| Scintillation materials are widely used in many fields such as high-energy physics,nuclear medicine imaging,and national defense security.The rapidly developing neutron imaging technology also requires the aid of scintillation materials,while the ytterbium-doped sulfur oxysulfide material,ie Gd2O2S:Tb?GOS:Tb?,due to a series of excellent properties including its ultra-high neutron absorption cross section,efficient internal energy transfer and CCD camera matching,a lot of attention has been paid to it.In the face of the current situation in which developed countries in Europe and the United States strictly keep the key preparation technology of GOS-based ceramics,we must research and optimize GOS ceramics from various aspects such as powder preparation,ceramics sintering and defect effects.Based on this,this study starts with the synthesis of high-sintering active GOS:Tb powder,explores the optimization of powder properties,and prepares high-density GOS:Tb ceramics by a two-step sintering method composed of pre-sintering combined with hot isostatic pressing?HIP?post-treatment.Scintillation ceramics were used to study the effect of sintering temperature on densification and defects.The effects of post-heat treatment on the properties of ceramics were studied,and the possible defects of ceramics were characterized by thermoluminescence technology.Finally,the scintillation performance of GOS:Tb ceramics under neutron excitation was tested.?1?Using commercial gadolinium oxide,cerium oxide raw material and concentrated sulfuric acid,the precursor was synthesized in a hot water bath environment,and the precursor was calcined and dehydrated in the air to obtain an intermediate product,and finally reduced in a tubular furnace H2 atmosphere to obtain GOS powder.XRD test showed that the precursor formed in the hot water bath was Gd2?OH?4SO4?nH2O.The air-calcination removed the bound water and hydroxyl water.The phase of precursor was changed to Gd2O2SO4,and the GOS was finally obtained by reducing Gd2O2SO4under H2 atmosphere.The layered structure of precursor is retained in the intermediate product and GOS powder.The effects of reduction temperature on the phase,agglomeration and luminescence properties of GOS:Tb powder were studied.The results showed that the pure phase GOS:Tb powder can be obtained by reduction in the temperature range of 600-1050°C,and with the increase of reduction temperature,the powder crystallinity became stronger,the particles grew,the agglomeration was weakened,and the luminescence intensity was enhanced under ultraviolet and X-ray excitation.Take into consideration of powder performance,H2 safety and other factors,750°C was finally selected as the reduction temperature,and the powder was used for subsequent ceramics sintering.?2?GOS:Tb scintillation ceramics were prepared by vacuum pre-singteing combined with HIP post-treatment.The effects of pre-sintering temperature on ceramics phase,densification process,luminescence properties and defects were studied.The results showed that the density of pre-sintering ceramics increased with the increase of sintering temperature,and the GOS phase was maintained.However,temperatures of1500°C and above caused ceramics to produce a large number of defects and turn black.HIP post-treatment removed residual pores to make the ceramics highly dense and had no effect on chemical composition.The luminescence properties of HIPed ceramics pre-sintering at 1250°C to 1450°C were characterized by fluorescence spectroscopy?PL?,X-ray excitation emission spectroscopy?XEL?and high temperature thermoluminescence?TSL?.The results showed that the GOS:Tb ceramics pre-sintered at 1350°C had the highest luminous intensity,which was the result of a combination of density and defect concentration.?3?Initially explored the performance changes of GOS:Tb ceramics after heat treatment in air and H2S atmosphere.The results showed that GOS ceramics would be oxidized by heat treatment in air at 800°C and above,and produced heterogeneous phases such as Gd2O3.The GOS phase can be maintained at 600°C,but the XEL intensity decreases significantly,which may be related to the large amount of S vacancies.The heat treatment of H2S atmosphere had a certain improvement of the luminous intensity of ceramics.The thermoluminescence test also showed that the number of defects had decreased,indicating that the H2S atmosphere has positive significance for the improvement of ceramics properties.?4?The spectral characteristics,fluorescence spectra,quantum yield,and temperature-changing XEL of GOS:Tb ceramics were studied.At the same time,the defects and luminescence behaviors in ceramics were preliminarily investigated by using low-temperature thermoluminescence technology.The results showed that the prepared GOS:Tb scintillation ceramics had good optical permeability.The total transmittance of the 600 nm band at 0.5 mm thickness was more than 35%,and the quantum yield was 45.3%.The variable temperature XEL indicated that the 5D3?7FJ transition of Tb3+in ceramics increased with the decrease of temperature,while the5D4?7FJ transition decreased with the decrease of temperature,which was related to the energy level structure of Tb3+.The low temperature thermoluminescence test showed that there were two main defects in the ceramics.The lower temperature thermoluminescence peak was weaker,which may correspond to the S vacancy,and the higher temperature thermoluminescence peak was stronger,which may be the corresponding O vacancy.?5?Characterization of neutron scintillation performance on ceramics.The results showed that the light yield of GOS:Tb scintillation ceramics with different thickness was between 368-377 photons/neutron.The difference was not big,mainly because of the good light transmittance and low optical loss of ceramics.The neutron excitation light yield of prepared GOS:Tb scintillation ceramics was about 2.5 times that of the commercial GOS:Tb powder scintillation screen,and thus GOS:Tb ceramics scintillator is a more promising neutron imaging scintillator. |
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