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Abstract: Physical structure of pixelated detectors provides a unique tool to evaluate the effects of different types of defects in the semiconductor material that is used to fabricate the detectors. The spectroscopic performance measured for individual pixels or groups of pixels can be used to correlate point defects or fields of inhomogeneities within the material with the charge collected from photoelectric events. A block of single crystal mercuric iodide of approximately 18×18mm2 area and between 6 and 10mm thick is prepared. The homogeneity of this material is then investigated with light in the transparent region for HgI2 using an optical microscope. Several types of defects can be identified in this way by the scattering of light, for example, single large inclusions or voids and areas of haziness consisting of fields of small inclusions. Standard procedures are used to fabricate from this block a pixelated detector with a 121-pixel anode structure. The performance of each pixel is measured, and differences in charge collection are correlated with the optical data. Measurement data are presented, and possible mechanisms of the interactions between the defects and the charge carriers are discussed. [Copyright &y& Elsevier]