原子核同核异能态诱发辐射研究进展

介绍了同核异能态的特点和诱发同核异能态的几种机制,如直接光激发、电子跃迁诱导核激发(NEET)和电子俘获诱导核激发(NEEC)。同核异能态的诱发辐射研究目前在国际上竞争激烈,尤其是对最近提出的NEEC的研究。同时探讨了在我国开展这些研究的可能性。     

原子核同核异能态诱发辐射研究进展

介绍了同核异能态的特点和诱发同核异能态的几种机制,如直接光激发、电子跃迁诱导核激发(NEET)和电子俘获诱导核激发(NEEC)。同核异能态的诱发辐射研究目前在国际上竞争激烈,尤其是对最近提出的NEEC的研究。同时探讨了在我国开展这些研究的可能性。     

Electron-stimulated defect formation in single-walled carbon nanotubes studied by hydrogen thermal desorption spectroscopy

Defects in single-walled carbon nanotubes introduced by low-energy electron irradiation at 8 K were sensitively detected by cryogenic thermal desorption of hydrogen molecules H₂ in the temperature range of 10-40 K. The thermal desorption spectrum was found to change significantly with sample annealing at temperatures as low as 40-70 K. Experimental results suggest that the H₂ physisorption sites responsible for the ‘defect’ peak at 28 K are interstitial channel space between nanotubes closely packed in bundles which becomes more easily accessible on damage. It is also suggested that the disordering provides groove sites for H₂ physisorption with smaller binding energy causing the damage-induced spectral component around 16 K, slightly lower than the desorption peak at 20 K that is observed in undamaged samples. The spectral change at 40-70 K could be interpreted by migration of adatoms at the low temperatures.     

Desorption of metastable species induced by electronic excitation at the surface of solid krypton with physisorbed hydrogen

The effect of the physisorbed hydrogen molecules on the electron stimulated desorption of metastables from the surface of solid Kr was investigated. At the clean surface of solid Kr, the desorption of Kr metastables occurs only through an excimer dissociation mechanism. Physisorption of hydrogen molecules causes the following effects: (1) the total desorption yield of the metastable species increased by an order of magnitude or more; (2) metastable Kr, which is probably desorbed through the cavity ejection mechanism, emerges; (3) metastable Kr by excimer dissociation is also enhanced; and (4) a new metastable species, which seems to be composed of metastable hydrogen molecule, appears.     

Mechanisms of mineral scaling in oil and geothermal wells studied in laboratory experiments by nuclear techniques

Two independent nuclear methods have been developed and tested for studies of mineral scaling mechanisms and kinetics related to the oil and geothermal industry. The first is a gamma transmission method to measure mass increase with a 30 MBq source of ¹³³Ba. The other method applies radioactive tracers of one or more of the scaling components. CaCO₃-precipitation has been used as an example here where the main tracer has been ⁴⁷Ca²⁺. While the transmission method is an indirect method, the latter is a direct method where the reactions of specific components may be studied. Both methods are on-line, continuous and non-destructive, and capable to study scaling of liquids with saturation ratios approaching the solubility product. A lower limit for detection of CaCO₃ with the transmission method in sand-packed columns with otherwise reasonable experimental parameters is estimated to be <1 mg in a 1 cm section of the tube packed with silica sand while the lower limit of detection for the tracer method with reasonable experimental parameters is estimated to <1 μg in the same tube section.     

原子数据库在研究同核异能态诱发辐射中的应用

如何人为地诱发同核异能态辐射,从而利用储存在其中的巨大能量,是个备受关注的交叉学科课题。利用原子物理过程进行触发是目前较为可行的方案,原子物理参数将在里面起到关键作用。介绍了一套高效率的原子数据库,目前该数据库可对原子序数小于96的元素所有电离度离子的能级结构信息进行快速扫描,从而筛选出一些可能通过原子过程触发的同核异能态体系。通过在197Au元素电子跃迁诱导核激发过程和178Hf元素电子俘获诱导核激发过程中的应用,展示了该数据库对寻找合适同核异能态体系的作用。     

