Luminescence Properties of Bi³⁺-Doped Gd₂O₃ Phosphor under UV and X-ray Excitation

X-ray excited luminescence in scintillators plays a crucial role in applications ranging from medical diagnostics to industrial quality inspection and scientific research. This study explores the potential of Bi³⁺-doped Gd₂O₃ phosphors as next-generation scintillators, leveraging their broad-spectrum emission characteristics to achieve higher light yields. We synthesized Gd₂O₃:Bi³⁺ phosphors in two crystalline phases─cubic and monoclinic─using a high-temperature solid-phase method. These phosphors demonstrate varied applications, from backlighting displays to X-ray imaging of opaque and biological samples. Under UV light excitation, each phase exhibits distinct luminescence properties due to different Bi³⁺ occupancy environments. The monoclinic phase, excited at λ_ex = 370 nm, showed stronger luminescence and a broader emission peak (fwhm = 70 nm) with a higher photoluminescence quantum yield (67.94%), compared to the cubic phase, which under optimal conditions (λ_ex = 377 nm) exhibited narrower peaks (fwhm = 40 nm) and a lower yield (44.41%). Notably, under X-ray excitation, the cubic phase outperformed with a higher light yield (48,900 photons/MeV) and lower detection limits (380 nGy·s^-1 at λ_em = 430 nm and 285 nGy·s^-1 at λ_em = 520 nm). Both phosphors were also embedded in PMMA to create thin films for X-ray imaging, achieving high-resolution outputs (14 lp·mm^-1).