## Charge Sharing Across Adjacent Units
Sub-pixel position sensitivity in an array of CZT hemispherical detectors can be achieved by exploiting charge sharing between neighboring detector units. When a gamma photon interacts near the boundary of two adjacent detectors, the generated electron cloud can be partially collected by each anode. By accurately measuring the relative pulse heights or integrated charge from each unit, the interaction position can be interpolated with spatial resolution finer than the physical unit size. This method relies on precise calibration of gain and linearity for each detector and careful characterization of inter-unit charge diffusion.
## Center-of-Gravity (CoG) Algorithms
The Center-of-Gravity method is commonly applied in arrays to enhance sub-pixel localization. For each detected event, the relative signal amplitudes across multiple neighboring hemispherical units are used to compute a weighted average position. The hemispherical geometry aids this process by producing localized weighting potentials near each anode, which minimizes contributions from distant events and enhances position fidelity. CoG algorithms require high signal-to-noise ratios and correction for variations in charge collection efficiency across the array.
## Pulse Shape Analysis (PSA)
Pulse shape analysis can provide additional sub-pixel sensitivity by leveraging differences in electron drift times and pulse rise characteristics. In hemispherical detectors, the radial electric field produces depth-dependent electron drift velocities. By analyzing the temporal profile of the detected pulses across multiple units, the lateral position of the interaction can be inferred with sub-unit precision. PSA is particularly effective when combined with charge-sharing information, allowing the extraction of three-dimensional interaction coordinates within the array.
## Cross-Talk Calibration and Correction
Achieving accurate sub-pixel resolution requires careful calibration of electrical cross-talk between adjacent units. Signals induced in neighboring anodes by capacitive coupling or long-range weighting potential effects can bias position estimation. Calibration procedures map the cross-talk contributions and correct measured signals to isolate true charge sharing. In arrays of hemispherical CZT detectors, precise modeling of the weighting potential distribution and inter-unit coupling is critical to maintaining reliable sub-pixel localization.
## Multi-Unit Event Reconstruction
In high-flux scenarios, multiple simultaneous interactions may occur across different hemispherical units. Advanced reconstruction algorithms can separate overlapping events based on their timing, pulse amplitude, and spatial distribution. By combining information from multiple units and employing statistical or machine learning-based interpolation techniques, sub-pixel interaction positions can be resolved even in densely packed arrays. This approach leverages the uniform radial electric fields of each unit to maintain consistent charge collection behavior across the array.
## Integration with Detector Electronics
Sub-pixel sensitivity relies on high-performance readout electronics capable of capturing fine amplitude and timing information from multiple units simultaneously. Low-noise preamplifiers, high-resolution ADCs, and fast timing circuits enable precise measurement of charge sharing and pulse shapes. Additionally, on-board digital signal processing can implement real-time CoG and PSA calculations, allowing sub-pixel positions to be reported directly for each detected gamma event.
## Impact on Imaging and Spectroscopy
By achieving sub-pixel position sensitivity, arrays of hemispherical CZT detectors can improve spatial resolution in gamma imaging applications without increasing the physical detector density. This enables finer localization of radioactive sources, more accurate event mapping, and reduced partial volume effects. In spectroscopy, sub-pixel sensitivity ensures that events near unit boundaries are correctly attributed, minimizing spectral distortions caused by charge sharing and enhancing both energy and position resolution.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-can-sub-pixel-position-sensitivity-be-achieved-in-an-array-configuration-of-multiple-czt-hemispherical-detector-units.html
