Energy and Position Reconstruction in Pixelated CdZnTe Detectors.

Abstract

Pixelated CdZnTe detectors can achieve 3D position reconstruction, which enables 4pi Compton imaging of gamma rays with a single detector and energy resolution better than 1% full width at half maximum (FWHM) at 662 keV. The detector configuration, readout electronics, event reconstruction algorithms, multiple-pixel event performance, depth reconstruction, and complications due to readout electronics are discussed.

In pixelated CdZnTe two dimensions of position sensitivity are determined based on which pixel collects charge. The third dimension comes from the signal ratio of the planar cathode to the pixelated anode, or, from the electron drift time for each pixel that collects charge. This work investigates the depth reconstruction using a collimated fan beam of gamma rays oriented such that counts are restricted to a narrow range of depths in a detector. The depth uncertainty is evaluated by measuring the fluctuation of the reconstructed depths of interaction. The depth accuracy is evaluated by calibrating the beam position and measuring the offset between the true depth of interaction and the reconstructed depth of interaction. Depth uncertainty and accuracy better than 1 mm FWHM is achieved at 662 keV for both single and multiple interaction events in 15 mm thick CdZnTe detectors.

The energy reconstruction of pixelated CdZnTe is challenging for gamma rays that deposit energy on multiple pixels, due to factors such as signal crosstalk and system nonlinearity. This work details the impact of such factors on multiple pixel event energy resolution and, when possible, uses experimental data to put an upper bound on their effect. A correction is developed for the peak-hold circuitry of the readout electronics, improving the performance of events with at least three pixels that collect charge. The linearity of the system was also studied, especially as a function of the number of pixels that collect charge, resulting in energy resolution of 0.36% FWHM at 2614 keV. Finally, a summary of the detector issues that can cause poor event reconstruction is presented, as well as means to overcome these problems.