Synthesis and Characterization of Novel Li-Containing Garnet Ceramic Scintillators for Nuclear Security Applications

In the field of nuclear security, the ability to detect neutrons is a critical part of the prevention of the smuggling of illicit nuclear materials. The use of dual-mode detector would drastically reduce the number of passive detector systems necessary to meet the detection needs of the nuclear security industry. Li-containing scintillators have been researched for over 70 years; however, relatively few efficient dual-mode detector materials have been discovered. The currently available Li-containing scintillators are relatively low density, very difficult to grow, and highly hygroscopic. These limitations make the wide-spread use of Li-containing scintillators as dual-mode detectors inefficient and expensive. The ideal dual-mode detector material should be a non-hygroscopic optically transparent compound with a high Z-effective, high density and a high Li content that can be produced using a scalable, cost efficient synthesis process. Currently, no such material is available, and, as such, one goal of this dissertation is to provide a foundation for discovering novel potential materials. In this pursuit, the author believes that new and emerging research on Li-containing garnets being done in the field of solid-state Li electrolyte materials could be the genesis of a novel set of dual-mode detector materials. Due to challenges related to the high melting point, and lithium volatility of the proposed materials, traditional single crystal growth methods commonly used for scintillation materials (e.g. Chzocralski and Bridgman growth) are not practical, and so an alternative approach that does not require high temperatures is necessary. The alternative approach chosen in this work is the synthesis of transparent ceramics, as the methods traditionally used in this approach provide a route that does not require high temperatures and can be viiimodified to limit or compensate for complications from the volatility of lithia. This work aims to initiate a critical step toward a practical dual-mode detector material by developing optically transparent, Li-containing ceramic bodies that could potentially be activated with Ce or Pr. In this dissertation, steric entrapment, co-precipitation and sol-gel synthesis methods are investigated for their applicability in the synthesis of Li-containing garnet precursor powders. Powders are then consolidated via uni-axial hot-pressing, and the conditions for this consolidation are studied to develop a procedure for synthesizing transparent Li₅La₃Ta₂O₁₂ ceramics. Ultimately, un-doped, Ce-doped, and Pr-doped Li₅La₃Ta₂O₁₂ transparent ceramics are produced and the first detection of gammas and alphas by a Li-containing transparent ceramic is reported.