On the cause of a contrast change in the SR images of micropipes in SiC

We propose a theoretical explanation for the experimentally observed feature in phase contrast images of micropipes in silicon carbide in a white synchrotron radiation (SR) beam, which consists in the fact that the contrast of various micropipes or various regions of the same micropipe is of different colors. The contrast is most often white (the intensity is higher than the background) at the center and black at the edges; however, the sign sometimes changes, and the contrast becomes black at the center and white at the edges. We discuss the results of experiments performed at the SR source in Pohang, Republic of Korea. The cause of the contrast change can be a change in the angle between the micropipe axis and the SR beam direction. At not overly small angles, the phase progression in a section of the micropipe is small and the contrast is standard. If the angle becomes very small, the size of the longitudinal section of the micropipe by the beam increases, which leads to wave field oscillations in the region of the section. At large distances in the white beam, these oscillations are averaged, and averaging results in black contrast.     

Computer simulation of phase-contrast images in white synchrotron radiation using micropipes in silicon carbide

A method of computer simulation of phase-contrast images in white synchrotron radiation has been developed to determine the section parameters of micropipes in silicon carbide. The experiments have been carried out using the third-generation synchrotron radiation source the Pohang Light Source (South Korea). The effective spectrum of the synchrotron radiation that forms of an image has been shown to have a relatively sharp maximum at an energy of 16 keV, which makes it possible to conserve coherency within the required limits. A computer program has been developed that automatically determines the diameters of an elliptic section of a micropipe from the condition of coincidence of calculated and experimental profiles. It has been shown that the studied micropipes have a strongly stretched elliptic section that can twist when moving along the pipe axis.     

The character of micropipes in silicon carbide crystals

There has been some question as whether micropipes in SiC crystals, giant dislocations that possess hollow cores and lie roughly along the hexagonal c-direction, are screw dislocations with Burgers vectors of nc, where n is an integer, or are dislocations of mixed character, with Burgers vector nc?+?ma, where m is equal to n or possibly different from n. X-ray topographs of the basal faces of PVT-grown 4H-SiC wafers containing micropipes taken in both back-reflection and grazing-incidence geometries are compared with computer simulations of micropipe images. The deviation from circular symmetry of micropipe images in such topographs may be explained as well by the micropipe's tilt from the c-direction as by an nc?+?ma nature. The contrast of micropipes in electron micrographs comprises of a pair of light and dark lobes consisting of a detailed system of subsidiary fringes. These are comparable with micropipe image computer simulations for Burgers vector nc. When an na edge component is introduced to the simulation, a small feature, a few hundred nm wide, appears, which is absent from the micrographs.     

Investigation of micropipes in 6H-SiC by Raman scattering

The spectra of 6H-SiC crystals including micropipes have been examined for the Si face using Raman scattering. The first-order Raman features reveal that the intensity of the transverse optical phonon band centered at ∼796 cm⁻¹ is sensitive to the micropipes. And the second-order Raman features of the micropipes in bulk 6H-SiC are well-defined using the selection rules for second-order scattering in wurtzite structure. It is found that there are some second-order peaks missing for the micropipe-including sample, which may be induced by the reduction of the incident laser intensity at around the micropipe, especially the uneven surface in the inner wall of the micropipe. These features might also be employed to characterize other structural defects such as screw-dislocations and threading edge dislocations.     

Quasi-stellar objects as Čerenkov sources

If a beam of ultrarelativistic charges enters a gas cloud ?erenkov radiation may be produced. The condition for such radiation is given, and the ?erenkov power radiated per charge is compared to the synchrotron power. Intense ?erenkov pulses will excite emission lines via stimulated Raman scattering. Due to anomalous dispersion in the presence of ac Stark effect the ?erenkov field will be most intense at frequencies well red-shifted from atomic resonances, and this red-shift will be a feature of the stimulated Raman line emissions. The intensity ratios and anomalous red-shifts of the broad emission lines of QSOs can be explained. ?erenkov emission froma collimated beam will be highly directional. The emission evidently does not occur in BL Lacs, which are pure synchrotron emitters.     

一款小角X射线散射原位测量专用简易样品加热装置

原位加热实验是同步辐射小角X射线散射领域的新热点。本文针对同步辐射小角X射线散射中需要加热并原位实时检测的实验而设计的一款样品简易原位加热专用装置,该装置主要由温控器、样品池、固定架组成,它具有结构简单、操作容易的特点。本文介绍该装置的基本结构、特点并采用该装置进行原位加热干燥褐煤实验,通过分析实验结果验证装置的可行性。     

Structural defects in 6H-SiC substrates and their effect on the sublimation growth of epitaxial layers in vacuum

The structural perfection of silicon carbide substrates and homoepitaxial layers grown on the substrates by sublimation has been studied by x-ray diffraction (topography and diffractometry) and optical microscopy. The optimum diffraction conditions (hkil reflections, radiation wavelength λ, and recording geometry) for revealing “micropipes” of the dislocation nature are determined. It is shown that the growth conditions used make it possible to obtain highly perfect epitaxial layers.     

