| Scintillation crystal is an important part of synthetic crytals. In recent years, the novel rare earth halide scintillation crystals with LaBr3:Ce3+as their representative have aroused widespread concern. They have excellent scintillation properties, especially have high efficiency, fast decay, and high energy resolution and other features, which well meet the demands of modern medical imaging, and showing great application potential. Since many rare earth halides are commonly hygroscopic, it’s very difficult to prepare the high purity anhydrous rare earth halides raw materials for the corresponding crystal growth, which is a serious bottleneck for the further development of these new rare earth halide scintillation crystals. In order to meet the growing needs of the promising LaBr3:Ce3+scintillation crystals, this thesis investigated the preparation of high purity LaBr3and CeBr3and their purification process in detail, focusing on the effective control of the key impurites of water and oxygen.The main research contents and the findings are as follows:(1) The dehydration mechanism of LaBr3·7H2O was investigated by differential thermal analysis (DTA) and thermogravimetric (TG) methods, and it was revealed that the dehydration is a three-step process. Based on this result, an ammonium bromide assisted method, stepped dehydration by firing LaBr3·7H2O under the protection of NH4Br, was raised to prepare the high purity anhydrous LaBr3. The reaction mechanism of the ammonium bromide assisted method was studied. The XRD results showed that there exists (NH4)2LaBr5intermediate product due to the reaction of LaBr3·7H2O and NH4Br. And combined with the DTA/TG analysis, the related reactions conditions during the ammonium bromide assisted preparation were also confirmed.(2) The effects of various process parameters such as blending manners of starting materials, the addition amount of NH4Br, the dehydration temperature and atmosphere were invesitigated. It was revealed that the desirable blending manner is by evaporating concentration of the mixed aqueous solution of LaBr3and NH4Br to co-crystallization, the appropriate addition amount of NH4Br is three times that of LaBr3·7H2O in molar ratio, and the optimized dehydration route is as follows:(a) removing the free water and a small amount of the crystal water at about100℃under vacuum conditions.(b) removing most of the crystal water at230℃under vacuum;(c) removing the residual crystal water at260℃ under the protection of Ar.(3) The effects of the atmoshere and temperature of the melting process were studied. Experiments showed that the melting process, at820℃under the protection of Ar, can make (NH4)2LaBrs decompose and then effectively remove NH4Br to yield high purity anhydrous LaBr3. For the obtained sample, the water content is8ppm, the oxygen content is850ppm, and its overall purity is above99.9%.(4) The preliminary study on preparation of anhydrous CeBr3through direct reaction of the metal cerium and simple substance of bromine was carried out. Theoretical analysis revealed that this combination reaction is mainly controlled by internal diffusion, and the appropriate reaction temperature is determined to be650℃by experiments after improving the starting material status and the experimental equipments.(5) The effective characterization methods of rare earth bromides were developed. The contents of metal ionic impurities, residual water and oxygen of were effectively measured by ICP-AES, water analyzer and oxygen-nitrogen analyzer, respectively. Raman spectroscopy was also developed to qualitative define and determine the LaOBr impurity in the anhydrous LaBr3. |
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