Bulk uniformity of Cadmium Zinc Telluride (CZT) crystals for large volume coplanar gamma spectrometers

Gamma ray spectroscopy for remote sensing applications requires a high sensitivity that can be achieved by a combination of high efficiency and high energy resolution for high energy gamma rays (up to 1 MeV) . While cryogenic germanium detectors or LaBr₃:Ce³⁺ scintillators could in principle satisfy the requirements of many such applications, the search for compact, portable systems that do not require cooling or the use of photomultiplier tubes are still underway. State-of-the-art CdZnTe spectrometers having an energy resolution exceeding 1% have been recently demonstrated, however they have a typical volume that is less than 1 cm³. Large volume (greater than 3 cm³) CdZnTe spectrometers with an energy resolution that is better than 3% are not yet commercially available, although the size of large single crystals that would satisfy the volume requirement can now be grown by various companies with reasonable yield. In this study we have investigated the feasibility of using commercially available large (1-6 cm³) detector grade CdZnTe single crystals to fabricate co-planar gamma spectrometers with a targeted performance that exceeds 3% energy resolution at 662 keV. The fabrication of optimal performance coplanar detectors requires a custom design that might vary for each crystal growth method. Besides the knowledge of the average values of charge carrier concentrations, mobilities and trapping times the physics-based model requires the knowledge of bulk nonuniformities and their effects on local variations in the electric field and electron mobility-trapping-time product. All crystals investigated had tellurium, as second phase. A new type of material inhomogeneity, and its influence on detector´s performance, became apparent only in large volume crystals and will be reported.