Fabrication And Properties Of Gd₂O₂S:Pr,Ce,F Scintillation Ceramics For X-ray Computed Tomography

Rare-earth oxysulfide?RE₂O₂S?has been widely concerned as a kind of well-designed host matrix for rare-earth ions doping to fabricate phosphors since its inception.Gadolinium oxysulfide?Gd₂O₂S,GOS?is prominent as a scintillator matrix because of its wide band gap structure?4.4⁴.8 eV?and excellent X-ray stopping power.Pr³⁺doped GOS translucent ceramic scintillators have been widely used in X-CT machines by several famous international companies,such as Siemens,Philips and so on.Scientists of Siemens have even compared its value to gold.However,domestic GOS scintillators still rely on import,and there has been no breakthrough in the studies on both material preparation and scintillation mechanism.Therefore,the research of GOS scintillation ceramics has important scientific and commercial value.For scintillation ceramics,powders with uniform distribution and light agglomeration may play a major role in ceramic fabrication process and directly affect its final optical and luminescent properties.Therefore,this study started with the optimization of powder preparation processes.First,the powder was prepared by the traditional flux method,and the best mole ratio of raw materials and calcination temperatures are investigated.Due to the large particle size of the powder prepared by the flux method,Gd₂O₂S:Pr?Ce?nano-powder with high sinterability was further prepared by reducing the precursor obtained by hot-water-bath method,intending to obtain high performance ceramics through improving the properties of powder.Finally,the ceramic sintering process was investigated,Gd₂O₂S:Pr?Ce,F?scintillation ceramics prepared with nano powders had excellent luminescent properties.The main work and results are described as follows:1)Using traditional flux method,a certain proportion of raw materials Gd₂O₃,Pr₆O₁₁,CeO₂,S,Na₂CO₃,etc.were thoroughly mixed and calcined,and pure Gd₂O₂S:Pr?Ce?powder was obtained after washing.The effects of different amounts of S in the raw materials on the final Gd₂O₂S:Pr powder properties were studied.The XRD pattern showed that pure Gd₂O₂S:Pr powders were obtained using different amounts of S.The XEL and PL spectra indicated that the powder prepared with the molar ratio of Gd₂O₃:S=1:4 had better luminescent performance.The SEM images showed that the powder that calcined at 1200°C for 3h had good morphology and dispersibility.2)PVB was added in the powder prepared by the flux method to assist forming.Then the Gd₂O₂S:Pr ceramic was prepared by sintering in vacuum followed by HIP post-treatment.Because of the low density of the ceramics pre-sintered at different temperatures,HIP post-treatment failed to improve the densification of ceramics.In addition,dense Gd₂O₂S:Pr,Ce ceramics prepared by hot pressing had strong green luminescence under ultraviolet excitation.3)Commercial Gd₂O₃,Pr₆O₁₁,CeO₂,and concentrated H₂SO₄ were used to synthesize precursors under 90?hot-water-bath.XRD and TG-DTA were used to analyze the phase transition of the precursor at different temperatures in air and H₂ atmosphere.When calcining at low temperatures in air,crystal water?200⁴00??and hydroxide?400¹100??were removed respectively,and cubic Gd₂O₃ appeared by decomposition above 1100?.Under H₂ atmosphere,the phase transformations below 400?were the same as that in air,and maintained GOS phase from 500?to 1100?.SEM images showed that the final Gd₂O₂S:Pr powder remained same layered structure as the precursor.XEL was used to analyze the luminescent performance of powder.With increasing the reduction temperature,the particles grew and crystallinity enhanced,and the luminescent intensity under X-ray excitation gradually increased.4)The Gd₂O₂S:Pr ceramics were prepared by sintering nano-powder in vacuum combined with HIP post-treatment.Under X-ray and ultraviolet excitation,bright green emission centered at511 nm could be observed in Gd₂O₂S:Pr ceramics due to the ³P₀-³H₄ transition of Pr³⁺.CIE chromaticity coordinates of?0.232,0.610?and?0.253,0.596?corresponded to the PL and XEL luminescence of the ceramics,respectively.The photoluminescent lifetime of Pr³⁺³P₀-³H₄transition was measured to be 6.73?s under 337 nm excitation.5)Gd₂O₂S:Pr?Ce,F?scintillation ceramics were prepared,and the densification process of ceramics after pre-sintering at different temperatures and the effect of HIP post-treatment on the microstructure of ceramics were analyzed by SEM characterization.Under X-ray excitation,the Gd₂O₂S:Pr ceramic after pre-sintering at 1350?followed by HIP post-treatment had better luminescent property.XEL spectra and pulse height spectra showed that Ce and F doped ceramics had better luminescent performance.The light yield of Gd₂O₂S:Pr,Ce,F scintillation ceramics is about 13000 ph/MeV with 2?s shaping time.