Influence of Gamma-Ray Irradiation on Absorption and Fluorescent Spectra of Nd：YAG and Yb：YAG Laser Crystals

We investigate the influence of gamma-ray irradiation on the absorption and fluorescent spectra of Nd^3＋：Y3A15 O12 （Nd： YAG） and Yb^3＋ ： Y3A15 O12 （Yb： YAG） crystals grown by the Czochralski method. Two additional absorption （AA） bands induced by gamma-ray irradiation appear at 255nm and 340nm. The former is contributed due to Fe^3＋ impurity, the latter is due to Fe^2＋ ions and F-type colour centres. The intensity of the excitation and emission spectra as well as the fluorescent lifetime of Nd：YAG crystal decrease after the irradiation of 100 Mrad gamma-ray. In contrast, the same dose irradiation does not impair the fluorescent properties of Yb：YAG crystal. These results indicate that Yb： YA G crystal possesses the advantage over Nd： YA G crystal that has better reliability for applications in harsh radiant environment.     

Effects of annealing treatment and gamma irradiation on the absorption and fluorescence spectra of Cr:GSGG laser crystal

The influence of annealing treatments and gamma-ray irradiation on the absorption and fluorescence spectra of Cr:GSGG crystals grown by the Czochralski method has been investigated. Two absorption bands located near 686 nm and 1050 nm were weakened markedly after the crystal was re-annealed in H₂ atmosphere, which is due to the Cr⁴⁺ ions being de-oxidized into Cr³⁺ ions. The other two weak additional absorption bands induced by gamma-ray irradiation appearing near 310 nm and 480 nm are ascribed to the Fe²⁺ ions and F-type color centers, respectively. In particular, the gamma-ray irradiation with a dose of 100 Mrad has an effect of improving slightly the luminescence properties of Cr:GSGG crystals. The improvement mechanism is analyzed and discussed.     

Digital positron lifetime spectroscopy at EPOS

As the digital equipment to measure positron lifetimes gets cheaper and more widely used, it is decided that EPOS, the ELBE positron source will sample the signals from the photomultipliers directly and evaluate it online or offline by digital means.Still using isotope sources, the EPOS lifetime spectrometer results in a timing resolution of around 170 ps (with ⁶⁰Co), which compares good to analog equipment. A distinct improvement is expected when a coincidence setup will be used at ELBE. However, also the software needs further improvement: while one of the goals is of course to achieve the best time resolution, there is also the aspect of runtime and expandability. Results of evaluations will be presented and compared with results from other groups.     

Effect of oxygen deficiency on the radiation sensitivity of sol-gel Ge-doped amorphous SiO2

We report experimental investigation by electron paramagnetic resonance (EPR) measurements of room temperature γ-ray irradiation effects in sol-gel Ge doped amorphous SiO₂. We used materials with Ge content from 10 up to 10⁴ part per million (ppm) mol obtained with different preparations. These latter gave rise to samples characterized by different extents of oxygen deficiency, estimated from the absorption band at ~5.15 eV of the Ge oxygen deficient centers (GeODC(II)). The irradiation at doses up to ~400 kGy induces the E'-Ge, Ge(1) and Ge(2) paramagnetic centers around g ~ 2 with concentrations depending on Ge and on GeODC(II) content. We found correlation between Ge(2) and GeODC(II) contents, supporting the suggestion that the latter defect is the precursor of Ge(2). Even if the concentration of E'-Ge and Ge(1) defects cannot be strictly related to GeODC(II) one, the concentration growth of these paramagnetic defects with irradiation evidences that the radiation sensitivity is enhanced by the oxygen deficiency for Ge doping above 1000 ppm mol and it is reduced below 100 ppm mol. Moreover, the investigation of samples with different GeODC(II) concentration but fixed Ge content has shown that the oxygen deficiency enhances the overall radiation sensitivity for [GeODC(II)]/[Ge] in the range 10^-3 10^-2.     

金属有机框架材料的结构、动力学行为和主客体相互作用的固体核磁共振研究

金属有机框架材料(MOFs)在绿色能源气体储存、二氧化碳捕获、化学分离、化学传感和多相催化等领域有着广泛的应用前景,与其分子结构、动力学行为以及与客体分子的相互作用密切相关。固体核磁共振(NMR)能提供原子水平的结构距离信息,能从多个时间尺度反映分子动力学行为,能通过极化转移揭示主客体相互作用。本文综述了近年来先进的固体核磁共振方法在研究MOFs的结构、动力学行为以及主客体相互作用等方面的研究进展。多核、多维固体NMR可给出MOFs材料的金属中心以及有机配体的局部配位状态,变温固体NMR可以反映MOFs的分子柔性以及有机配体在不同温度下的运动模式及速率。固体NMR还可用来研究MOFs与吸附客体分子(如甲烷、二氧化碳等)之间的相互作用模式。通过固体NMR技术获得的结构信息有助于人们理解MOFs材料的构效关系,并为合理设计新型的MOFs材料提供依据。