高能电子在强激光场中的同步辐射获得超短X射线脉冲

 研究了逆流相对论高能电子与强激光脉冲相互作用的同步辐射过程,当电子具有合适的初速度且传播方向与激光的传播方向相反时,电子在激光脉冲中心作圆周运动。由于电子的运动半径比传统同步辐射环中电子的运动半径小几个数量级,因此电子的辐射能量大大增加。研究发现此过程可以获得阿秒和泽秒X射线脉冲;同时发现随着入射电子初能量的增加和激光能量的增强,获得X射线脉冲脉宽越来越短,强度越来越大。这使得激光同步辐射可以成为一种强大的短波长短脉宽的辐射光源。     

Observation of trapped and passing runaway electrons by infrared camera in the EAST tokamak

In EAST, synchrotron radiation is emitted by runaway electrons in the infrared band, which can be observed by infrared cameras. This synchrotron radiation is mainly emitted by passing runaway electrons with tens of Me V energy. A common feature of radiation dominated by passing runaway electrons is that it is strongest on the high field side. However,the deeply trapped runaway electrons cannot reach the high field side in principle. Therefore, in this case, the high field side radiation is expected to be weak. This paper reports for the first time that the synchrotron radiation from trapped runaway electrons dominates that from passing runaway electrons and is identifiable in an image. Although the synchrotron radiation dominated by trapped runaway electrons can be observed in experiment, the proportion of trapped runaway electrons is very low.     

New techniques for new sources: A fresh look at energy-dispersive diffraction for high-pressure studies

Abstract

Energy-dispersive diffraction is the most frequently used technique for high-pressure studies with synchrotron radiation. For optimum performance it requires high-energy radiation and few existing sources are able to meet this requirement. This is also important with large volume devices which demand even higher energies in order to obtain sufficient transmission. When working with diamond-anvil cells, the main experimental difficulties arise from the very small sample sizes. The use of a conical diffraction geometry increases the diffracted intensities, improves the signal-to-noise ratio and largely overcomes the crystallite statistics problem. This technique can also be used to greatly simplify high-pressure single-crystal studies. Combining these developments with recent progress in detectors and electronics will open up the field of high-pressure kinetics, but it is only with the operation of third generation synchrotron sources such as the ESRF that the present experimental limitations will be overcome.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for High Pressure Crystallography’. Daresbury Laboratory 20-21 July 1991     

On the added mass and radiation damping of rod bundles oscillating in compressible fluids

This paper presents a theoretical analysis as well as numerical results for the added mass and radiation damping coefficients of a group of two-dimensional circular cylinders oscillating harmonically in an infinite compressible fluid. The fluid reaction force on these vibrating cylinders is obtained by solving the two-dimensional acoustic wave equation with Neumann conditions on the cylinders and the radiation condition at infinity. Numerical results show that when the acoustic wavelength is large compared with the cylinder radius, the added mass predominates over the radiation damping, and both are independent of the dimensionless wavenumber. On the other hand, when the acoustic wavelength is small compared with the cylinder radius, the radiation damping predominates over the added mass, and both are small.     

Yerevan synchrotron in the mode of stretcher for injected electron beam of 75 MeV energy

At the Yerevan synchrotron, an experiment is planned on the investigation of a clustered structure of nuclei, when the synchrotron is working in the mode of stretcher of the injected electron beam with energy of 75 MeV. The results of calculation of the circulating beam dynamics in the vacuum chamber of the synchrotron during 5 ms and changes of the beam parameters due to scattering of electrons by nuclei of the residual gas are presented. It is shown that the increase in the beam sizes due to scattering of electrons is permissible for available sizes of the synchrotron vacuum chamber.     

Micro scanning X-ray diffraction study of Gallo-Roman Terra Sigillata ceramics

The red glaze (slip) that characterizes the Terra Sigillata potteries, greatly contributed to their success during the Roman period. This feature can in fact be partially explained by the microstructure (crystalline phases and grain sizes) and the physico-chemistry (compositions) of the ceramics. In this paper, we describe how the technique of synchrotron micro scanning X-raydiffraction could contribute to the understanding of the elaboration process and origins of these ceramics. The small (micron) size of the X-raybeam coupled with the use of a sample scanning stage allows one to spatially resolve the distribution and other characteristics of the constitutive mineral phases. PACS 61.10.Nz; 07.85.Qe     

Ultrafast X-ray science at the advanced light source

On May 19, 2004, 250 guests from all over the world joined the DESY research center to celebrate 40 years of research with synchrotron radiation at DESY in Hamburg. “The first measurements with the light beam from the DESY ring accelerator started in 1964. DESY was one of the seed laboratories in which the worldwide success story of research with synchrotron radiation began,” Albrecht Wagner, chairman of the DESY Board of Directors, explained in his welcoming address. “Today, more than 1,900 scientists from 31 countries come to DESY every year to carry out experiments with synchrotron radiation.”

