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Co-doping with divalent elements is known to improve the light yield and decay times of some cerium-activated scintillators, despite the stabilization of tetravalent Ce4+, previously believed to be non-luminescent, from the Ce3+ state. Ce4+ stabilization is a charge compensation mechanism which results from divalent ion substitution of a 3 + site. To elucidate the underlying mechanisms, which remain poorly understood, we have grown three Ce,Ca:YAG crystals with different amounts of calcium co-dopants by the Czochralski method and characterized their scintillation and defect properties. Calcium co-doping reduces the decay times and stabilizes the formation of Ce4+ as expected. Interestingly, X-ray fluorescence analysis reveals a decreased concentration of cerium within the YAG crystal for the sample doped with high levels of Ca, which could contribute to the observation that scintillator properties are improved only for low levels of Ca co-doping. Additionally, positron annihilation spectroscopy reveals an increase in the concentration of vacancies with increasing Ca concentration, while thermoluminescence is observed to show no detectable signal. Furthermore, room temperature photoluminescence of the Ce 4f to 5d1 transition demonstrates decreased emission with increasing Ca co-doping. These data suggest that Ca co-doping decreases the decay time by creating defects with non-radiative decay pathways.

In this study, four yttrium aluminum garnet single crystals co-doped with cerium and lithium were produced by the Czochralski method and the scintillation and defect properties were investigated. Our results demonstrated an increase in luminescence with Li co-doping as well as elimination of longer decay times. Surprisingly, although Li is monovalent, no oxidation of cerium from Ce3+ to Ce4+ was found as would be expected to maintain charge neutrality. Additionally, thermoluminescence results indicated a reduction in the trapping of charge carriers by shallow and deep traps, and room temperature photoluminescence measurements showed an improvement in the Ce3+ 5d to 4f transition efficiency.