Showing papers on "Resolution (electron density) published in 1985"

High resolution NMR imaging in solids

Abstract: A method is described to obtain high resolution NMR images in solids. Line broadening effects are eliminated by observing the free induction decay signal at a fixed time while the experiment is repeated each time with incremented field gradient in equal steps for the spatial resolution. Solid images with excellent resolution are shown.

Nuclear magnetic resonance imaging

Abstract: An NMR chemical shift imaging method and apparatus wherein a number of experiments are performed with different encoding gradients sufficient to obtain a desired spatial resolution in the image, and each of said number of experiments is repeated for each of a number of different delay times between excitation and the start of data collection sufficient to obtain a desired chemical shift resolution in the image.

Protein Conformation by Infrared Spectroscopy: Resolution Enhancement by Fourier Self-Deconvolution

Abstract: Fourier self-deconvolution (FSD) has been employed to enhance the resolution of the infrared spectra of proteins in the solid state and in D2O solution. The feasibility of using diffuse reflectance spectrometry for measuring the infrared spectra of solid proteins has been demonstrated. FSD permits inherently broad absorption bands to be resolved into distinct peaks which can be associated with specific protein secondary structures. Because the areas of the resolved peaks are the same as the areas of the previously unidentifiable components, this new method should enable quantitative estimates of the proportion of each conformation in a protein to be calculated.

Imaging of atomic clouds outside the surfaces of gold crystals by electron microscopy

Abstract: The structure of small metal crystals has been intensively examined by high-resolution electron microscopy (HREM). In particular, multiply-twinned gold and silver crystals have been characterized using the profile-imaging method1,2 at atomic resolution, and reconstructed metal surfaces observed2. Crystal structure images of large gold clusters consisting of 55 gold atoms arranged in a cubeoctahedron have been recorded3 using 2.5 A resolution, and crystal growth, row by row, on a {111} surface has been documented4 using a low-light-level silicon-intensified target television (TV) camera and video system with an on-line image processor. Direct imaging of rearrangements of atomic columns on extended gold surface5 established that profile imaging can provide information about surface self-diffusion. The motion of surface atoms, recorded with a real-time video tape-recorder (VTR) system, and the formation of surface atom steps on {100} surfaces, although not {111}, has also been reported recently6. Dynamic HREM observations at TV rate showing defect motion in gold7 and CdTe (refs. 8, 9) has given information on ‘in-lattice’ rearrangements of columns of atoms. We report here surface profile images recorded with the electron beam along the 〈110〉 direction with spatial resolution of ∼2.0 A which reveal changes in occupancy of the atom columns often within periods of less than 0.1 s. Surfaces with several adjacent atom columns involved in rapid structural changes frequently interacted with a cloud of atoms extending out to 9 A off the crystal and changes in shape and density of the clouds were recorded. Although these clouds have already been described6,10, the present work is the first to analyse these events properly and to describe them in detail. The motion of atomic columns and the existence of atom clouds revealed here may have important consequences for crystal growth, surface science and catalysis studies.

Peak Shape Variation in Fixed-Wavelength Neutron Powder Diffraction and its Effect on Structural Parameters Obtained by Rietveld Analysis

Abstract: Analyses of high-resolution neutron powder diffraction data, using both the Pearson VII and pseudo-Voigt peak shape functions, have revealed a range of peak shapes from essentially Gaussian to Lorentzian and beyond. Moreover, the refinements show that the Lorentzian character of the peaks in each pattern increases with increasing diffraction angle. Both kinds of shape change are associated with varying relative contributions to the peak profiles of the instrumental resolution, isotropic crystallite strain and crystallite size effects. Rietveld analysis of powder data with the standard Gaussian form when the peaks have significant Lorentzian character has little effect on atomic positional parameters, but it leads to an overestimation of the thermal vibration coefficients and higher least-squares residuals.

