## Principle of Depth-of-Interaction (DOI) in Hemispherical CZT Detectors
Depth-of-interaction (DOI) refers to the position along the radial axis of a hemispherical CZT detector where a photon interaction occurs. Because the detector employs a small anode and a large hemispherical cathode, the radial electric field varies across the detector volume. The drift time and signal formation characteristics of electrons depend on the distance from the anode, allowing DOI to be inferred from the pulse shape.
## Pulse Rise Time Analysis
The rise time of the preamplifier output pulse is proportional to the drift time of electrons from the interaction site to the small anode. Interactions near the cathode generate electrons that traverse the full radial distance, resulting in longer rise times, whereas interactions near the anode produce shorter rise times. By measuring the pulse rise time for each event, the relative radial position of the interaction can be estimated.
## Charge Collection and Pulse Amplitude Correlation
In addition to rise time, the pulse amplitude can provide DOI information, especially when electron trapping is non-negligible. Events occurring farther from the anode may experience more trapping, slightly reducing the total collected charge. By correlating pulse amplitude with rise time, more accurate depth estimates can be achieved, compensating for variations in charge collection efficiency.
## Digital Signal Processing Techniques
Modern systems employ high-speed digitization of the preamplifier pulse to capture the full waveform. Digital filters, derivative analysis, and deconvolution techniques are applied to extract precise timing characteristics, including rise time, pulse slope, and asymmetry. These parameters are then mapped to interaction depth using calibration curves derived from known collimated sources or simulated electric field models.
## Calibration and Field Mapping
To accurately relate pulse shape to DOI, the detector is calibrated using interactions at known depths, often using narrow collimated beams or layered absorbers. The radial electric field in the hemispherical geometry is modeled numerically, allowing conversion of measured pulse rise times and shapes into absolute radial positions. This calibration accounts for field non-uniformities and the small-pixel effect that dominate charge induction in hemispherical detectors.
## Applications of DOI Information
DOI extraction improves energy resolution by correcting for depth-dependent trapping and tailing effects. It also enables three-dimensional imaging, gamma-ray event reconstruction, and Compton scattering analysis, enhancing performance in spectroscopy and portable imaging systems.
## Summary
Depth-of-interaction in a CZT hemispherical detector is inferred from pulse shape characteristics, primarily electron drift-induced rise time, combined with amplitude information. High-speed digitization, digital signal processing, and careful calibration allow mapping of pulse features to interaction depth. This capability improves energy resolution, enables depth correction for charge trapping, and supports advanced imaging and spectroscopic applications in electron-dominated hemispherical detector designs.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-can-depth-of-interaction-information-be-extracted-from-pulse-shape-analysis-in-a-czt-hemispherical-detector.html
