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Large-volume CdZnTe (CZT) single crystals with electron lifetimes exceeding 10 μs recently became available commercially for making large effective area gamma-ray arrays. However, significant variations were observed in the performance of detectors made from such arrays. We evaluated the spectroscopic performance of such a large-volume, 20 × 20 × 15 mm3, coplanar-grid (CPG) CdZnTe detector, intended for use in a handheld radioisotope-identifier, which unexpectedly showed a poor spectral performance with an energy resolution of 3.5-4% FWHM measured with 662-keV gamma-rays. To understand the factors affecting its performance, we applied several characterization techniques, viz., white X-ray diffraction topography measurements, IR microscopy, and micron-spatial resolution X-ray mapping. They allowed us to identify major crystal defects in the device that limit its energy resolution and detection efficiency.