The Study On Preparation And Spectral Modulation Of High-Performance Lutetium Oxide Based Nanoscintillators And Their Applications

In recent years,inorganic scintillator materials have been widely used in non-destructive testing,safety inspections and medical diagnostic imaging due to their ability to convert absorbed high-energy particles or rays into ultraviolet or visible light,and combined with the back-end photoelectric converter to generate visualized images and detection data.Currently,the most commonly used inorganic scintillators are mainly oxygen compounds and halide materials,which have good energy resolution and high light yield,but have disadvantages such as being easily hygroscopic,toxic and poor stability.Furthermore,they are mostly in the form of single crystals,transparent ceramics and glass,which are difficult to meet the requirements for manufacturing non-flat panel detectors.Rare earth doped fluoride and organic materials can satisfy the above requirements,but the fluoride materials tend to exhibit strong afterglow luminescence under excitation,which have negative affects on image quality and signal-to-noise ratio.Meanwhile,the narrow-band luminescence of most rare earth ions are unable to meet the spectral analysis and imaging requirements of objects containing multiple wavelength bands（e.g.blue and green light）.In order to solve the above problems,we strive to achieve low afterglow luminescence and broadband spectral modulation by means of cation-doped oxide scintillators,and to design inorganic-organic composite films for non-flat panel detection,which can be better adapted to the requirements of diversified digital X-ray imaging applications.To solve the above problems,a series of Pr³⁺,Sm³⁺,Bi³⁺and Gd³⁺ion doped Lu₂O₃nanoscintillators were synthesized by co-precipitation method combined with subsequent heat treatment process.By changing the doping concentration of cations,the structure,morphology,optical properties,afterglow and luminescence mechanism were systematically analyzed.Subsequently,we also prepared composite films by blending inorganic scintillators with organic polymers,and tested the related imaging performance of the scintillation composite films in the constructed X-ray imaging system,including static and dynamic X-ray applications.Our main research results are as follows:Firstly,we successfully prepared Lu₂O₃:Pr³⁺nanoscintillators with different Pr³⁺doping concentrations by co-precipitation method combined with subsequent heat treatment process.The nearly spherical Lu₂O₃:Pr³⁺nanoparticles have uniform size and good dispersion.The co-existence of Pr³⁺and Pr⁴⁺is confirmed by the XPS results.By judging the average distance between Pr and O,it can be determined that the simultaneous existence of the ³P₁,³P₀,¹D₂-³H₄transitions,and that the non-radiative transitions from the³P₁,³P₀energy levels to the¹D₂energy level occurs.The experimental results show that the photoluminescence and radioluminescence intensity of Lu₂O₃:Pr³⁺samples are enhanced and then weakened with increasing Pr³⁺doping concentration,and the maximum value is reached at x=0.001.At this point,the afterglow level of Lu₂O₃:Pr³⁺nanoscintillators are 2190 ppm,which are much higher than that of commercial scintillators CsI:Tl（several hundred ppm）.Sm³⁺ions co-doped Lu₂O₃:Pr³⁺nanoscintillators can effectively improve the red luminescence intensity and reduce the afterglow level（128 ppm）,which is comparable to that of commercial scintillator CsI:Tl.The defect formation energies of cubic structure（Ia3）Lu₂O₃containing different defects were learned from our previous work,and Lu₂O₃:Pr³⁺,Sm³⁺（x=0.001,y=0.005）nanoscintillators are heat treated with O₂atmosphere to investigate the variation of its radioluminescence intensity and afterglow intensity.The formation energy and charge density difference of the Pr³⁺/Sm³⁺/Pr³⁺,Sm³⁺samples with varying valence states are studied based on the first principle calculations,and the change of charge gain and loss in the surrounding environment are observed,and the elemental valence of the Pr³⁺/Sm³⁺/Pr³⁺,Sm³⁺samples are analyzed by XPS results.Finally,the afterglow mechanism of Lu₂O₃:Pr³⁺nanoscintillators is explained in detail based on the experimental phenomena and theoretical calculation results.It is determined that the afterglow of Lu₂O₃:Pr³⁺,Sm³⁺（x=0.001,y=0.005）nanoscintillators is due to the presence of interstitial oxygen and positive charge defects Pr_Lu^·.Meanwhile,mechanism analysis indicates that Pr³⁺and Sm³⁺co-doped Lu₂O₃nanoscintillators not only produce superposition emission due to the similar energy level transition positions corresponding to Pr³⁺and Sm³⁺ions,but also cause energy transfer between Pr³⁺ions and Sm³⁺ions,resulting in Lu₂O₃:Pr³⁺,Sm³⁺nanoscintillators exhibit efficient red luminescence under both ultraviolet and X-ray excitation.Then,we also synthesized Lu₂O₃:Bi³⁺nanoscintillatiors with different Bi³⁺doping concentrations by co-precipitation method combined with air heat treatment process.With increasing Bi³⁺doping concentration（x=0.005-0.090）,the average particle