Microscale luminescence imaging of defects, inhomogeneities, and secondary phases in halide scintillators

A microscale luminescence system was custom-built to investigate crystallinity, crystal quality, and emission homogeneity in scintillator crystals. This system consists of a fluorescence microscope that has been integrated with a spectrometer and custom-software for both manual and automated collection of two-dimensional reflection and emission images and maps of scintillators under excitation. The system’s capabilities are demonstrated through imaging studies on samples of CsBa2I5 (2%Eu) (CBI), CsI (5%Ba, 3%Eu), and SrI2 (5%Eu). Emission images obtained under 365 nm excitation reveal features that cannot be visualized using light microscopy alone. In the CBI samples, rod-like structures of 100–200 μm in diameter were observed. Using electron probe microanalysis (EPMA), these rods were found to be rich in barium and poor in cesium and europium. In CsI (Ba, Eu), oblong features were observed. Electron probe microanalysis confirmed that these regions varied in composition. Finally, an emission map of a one-inch diameter disk of SrI2 (Eu) indicated a uniform distribution of the dopant. This study demonstrates that the microscale luminescence system is a valuable complement to the current suite of scintillator characterization tools. Its capabilities for evaluating crystal quality and homogeneity will provide useful feedback for crystal growth optimization.