This research aims to fabricate and characterize new inorganic borate phosphors and single crystal scintillators for medical diagnostic, nuclear and high energy physics experiments, and well logging. The rare earth doped phosphors are drawing attention due to their applications in various fields such as X-ray imaging, neutron imaging, light emitting diodes (LED), plasma display panels (PDP), laser and other fields of medical imaging . The presence of Lu contributes high effective atomic number (Zeff = 52) and high molecular weight (M.Wt = 393 g/mol) that make LLBO a potential candidate for X- and γ-ray imaging detector. On the other hand the presence of 6Li (Lithium) and 10B (Boron) could make LLBO as a promising neutron detector. For this work Li6Lu(BO3)3: RE (where RE = Ce3+, Pr3+, Tm3+, Dy3+ and Tb3+)are synthesized by solid state reaction method. The crystallinity and particle morphology of the synthesized phosphors are characterized by XRD (X-ray diffraction) and FE-SEM (Field emission scanning electron microscope). The rare-earth elements are doped and optimized i.e. Ce3+ (3 mol%), Pr3+ (7 mol%), Tm3+ (3 mol%) , Tb3+ (2 mol%) and Dy3+ (5 mol%). Luminescence properties such as X-ray induced luminescence, proton induced luminescence and UV photoluminescence are measured for the synthesized phosphors. The obtained relative light yield for LLBO: Dy3+ is 75% of that of the commercially available (PDP) powder. The decay time measured for it is 1.38 ms. A series of high quality single crystals are grown by Czochralski technique i.e. Li6LuxGd1-x(BO3)3:Ce3+ (LLGBO) (where, x = 0.0, 0.2, 0.5, 0.8, 1.0). The reason behind the development of Ce3+ contained halide scintillators are: the allowed 5d → 4f transition of Ce3+ ions makes response time in the 10 – 100 ns range possible. The light emission is in the near ultraviolet (UV) and visible (VIS) spectral regions, which matched well with the spectral response curve of the modern photo-sensors. Detailed description of the crystal growth procedure and the solutions of the difficulties during the growth process are presented. In general our crystal growth procedure includes: starting material preparation, growth procedures and characterization of the grown crystals.
Structures of all grown crystals are analyzed by powder x-ray diffraction technique (XRD). X-rays and visible laser induced luminescence of the grown scintillators are measured at room temperature and the emission peak obtained is in the range of 350 ~ 500 nm. Measurement of scintillation properties includes; energy resolution, α/γ ratio (from pulse amplitude ratio for 241Am and 137Cs), scintillation decay time and light yield. The energy resolution for LLGBO crystals is in the range of 15% ~ 32% (FWHM) and relative light yield is in the range of 6% ~ 9% of that of LYSO crystals, when excited by 137Cs. The α/γ ratio obtained is 0.25 when excited by 241Am and 137Cs sources. Three decay time components are observed for LLGBO crystals when excited by 662 KeV 137Cs sources. The synthesized LLBO phosphors in this work can be used in X- and γ-ray imaging while the grown LLGBO crystals can be used for neutron imaging and γ-ray detection.
Thesis Advisor: Prof. Hongjoo Kim