Forty years ago, synchrotron radiation at DESY started from scratch. At the beginning of the 1960s, the radiation generated by the electrons in the bending magnets of their new 6 GeV electron synchrotron was regarded by DESY particle physicists as an unwanted, disruptive effect.     

Luminescence excitation and its relation to the structures of natural crystalline diamond and diamond-like films

The luminescence excitation spectra (LES) of natural crystalline diamond and amorphous diamond-like films are measured at energies from 4 to 20 eV at the LOCUS station for luminescence and optical studies of the Kurchatov Institute synchrotron radiation source. Two sharp peaks at 5.5 and 10.5 eV are observed in the LES of diamond, while the LES of the film demonstrate only one peak at 10.5 eV. For crystalline diamond, the peaks can be explained within the framework of the diamond band structure and the processes of inelastic scattering of photoexcited holes. Dark-field electron microscope images of the diamond-like film show that, in spite of its amorphous structure, this film contains regions of coherent electron scattering whose dimensions are on the order of 2–5 nm, which is sufficient to describe the film LES using a model for crystalline diamond. The absence of the first peak in the film LES is related to the fact that the absorption coefficient of diamond is very small at 5.5 eV and radiation is only slightly absorbed by the thin film.     

Pressure-Induced Phase Transition in BaTiO3 Nanocrystals

The crystal structure and electric properties of BaTiO3 nanocrystals are studied by in situ high-pressure synchrotron radiation x-ray powder diffraction. The phase transition takes place not only in the samples of BaTiO3 nanocrystals that are tetragonal phase with grain sizes more than 100nm, but also in the samples of BaTiO3 nanocrystals that are cubic phase with grain sizes less than 100nm. The pressures of phase transition are found to increase with decrease of the grain size from about 4 to 10GPa for crystallites ranging from 200 to 10nm in radius. The bulk moduli are calculated according to Birch-Murnaghan state equation before and after the phase transition.     

Surface magnetism of iron and cobalt on W (110) probed with polarized synchrotron radiation

I report investigations on the magnetic properties of epitaxially ordered ultrathin metal films and nanoscaled islands on a tungsten surface. The systems are fabricated by thermal evaporation under ultrahigh vacuum conditions and characterized using standard surface science techniques, the 3D iron islands are created by a thermally stimulated self-organization process on the surface. The magnetic properties of these systems are investigated using polarized synchrotron radiation ranging from the VUV region in valence band photoemission to the X-ray regime (14.4 keV) for synchrotron-based M?ssbauer spectroscopy. In angle resolved photoemission the effect of magnetic dichroism gives rise to different photoelectron intensities when the magnetization states are reversed. This phenomenon is used to analyze the electronic and magnetic properties. On the other hand, synchrotron-based M?ssbauer spectroscopy is applied to an oriented iron island structure in order to obtain information on the strength and direction of the magnetic fields from the nuclear energy levels. Received: 28 April 2000 / Accepted: 4 September 2000 / Published online: 7 March 2001     

双压电片镜在同步辐射光源光学系统中的应用

介绍了双压电片镜自适应光学技术,同时为其在同步辐射光学领域中的应用与进一步发展提供前瞻性的思考与探索。根据目前已公开发表的相关文献资料,总结介绍了双压电片镜自适应光学技术,阐述了该技术的工作机理与关键参数,并对其在国际上具有代表性的同步辐射机构中的应用情况作出描述,并指出涉及的关键技术问题与未来的发展趋势:不仅要有效地解决"连接点效应"对双压电片镜技术的负面影响,还要实现亚微米乃至纳米级的聚焦光斑,这两项内容都是双压电片镜技术需要进一步解决的重要问题。未来,双压电片镜自适应光学技术可望在我国先进的第三代同步辐射装置—"上海光源(Shanghai Synchrotron Radiation Facility,SSRF)"二期工程建设中得到应用。     

同步辐射小角x射线散射方法研究由城市固体垃圾制备的活性炭

应用同步辐射小角x射线散射方法研究了由不同城市固体垃圾制备而成的活性炭的孔结构-结果发现利用木类、纸张、塑料这三类典型垃圾组分的热解残余物为原料制备中孔发达的活性炭是可行的-活性炭的形态和结构取决于垃圾热解残余物的组分和热解程度等因素- 关键词： 小角x射线散射 活性炭 分形维数 平均孔径     

Nanometer linear focusing of hard x rays by a multilayer Laue lens

We report on a type of linear zone plate for nanometer-scale focusing of hard x rays, a multilayer Laue lens (MLL), produced by sectioning a multilayer and illuminating it in Laue diffraction geometry. Because of its large optical depth, a MLL spans the diffraction regimes applicable to a thin Fresnel zone plate and a crystal. Coupled wave theory calculations indicate that focusing to 5 nm or smaller with high efficiency should be possible. Partial MLL structures with outermost zone widths as small as 10 nm have been fabricated and tested with 19.5 keV synchrotron radiation. Focal sizes as small as 30 nm with efficiencies up to 44% are measured.