Showing papers on "Resolution (electron density) published in 1972"

The Reconstruction of a Three-Dimensional Structure from Projections and Its Application to Electron Microscopy. II. Direct Methods

Abstract: Under certain conditions it is possible to obtain the approximate projected density of a specimen from an electron micrograph, and by taking electron micrographs in different directions a number of different projections of a specimen can be determined. Several methods have been proposed for reconstructing the three-dimensional density distribution of an object from a series of projections whose planes are equally spaced in angle about an axis. One of these methods uses the Fourier transforms of the projections, while other methods operate directly on the projection data. The limitations of the direct method of 'back-projection' are discussed, and it is shown how a valid reconstruction may be achieved by back-projection from modified projected densities. This direct method is shown to be equivalent to the Fourier method and also to certain methods of reconstruction developed for the solution of analogous problems in other fields. An expression is given for the resolution of the reconstruction obtainable from a finite number of projections. Because of its computational simplicity this method is very suitable for reconstructing the density of objects with rotational or helical symmetry.

Optimal Experimental Measurements of Anisotropic Failure Tensors

Abstract: The resolution of the components of strength tensors derived from experimental data is discussed. It is shown that the variations in the non- interaction components are independent of the tension-c...

Diffraction studies of glass structure: II. The structure of vitreous germania

Abstract: The effects of pile irradiation on the polymorphs of germania are described and it is shown that the structural effects are small. For the glass, density changes on irradiation are smaller than those resulting from different methods of preparation. The X-ray diffraction pattern of vitreous germania has been measured for Q between 0.8 and 16 A −1 . The experimental method and data reduction procedures are described. The structure of vitreous GeO 2 is discussed, using the correlation function D X ( r ) obtained by fourier transformation of the X-ray intensity data. This is compared with correlation functions derived from neutron diffraction experiments and from quasi-crystalline models based on the various crystalline polymorphs of GeO 2 . The structure is shown to be built of GeO 4 tetrahedra: the coordination number of oxygen about germanium is 4.0 to within one or two per cent, the GeO distance is 1.74 A and the average GeOGe angle is 133°. The distribution of GeOGe angles is fairly narrow but cannot be determined quantitatively with data of present resolution. The average structure of the glass closely resembles that of the quartz modification as smeared out in the quasi-crystalline model. The remaining differences between model and experiment may be accounted for in terms of an unwinding (and disordering) of the crystal structure by relative rotation of tetrahedra, keeping the average intertetrahedral angle of 133°.

Determination of the Electron Density Matrix from X-Ray Diffraction Data

Abstract: We present a method for the conversion of x-ray diffraction data into an electron density which reflects the antisymmetry of the $N$-electron wave function.

An application of high energy-resolution scattering measurements in channeling studies

Abstract: One of the major advantages of using wide-angle scattering-yield measurements to study channeling is the possibility for confining the observed yield to a thin zone at a chosen, variable depth beneath the target surface The thickness of this thin zone is, at least at a not too large depth, proportional to the energy resolution of the experimental apparatus used, ie that it is usually proportional to the energy resolution of the applied particle detector In the present paper is described the application of a magnetic particle spectrometer which, in a random substance, yields a depth resolution of ∼15 −30A Although this depth resolution loses some of its relevance when channeling occurs, it is possible to study details of channeling which are inaccessible in measurements where a solid-state detector is used An example of this is demonstrated in an investigation of one single collision between particles and atomic strings and planes

The photo‐emission electron microscope: Its technique and applications

Abstract: SUMMARY This article gives a survey of the actual experience with the relatively new technique of photo-emission electron microscopy. After describing the principal technique and the features of a modern high resolution instrument, the questions of image generation, contrast formation and resolution are considered. In particular the influence of the depth of information on resolution for flat polished sections is studied and some comparisons with the scanning electron microscope (SEM) are made.* The different fields of application of the photo-EEM are shown by examples.

The Three-Dimensional Structure of Yeast Phenylalanine Transfer RNA: Shape of the Molecule at 5.5-Å Resolution

Abstract: Three isomorphous heavy-atom derivatives have been obtained of orthorhombic crystals of phenylalanine transfer RNA from yeast. These derivatives contain osmium, samarium, and platinum. The positions of the heavy atoms have been determined; these have been used to calculate a three-dimensional electron-density map of transfer RNA at a resolution of 5.5 A. The map shows a high contrast between the molecular boundaries and the solvent areas, so that most of the external shape of the molecule can be determined. The molecule appears to be 92 A long and to have a width varying from 16 A to 34 A. There are some narrow regions in the molecule that connect more globular regions. The electron density map shows chains of dense objects approximately 6 A apart that are probably due to adjacent phosphate groups on the polynucleotide chain. At the present stage of the analysis it is not possible to trace the entire backbone unambiguously; however, the data at this resolution suggest no apparent similarity between the folding of the molecule and any of the tertiary structure models proposed for transfer RNA.

A high energy resolution Auger electron spectrometer using concentric hemispheres

Abstract: A design study of a high energy resolution Auger electron spectrometer using retarding electron optical lenses is presented. Important design parameters and constructional details of such an instrument are given. The performance of the spectrometer is assessed and the results show that resolving powers of up to 1200 and scan speeds of up to 1000 eV s-1 have been achieved. Finally the Auger spectra of KCl and MgO are given as examples.

A spatially-coded x-ray source.

Abstract: A new type of x-ray tube which permits three-dimensional imaging on a single film and high spatial resolution in spite of a large focal spot is described. The resolution is approximately the reciprocal of the spatial frequency of a half-tone screen.