CZOCHRALSKI GROWTH AND SCINTILLATION PROPERTIES OF LITHIUM, SODIUM, AND POTASSIUM CODOPED LUTETIUM, YTTRIUM ALUMINUM GARNET SINGLE CRYSTALS FOR APPLICATIONS IN MEDICAL IMAGING

In medical imaging systems, like Positron Emission Tomography (PET), scintillators detect the ionizing radiation emitted from the radioactive tracer within the patient, helping to create the diagnostic image. Promising characteristics like high density, ease of growth and machinability, as well as, bright and fast scintillation have thrusted Ce3+ or Pr3+ activated rare-earth aluminum garnet scintillators like (Lu, Y)3Al5O12 into near operation in the field of medical imaging; however, there is still much room for improvement due to defects inherent in the garnet matrix. One approach to improving the scintillation light yield and decay time, by removing defects, is codoping. Recently the impact of codoping with monovalent ions (i.e. Li+, Na+, K+) has shown potential and requires more investigative efforts into the effects of monovalent ions on garnet scintillator performance. In this dissertation, we explore the strategy of Li+, Na+, and K+ codoping to alter the scintillation properties of praseodymium-doped (Lu0.75Y0.25)3Al5O12 (LuYAG: Pr3+) single crystals. A successful effort was made to optimize the Czochralski single crystal growth process by altering the translation rate, rotation rate, cool down duration, and boule geometry in order to mitigate cracking. After thorough structural and elemental analysis of grown crystals, the scintillation light yield, energy resolution, and scintillation decay time were measured for each codoped crystal and their performance was compared to a non-codoped LuYAG: Pr control. Additionally, we examined the importance of dopant and codopant concentration through analysis of the optical and scintillation properties. A definition of the complex defect structure of monovalent codoped LuYAG: Pr single crystals was presented given the results from thermoluminescence, afterglow, and light sensitivity measurements; before and after annealing treatments. Finally, quantitative analysis of the limiting factors of energy resolution of monovalent codoped LuYAG: Pr was investigated determining the main contribution lies in the high proportional light yield response of LuYAG: Pr to gamma energies and the increased photoelectron production in the PMT.