硫化锰电子结构的软X射线共振非弹性散射研究

利用软X射线共振非弹性散射谱(resonant inelastic soft X-ray scattering, RIXS)对3d过渡金属硫化物中的硫化锰(MnS)电子结构进行了研究.通过分析Mn²⁺的2p⁶3d⁵→2p⁵3d⁶→2p⁶3d⁵二次光子过程，得到了共振非弹性散射谱中的两类非弹性峰，d-d电子跃迁和电荷转移(charge-transfer)跃迁.这两部分跃迁分别共振增强于L边附近及伴随峰附近.基于Hartree-Fock方法的多重态计算分别模拟了原子近似下和立方体O_h对称群下共振非弹性散射谱及吸收谱.计算得MnS实际晶体场10Dq值介于0.80eV—0.85eV之间.对MnS和MnO CT跃迁差异的讨论表明MnS较强的CT跃迁来源于其较窄的能隙宽度. 关键词： 软X射线共振非弹性散射 软X射线吸收谱 d-d跃迁 电荷转移     

原子数据库在研究同核异能态诱发辐射中的应用

 如何人为地诱发同核异能态辐射,从而利用储存在其中的巨大能量,是个备受关注的交叉学科课题。利用原子物理过程进行触发是目前较为可行的方案,原子物理参数将在里面起到关键作用。介绍了一套高效率的原子数据库,目前该数据库可对原子序数小于96的元素所有电离度离子的能级结构信息进行快速扫描,从而筛选出一些可能通过原子过程触发的同核异能态体系。通过在¹⁹⁷Au元素电子跃迁诱导核激发过程和¹⁷⁸Hf元素电子俘获诱导核激发过程中的应用,展示了该数据库对寻找合适同核异能态体系的作用。     

掺稀土离子激光玻璃荧光光谱的测量

介绍了荧光光谱测量系统的测量原理,讨论了激发光源、单色仪光栅、样品的制备、激发光的激发位置和入射角、探测器、荧光光谱波段等测量条件的选取对激光玻璃荧光测量结果的影响。实验表明,在掺稀土离子激光玻璃荧光光谱的测量中,只有选取合适的测量条件,才可获得准确、可靠的测试结果。     

Crystallinity of pure silica glass studied by positron annihilation

The crystallinity of silica glass fused from Brasilian quartz is demonstrated by positron annihilation lineshape measurements using a high resolution Ge(Li)-detector as well as by positron lifetime measurement.     

Absolute desorption yields of metastable atoms from the surface of solid rare gases induced by exciton creation

Absolute yields of the metastable excited atoms desorbed from the surfaces of solid Ne and Ar by the creation of surface and bulk excitons have been measured using monochromated synchrotron radiation as a selective excitation source. We have obtained the absolute yields of (2.3 ± 0.7) × 10⁻³, (1.4 ± 0.4) × 10⁻³, and (7.8 ± 2.3) × 10⁻⁴ atoms/photon at the excitation of S1, B1 and S′ exciton for Ne, respectively, and 1 × 10⁻⁵ atoms/photon at S1 excitation for Ar. The probability for metastable atom desorption is found to be about 2 to 10% at the excitation of S1 exciton on the surface of solid Ne.     

Heat treatment of pure silica glass studied by positron annihilation

The crystallinity of silica glass fused from Brazilian quartz was studied by Doppler-broadening techniques after heat treatment in the temperature range between 1000°C and 1500°C. Paper F13 presented at 4th Internat'l Conf. Positron Annihilation, Helsing?r, Denmark (August 1976)     

Progress of microscopic thermoelectric effects studied by micro- and nano-thermometric techniques