Diffraction effects in moiré deflectometry

Abstract: The bounds on the sensitivity and resolution of moire deflectometry caused by diffraction effects are derived from the wave equation. It is shown that Fresnel diffraction is a necessary and sufficient framework for treating these effects. Expressions are given for angular resolution, spatial resolution, and dynamic range. The proper selection of experimental parameters involves a trade-off between the angular resolution on the one hand and spatial resolution and dynamic range on the other.

Lifetime of image surface states.

Abstract: A quantitative evaluation of the lifetime of image-potential-induced surface states is presented for the first time. It is shown that Auger processes give contributions to those linewidths of a few thousandth of an electronvolt; these widths are much narrower than the values experimentally reported, which seem to be limited by the experimental resolution. Increased resolution could provide experimental information on inelastic interactions at surfaces.

Performance of a position-sensitive scintillation detector.

Abstract: The spatial resolution of a NaI(Tl), 25 mm thick bar detector designed for use in positron emission tomography has been studied. The position along the 500 mm long detector is determined from the centroid of the light distribution in the crystal as measured by a linear array of photomultiplier tubes. A Monte Carlo computer simulation was performed to investigate the factors limiting the spatial resolution. The program allowed the authors to study the effect of various phototube configurations and crystal surfaces. Since the resolution is affected by the width of the light distribution, the authors studied the effect of sharpening the distribution by modifying the front crystal surface with grooves cut perpendicular to the long axis of the crystal and by using nonlinear preamplifiers. The simulation predicts a spatial resolution (FWHM) of 3 mm with this crystal. Experimental measurements of spatial resolution were performed concurrently with the simulations. In particular, a modified grooved crystal was measured to have 4.0 mm spatial resolution, an improvement over the original crystal without grooves. With delay line pulse shortening, which increases the count rate capability of the detector, the grooved crystal was measured to have 5.5 mm spatial resolution.

Image processing in high-resolution electron microscopy using the direct method.I.Phase extension

Abstract: A procedure to combine the information from an electron micrograph (EM) and the corresponding electron diffraction (ED) pattern is proposed. Here the ED data will be used to obtain a set of amplitudes of the structure factors, while the EM will be used to obtain a set of starting phases assuming the weakphase-object approximation. A direct method is then used to extend the phase information from a resolution of about 2 to 1 A. The efficiency of the procedure has been verified by the test calculation on the model structure of copper perchlorophthalocyanine.

The structure of the stalk surface layer of a brine pond microorganism: correlation averaging applied to a double layered lattice structure.

Abstract: SUMMARY The surface layer of the stalk of a prosthecate halophilic microorganism is a periodic array (space group p3ml) comprised of electron dense trimers with a centre to centre spacing of 9.0nm. The structure is reminiscent of E. coli porin. We have demonstrated that the method of correlation averaging can be effectively used to separate overlapping lattices, and that this results in a higher fidelity reconstruction, when compared to the established method of Fourier filtration. Two statistical methods are used to determine the resolution of the correlation average as a function of the number of ‘windows’ averaged, (i) The phase residual of spatial frequencies in Fourier space is computed between independently obtained subaverages, and (ii) a new method of Q factor analysis examines the cumulative vector sum in Fourier space as a function of the number of windows averaged. Both methods give a resolution of 1/2.1−nm.

Subwavelength resolution far-infrared microscopy.

Abstract: The technique of scanned evanescent wave imaging can be used to obtain microscopic resolution well below the illumination wavelength. This paper describes the first application of this method to imaging in the far infrared using the 118.8-μm emission of an optically pumped methanol vapor laser. Resolution down to 30 μm has been demonstrated. Limitations set by available signal power are discussed.

