sources, one can quantify how trapping of carriers, particularly holes, reduces the fraction of charge reaching the collecting electrode. Depth-dependent measurements using cathode-to-anode ratio analysis can further localize the regions within the detector most affected by polarization.
## Temperature and Flux Dependence
Polarization is strongly dependent on operating temperature and incident photon flux. Higher temperatures increase carrier mobility, which can partially mitigate local charge accumulation, while higher flux accelerates trapping due to increased carrier generation. Characterizing polarization requires controlling or accounting for these variables to separate intrinsic material effects from external operational conditions.
## Mitigation Assessment through Bias Cycling
Periodic bias voltage reduction or reversal is used to study recovery from polarization. By comparing spectral responses before and after bias cycling, the degree of polarization and the timescale for trapped charge release can be quantified. This approach allows estimation of the detector’s operational limits and informs design of voltage protocols to maintain long-term stability.
## Summary Metrics for Polarization Characterization
Key parameters used to characterize polarization include:
* Peak position drift (measured in keV or percent shift over time)
* FWHM broadening and low-energy tailing
* Leakage current variation under constant bias
* Depth-dependent charge collection efficiency degradation
* Time constant for recovery after bias removal or reversal
These metrics collectively quantify the impact of polarization on long-term detector performance, providing guidance for operating conditions, detector material selection, and correction strategies to preserve spectroscopic fidelity.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-is-polarization-characterized-during-long-term-operation-of-a-biased-czt-planar-detector.html