size of Lu₂O₃:Bi³⁺nanoparticles does not change significantly.This is due to the fact that the particle size is affected by the precipitant amount and is independent of dopant ions.The morphology of Lu₂O₃:Bi³⁺nanoparticles remains nearly spherical without any agglomeration phenomenon.The spectral analysis shows that the photoluminescence intensity of Lu₂O₃:Bi³⁺samples increases significantly with increasing Bi³⁺doping concentration up to x=0.030.However,the dipole-dipole and quadrupole-quadrupole interactions between Bi³⁺ions lead to luminescence concentration quenching as Bi³⁺doping concentration continues to increase.The photoluminescence spectra with different peak positions and shapes can be obtained by changing the excitation wavelength,which are attributed to the charge transitions of Bi³⁺（S₆,centrosymmetry）and Bi³⁺（C₂,non-centrosymmetry）ions,respectively.Meanwhile,comparing the luminescence integrated intensity ratio(I₄₈₅（C₂）/I₄₁₀（S₆）)between Bi³⁺（C₂）and Bi³⁺（S₆）ions combined with the results of luminescence decay curves,it is confirmed that the existence of energy transfer from Bi³⁺（S₆）ions to Bi³⁺（C₂）ions at high Bi³⁺doping concentrations.The charge transitions of Bi³⁺（C₂）and Bi³⁺（S₆）ions are induced simultaneously under X-ray excitation,which shows different broadband radioluminescence compared with ultraviolet excitation.In addition,it shows high X-ray absorption capacity and quantum efficiency.Subsequently,we mixed Lu_1.98Bi_0.02O₃nanoscintillators,which possess strong radioluminescence,low afterglow level（60 ppm）and fast response time（465 ns）,with the organic polymers polymethyl methacrylate（PMMA）and acetone in a certain proportion to prepare a well-blended colloidal solution,and a serious of Lu_1.98Bi_0.02O₃-PMMA scintillation composite films（thickness range from 75 to 160μm）were obtained by rotating coating method.The organic polymer PMMA has a high transmittance and does not produce extra luminescence.In addition,PMMA can passivate surface defects of naked Lu_1.98Bi_0.02O₃nanoparticles,so that the radioluminescence intensity and radiation resistance of scintillation composite films are significantly improved compared with the naked Lu_1.98Bi_0.02O₃nanoparticles.Meanwhile,Lu_1.98Bi_0.02O₃-PMMA scintillation composite film shows a good linear response to radioluminescence intensity over a wide range,and shows good thermal stability and environmental stability.Lu_1.98Bi_0.02O₃-PMMA scintillation composite film can obtain static X-ray imaging with a spatial resolution of 6.0 lp/mm at a safe X-ray dose（4.6μGy）and exhibits good mechanical properties to fulfil the requirements of both flat panel and non-flat panel detection applications.Finally,the doping strategy of Gd³⁺ions replacing Lu³⁺ions was adopted to prepare Lu₂O₃:Bi³⁺nanoscintillators with different Gd³⁺doping concentrations.Radioluminescence intensity of the samples are significantly improved with increasing Gd³⁺doping concentration,which is due to the doping of Gd³⁺reduces the electronegativity of the samples and the energy transfer from Gd³⁺ions to Bi³⁺ions occurs.However,with increasing Gd³⁺doping concentration,problems such as deterioration of thermal stability and radiation resistance will also occur,so the Gd³⁺doping concentration is selected as x=0.5.We know that Bi³⁺occupies two Lu³⁺sites of C₂and S₆in Lu₂O₃,by observing the variation of luminescence intensity of Bi³⁺（C₂）and Bi³⁺（S₆）ions with increasing Gd³⁺doping concentration,it indicates that the addition of Gd³⁺ions is more favourable to enhance the luminescence intensity of the relevant emission peaks of Bi³⁺（C₂）ions.This is due to the fact that the new absorption peak at 315 nm generated by the addition of Gd³⁺ions can more improve the ¹S₀-³P₁transition absorption of Bi³⁺（C₂）ions.Meanwhile,the introduction of Gd³⁺ions makes the emission peaks of the nanoscintillators red-shifted,which is caused by the lower symmetry of the Lu₂O₃lattice structure.The addition of Gd³⁺ions does not affect the low afterglow level（94 ppm）and fast response time（535 ns）of Lu₂O₃:Bi³⁺nanoscintillators.Meanwhile,Lu_0.98Gd Bi_0.02O₃nanoscintillators have higher X-ray sensitivity,wider response range and stronger radioluminescence intensity compared with the BGO single-crystal scintillators.The Gd³⁺ions doped Lu_1.98Bi_0.02O₃nanoscintillators still maintain good dispersion and nearly spherical morphology.Therefore,Lu_0.98Gd Bi_0.02O₃-PMMA scintillation composite films with high transparency,easy bending and non-cracking are synthesized by using the method of preparing inorganic-organic composite films.The experimental results show that the scintillation composite films have good environmental stability,non-toxic and harmless,and achieve static X-ray imaging with spatial resolution of 7.0 lp/mm and real-time dynamic X-ray imaging of uniformly rotating object without ghosting at a safe X-ray dose.All these characteristics combined make the prepared Lu_0.98Gd Bi_0.02O₃nanoscintillators to replace conventional scintillators in fields such as non-destructive testing,safety inspections and medical diagnostic imaging.