Facilities for solution scattering and fibre diffraction at the Daresbury SRS

Abstract: The small-angle scattering facility at Daresbury has been constructed for diffraction studies of a wide range of naturally occurring and synthetic materials The high brightness of the SRS is combined with focusing optics, resulting in exposure times that can be two or three orders of magnitude less than those required on a conventional source Spacings of 2000 A in the vertical direction and 300 A in the horizontal direction can be observed, while the resolution between diffraction orders is 5000 and 600 A In addition, preliminary results have been obtained on a double-crystal diffractometer that has a resolution, in one dimension, of better than 26 000 A For high-angle fibre diffraction studies, a camera with pinhole collimation has been constructed Examples from solution scattering and fibre diffraction are used to illustrate the performance of these facilities

Solving the phase problem in fiber diffraction. Application to tobacco mosaic virus at 3.6 Å resolution

Abstract: The diffracting particles that give rise to a fiber diffraction pattern are randomly oriented about the fiber axis and, in consequence, the diffraction pattern is cylindrically averaged. The phase problem in fiber diffraction is not only to determine the phase in the usual crystallographic sense, but to overcome the loss of information from this averaging. This has been done by a multi-dimensional analog of protein crystallographic isomorphous replacement, combined with the use of information from the fine splitting of layer lines that occurs when a helical structure repeats approximately, but not exactly, in a given number of turns. The phases thus determined have been refined by a solvent-flattening procedure. They have been further refined by assuming the separation of cylindrically averaged Bessel-order terms (from multi-dimensional isomorphous replacement at an early stage and from a model at a later stage) and applying conventional isomorphous replacement (two-dimensional isomorphous replacement) to determine the phases of the terms. Cycles of model building and two-dimensional isomorphous replacement were found in the case of tobacco mosaic virus to improve greatly the quality of the electron density map, and enabled an atomic model of the virus to be built based on a highly interpretable map at 3.6 A resolution with five Bessel orders (terms overlapping because of cylindrical averaging) separated.

IMPROVEMENTS IN PHOTODIODE READOUT FOR SMALL CsI(Tl) CRYSTALS

Abstract: Measurements of photon energy resolution in the region around 1 MeV with a small CsI(Tl) photodiode detector will be presented.

The electron storage ring as a source of infrared radiation

Abstract: The characteristics of synchrotron radiation in the infrared region have been investigated at the Berlin electron storage ring BESSY using Fourier spectroscopy. In accordance with calculation, it was found that BESSY at 100 mA beam current delivers higher fluxes than a conventional source only in the spectral region below 30 cm−1, provided the interferometer is the throughput-limiting element. If, however, the throughput is limited by small sample size or, if the experiment requires a large f-number, the high brightness of synchrotron radiation could already yield flux advantages at several hundred wavenumbers. The possible application in one particular experiment - infrared reflection-absorption spectroscopy of adsorbed molecules on metal single crystal surfaces - is discussed. In the course of the investigations described here, it was also established that the short light pulses of the synchrotron radiation (∼ 100 ps) do not adversely affect the resolution in Fourier transform spectroscopy.

MRI of chemical shift spectra within limited inner volume

Abstract: Inner-volume MRI techniques are extended to provide chemical shift NMR spectra for each voxel of an imaged inner-volume of a larger object. Spatial resolution is achieved within a selected inner-volume using multiple phase-encoded NMR measurement cycles with 90° and 180° RF nutation pulses being applied in the presence of inner-volume selective B o magnetic gradients. However, NMR spin echo RF responses are recorded during readout periods without application of any magnetic gradients. The resulting NMR data is then subjected to one dimension of Fourier Transformation to resolve conventional NMR chemical shift spectra while additional dimensions of Fourier Transformation are used to provide desired spation resolution within the inner-volume. Thus, for spectroscopic imaging, the signal readout period is used for chemical shift information while the tissue volume is spatially resolved with phase encoding on two or three spatial axes which have reduced fields of view using three orthogonal selective RF irradiations.

Atomic Structure Images Formed by Plasma-Loss Electrons

Abstract: Two kinds of energy filtered atomic structure images of Si crystal in (011) orientation formed by no-loss and plasma-loss electrons were photographed using the energy analyzer of a sector type magnet which was attached to the bottom of a high resolution electron microscope. The behavior of plasma-loss electrons in the crystal was discussed on the basis of the dynamical theory of electron diffraction. The contrast of the crystal lattice image formed by the plasma-loss electrons was calculated and compared with the observed image.