Heat dissipation is one of the most serious problems in modern integrated electronics with the continuously decreasing devices size. Large portion of the consumed power is inevitably dissipated in the form of waste heat which not only restricts the device energy-efficiency performance itself, but also leads to severe environment problems and energy crisis. Thermoelectric Seebeck effect is a green energy-recycling method, while thermoelectric Peltier effect can be employed for heat management by actively cooling overheated devices, where passive cooling by heat conduction is not sufficiently enough. However, the technological applications of thermoelectricity are limited so far by their very low conversion efficiencies and lack of deep understanding of thermoelectricity in microscopic levels. Probing and managing the thermoelectricity is therefore fundamentally important particularly in nanoscale. In this short review, we will first briefly introduce the microscopic techniques for studying nanoscale thermoelectricity, focusing mainly on scanning thermal microscopy (SThM). SThM is a powerful tool for mapping the lattice heat with nanometer spatial resolution and hence detecting the nanoscale thermal transport and dissipation processes. Then we will review recent experiments utilizing these techniques to investigate thermoelectricity in various nanomaterial systems including both (two-material) heterojunctions and (single-material) homojunctions with tailored Seebeck coefficients, and also spin Seebeck and Peltier effects in magnetic materials. Next, we will provide a perspective on the promising applications of our recently developed Scanning Noise Microscope (SNoiM) for directly probing the non-equilibrium transporting hot charges (instead of lattice heat) in thermoelectric devices. SNoiM together with SThM are expected to be able to provide more complete and comprehensive understanding to the microscopic mechanisms in thermoelectrics. Finally, we make a conclusion and outlook on the future development of microscopic studies in thermoelectrics.     

Silver diffusion and precipitation of nanoparticles in glass by ion implantation

Silver particles in soda-lime glass, less than 10 nm in size, were prepared by ion implantation. The implantation dose was in the range of 0.5 to 2×10¹⁶ Ag ions/cm² and the beam current density was varied from 0.5 to 2A/cm². Here, the beam current density strongly influences ion diffusion and particle precipitation as well as compressive stress generation around the particles due to thermal effects resulting from the deceleration of silver ions. Stress relaxation can be achieved by increased dose rates or thermal processing at elevated temperatures. Based on RBS and HREM results, a possible route to homogeneous distribution of Ag nanoparticles within the glass is discussed with respect to their interesting optical properties.     

Performance of lead iodide nuclear radiation detectors with the introduction of rare earth elements

Lead iodide has been recognized as a promising material for room temperature radiation detectors. It has a wide band-gap (∼ 2.3 eV), high atomic numbers (82, 53) and it is environmentally very stable compared to mercuric iodide. Electrical and optical properties of lead iodide grown crystals purified under the influence of selected rare earth elements have been investigated. Photo-luminescence and capacitance-voltage measurements have been performed using different rare earth elements.     

Metal interface formation studied by high-energy reflection energy loss spectroscopy and electron Rutherford backscattering

We demonstrate that high-energy, high-resolution reflection electron energy loss spectroscopy can provide unique insights into interface formation, especially for the case where an extended interface is formed. By changing the geometry and/or electron energy the electronic structure can be probed over a range of thicknesses (from 10s of Å to more than 1000 Å). At the same time one resolves the elastically scattered electrons into different components, corresponding to scattering of atoms with different mass (so-called ‘electron Rutherford backscattering’). Thus these high-energy REELS/elastic scattering experiments obtain information on both the electronic structure and the atomic composition of the overlayer formed.     

带玻璃衬垫的硅泡沫薄膜压痕理论的近似解析与数值研究

基于弹性和多胞/泡沫固体理论,用近似解析和数值方法研究了带玻璃衬垫的硅多胞/泡沫薄膜压痕问题.研究的重点在于考察接触区的压力分布和压痕临界深度ε_c,并将计算得到的理论结果与实验结果进行了对比,两者基本一致.为了对这类薄膜结构系统完整性作合理的评估,分析了多胞/泡沫材料变形和破损机理,由此提出了压痕能概念并给出了相应的解析表达式. 关键词： 薄膜 材料试验 固体的力学性质     

Optical properties of anatase and rutile TiO_2 studied by GGA + U

The optical properties of thermally annealed TiO_2 samples depend on their preparation process, and the TiO_2 thin films usually exist in the form of anatase or rutile or a mixture of the two phases. The electronic structures and optical properties of anatase and rutile TiO_2 are calculated by means of a first-principles generalized gradient approximation(GGA) +U approach. By introducing the Coulomb interactions on 3d orbitals of Ti atom(U~d) and 2p orbitals of O atom(U~p), we can reproduce the experimental values of the band gap. The optical properties of anatase and rutile TiO_2 are obtained by means of the GGA+U method, and the results are in good agreement with experiments and other theoretical data. Further, we present the comparison of the electronic structure, birefringence, and anisotropy between the two phases of TiO_2. Finally,the adaptability of the GGA+U approach has been discussed.