Improving performance of a CdZnTe imaging array by mapping the detector with gamma rays

We can greatly reduce image artifacts in our pixellated CdZnTe arrays by mapping imperfect regions with a narrow collimated beam of gamma rays. Portions of our detectors produce signals that agree well with simulations of gamma-ray interactions, but there are many examples of structures in the material that respond unpredictably to gamma rays. We mapped some of these imperfect regions using 60 and 140 keV gamma-ray beams, recording a 7×7 set of pixel signals for each interaction. The pixel pitch was 380 μm. We used the mapped data to estimate the probability density function (PDF) of the pixel signals for each interaction position. Images were taken on the mapped sections, storing each gamma ray as a list of pixel signals. Images could be formed by either estimating each gamma-ray interaction position individually or using the entire set of image data in a single iterative computation using the expectation-maximization (EM) algorithm. At 60 keV individual interaction positions were estimated by fitting the data to a Gaussian PDF, correcting the artifacts and giving sub-pixel resolution of less than 150 μm in some regions. At 140 keV applying the EM algorithm was necessary for improving the images.