Introduction X-ray detectors could be divided into direct and indirect types, the latter has cheaper and more stable properties. As the key component of indirect detection, scintillators could convert high-energy photons into low-energy ultraviolet or visible photons. However, the preparation process of traditional scintillation single crystals is relatively complex and costly, and the hygroscopicity of materials also puts strict requirements on their working environment and packaging technology. Moreover, the Tl and Cd are toxic and not environmentally friendly. Therefore, it is very necessary to find new scintillators with simple preparation, low-cost, chemically stable, and environmentally stable properties. The glass-ceramics combines the excellent optical properties of nanocrystals with the high stability and transparency of inorganic glass. High-quality and large-area glass-ceramics could also be easily obtained. Rare earth ions have special energy level structures and luminescence characteristics. Under optical excitation, they can absorb energy and emit light of specific wavelengths, thereby enhancing their scintillation performance and luminescence characteristics, and are resistibility to radiation damage. When the scintillation glass is irradiated by external excitation light sources, the rare earth ions absorb the energy and transfer to high energy level state, and then release energy through radiation transitions. In this paper, rare earth ions are doped into glass-ceramics to introduce efficient luminescent centers. The luminescence performance of ZnGa2O4:Tb3+ glass-ceramics under X-ray irradiation and its indirect imaging ability are studied.
Methods To prepare ZnGa2O4 glass-ceramics composing of 52SiO2-10Na2CO3-22ZnO-16Ga2O3-x%Tb4O7 (mole fraction), the raw materials were weighed and ground in an agate mortar for 15 min. After uniformly mixing, the powder was transferred to a 30 ml alumina crucible and melted at 1 600 ℃ for 60 min. The sample was poured onto a preheated copper plate at 350 ℃, pressed into shape, and cooling naturally to form the precursor glass. The precursor glass was treated by heating at 600 ℃ for 4 h to release internal stresses. Subsequently, the samples were heat treated at 700, 750, 800, 850, 900 ℃ for 6 h to form glass-ceramics (GC) with different degrees of crystallinity. Finally, these GC samples were polished or ground into powder for further characterization.
The phase of the samples was determined by X-ray diffraction analysis, transmission electron microscopy, and scanning electron microscopy. The photoluminescence properties were analyzed by using the excitation spectrum and photoluminescence spectrum of the Hitachi F-7000 fluorescence spectrophotometer with a 150 W Xe lamp as the excitation source. The transmission spectra were determined by using a TU1810 UV/visible spectrophotometer. A miniaturized X-ray tube with Cu-Kα (λ=0.154 05 nm) radiation source was used for X-ray imaging, maintaining a tube voltage and current of 40 kV and 30 mA, respectively, with a Nikon D500 camera for imaging capture. The radiation luminescence spectra of the samples under X-ray irradiation were measured using a fluorescence spectrophotometer (Ocean Optics, QE Pro).
Results and discussion For the precursor glass treated at the temperature ranging from 700 ℃ to 800 ℃, the samples exhibit a transmittance of over 80% in the wavelength range of 400 nm to 800 nm. However, when the heating temperature exceeds 850 ℃, the transmittance of the samples gradually decreases attributing to the increase of the size of ZnGa2O4 nanocrystals precipitated within the glass matrix, resulting in an increase in Rayleigh scattering and the decrease in transparency. Under different doping concentrations of rare earth ion, there is no impurity observed in the XRD diffraction patterns. Additionally, the samples maintain a transmittance of over 80% under different concentrations of Tb3+ doping. Based on the intensity of photoluminescence spectra, the optimal Tb3+ doping concentration is determined to be 1.5%.
The ZnGa2O4:1.5%Tb3+ glass-ceramics exhibits a high absorption coefficient in the range of 0 to 50 keV, with a radiation luminescence yield of 13 000 photons/MeV, which is superior to that of commercial BGO in both absorption coefficient and radiation luminescence yield. Additionally, it exhibits a linear response relationship with X-ray dose rates and demonstrates long-term stability under X-ray irradiation. Indirect X-ray imaging by using this material could clearly visualize the internal structure of printed circuit boards with an imaging resolution of 16 lp/mm. Furthermore, the ZnGa2O4:1.5%Tb3+ glass-ceramics could restore its radiation luminescence performance to its initial state via thermal treatment.
Conclusions ZnGa2O4:1.5%Tb3+ glass-ceramics exhibits characteristic emissions of Tb3+ under both UV and X-ray irradiation, with a high transmittance of over 80%, which is beneficial for achieving high-resolution X-ray imaging (16 lp/mm). Additionally, under high-energy radiation exposure for a long duration (1 000 h), the RL intensity of ZnGa2O4:1.5%Tb3+ glass-ceramics only decreases by 20%, and the radiation damage caused by high-dose X-ray can be restored through simple low-temperature thermal treatment, which reduce the cost of scintillators and meet the long-term lifetime. This study demonstrates that ZnGa2O4:1.5%Tb3+ could provide high-quality, efficient, and precise imaging results, and making it promising for applications in medical imaging and non-destructive testing.