Showing papers on "Resolution (electron density) published in 1984"

Structure, stability and evolution of Saturn's rings

Abstract: Recent data obtained from the Voyager spacecrafts and ground-based measurements indicate: (1) the rings have a thickness of at most 150 m (ref 1) and probably several times less2,3; (2) the rings are mostly composed of ice particles ranging from centimetres to metres in size4; (3) the rings are subdivided into a large number of ringlets with a radial dimension ranging from 10-km down to the several metres resolution of the Voyager spacecraft's camera5; (4) the B ring contains very many optical depth variations (06–3)3 This behaviour is essentially determined by the collisional properties of the rings' ice particles Here we report some preliminary results from an experiment designed to measure the coefficient of restitution of ice particles colliding at impact velocities relevant to Saturn's rings We apply these results to simple dynamical models for Saturn's rings and deduce the rings' thickness to be ≺5 m We also show that regions with optical depth <05, such as the B ring, are unstable to viscous diffusion Such an instability may be the cause of optical depth variations in the B ring

A Novel Method Capable of Resolving Rotational Ionic States by the Detection of Threshold Photoelectrons with a Resolution of 1.2 cm -1

Abstract: Presentation de cette methode realisee en combinant une technique de champ pulse avec un analyseur steradiant. Application a la mesure du seuil des etats rotationnels de NO +

Thin film and depth profile analysis

Abstract: 1. Introduction..- 1.1 Requirements for Thin Film and In-Depth Analysis.- 1.2 Object and Outline of the Book.- References.- 2. The Application of Beam and Diffraction Techniques to Thin Film and Surface Micro-Analysis..- 2.1 Methods to Determine Chemical Structures in Material Research.- 2.2 Selected Analytical Features Used to Determine Chemical Structures.- 2.2.1 Depth Profiling.- a) Destructive Depth Profiling.- b) Nondestructive Methods for Depth and Thin Film Analysis.- 2.2.2 Microspot Analysis and Element Imaging.- 2.3 Determining Physical Structures in Material Research.- 2.3.1 X-Ray Diffraction.- 2.3.2 X-Ray Double Crystal Diffraction.- 2.3.3 Ultrasonic (Acoustic) Microscopy.- 2.4 Application of Different Microanalytical Techniques to Specific Analytical Problems.- 2.4.1 AES and TEM-EDX in Interface Analysis of MnZn Ferrites.- 2.4.2 Interfaces of SrTiO3 Boundary Layer Capacitor Material Studies by TEM and Selected-Area EDX.- 2.4.3 Analysis of GaAlAs with SIMS, X-Ray Diffraction and AES.- 2.5 Future Prospects.- References.- 3. Depth Profile and Interface Analysis of Thin Films by AES and XPS.- 3.1 Quantification from First Principles.- 3.2 Initial Transient Layer.- 3.3 Steady-State Region.- 3.4 Film-Substrate Interface.- References.- 4. Secondary Neutral Mass Spectrometry (SNMS) and Its Application to Depth Profile and Interface Analysis..- 4.1 Background.- 4.2 Experimental Method.- 4.2.1 Related Techniques.- 4.2.2 Performance of SNMS.- a) The Postionizing Method.- b) Operation Modes of SNMS.- 4.3 Quantification of SNMS.- 4.3.1 Quantification for Atomic Sputtering.- 4.3.2 Quantification Using Molecular SNMS Signals.- 4.3.3 Sensitivity of SNMS.- 4.4 Applications of SNMS to Depth Profile Analysis.- 4.4.1 General Considerations.- 4.4.2 Examples of Depth Profiling by SNMS.- 4.5 Concluding Remarks.- References.- 5. In-Situ Laser Measurements of Sputter Rates During SIMS/AES In-Depth Profiling..- 5.1 Background.- 5.2 Principles of Laser Technique.- 5.2.1 Laser Optical Arrangement.- 5.2.2 Phase and Reflectance Measurement.- 5.2.3 Fundamentals of Sputter-Rate Determination.- 5.3 Experiments.- 5.4 Results and Discussion.- 5.4.1 Signal Artefacts.- 5.4.2 Phase and Reflectance Measurements During Sputtering.- a) Sputtering of Silicon Surfaces.- b) Doped Silicon.- c) Metal Film Material.- d) Transparent Material.- e) Opaque and Transparent Multilayers.- 5.5 Conclusion.- 5.A. Appendix.- A.1. Opaque Material.- A.2. Transparent Material.- References.- 6. Physical Limitations to Sputter Profiling at Interfaces - Model Experiments with Ge/Si Using KARMA..- 6.1 Background.- 6.1.1 General Problems Encountered in Sputter Profiling.- 6.1.2 Requirements for a Model Experiment.- 6.2 Experimental Approach.- 6.2.1 KARMA.- 6.2.2 Sample Preparation.- 6.3 Conversion of Raw Sputter Profiles into Depth Profiles.- 6.3.1 Establishing the Depth Scale.- 6.3.2 Escape-Depth Correction.- 6.3.3 Self-Consistent Determination of Effective Mean Free Paths.- 6.4 Depth Profiles of the Ge/Si Interface.- 6.4.1 Asymmetry of Depth Profiles.- 6.4.2 Broadening as a Function of Ion Mass and Energy.- 6.5 Dose Effects and Preferential Sputtering.- 6.5.1 Dose Effects.- 6.5.2 Preferential Sputtering.- 6.6 Depth Resolution in Sputter Profiling.- 6.6.1 Depth Resolution Limits.- 6.7 Summary and Outlook.- References.- 7. Depth Resolution and Quantitative Evaluation of AES Sputtering Profiles.- 7.1 Background.- 7.2 Calibration of the Depth Scale.- 7.3 Calibration of the Concentration Scale.- 7.4 Depth Resolution in Sputter Profiling.- 7.5 Determination of the Resolution function.- 7.5.1 Definition of Depth Resolution.- 7.5.2 Experimental Determination of Depth Resolution.- 7.5.3 Model Descriptions of Depth Resolution.- 7.6 Deconvolution Procedures.- 7.7 Conclusion.- References.- 8. The Theory of Recoil Mixing in Solids.- 8.1 Background.- 8.1.1 Nomenclature.- 8.2 Review of Recoil Mixing Models.- 8.2.1 Primary Recoil Implantation and Mixing.- 8.2.2 Cascade Mixing.- a) Random-Walk Models.- b) Transport Theory Approach.- c) Miscellaneous Approaches.- 8.3 General Formulation of Atomic Relocation Phenomena.- 8.3.1 Target Description.- a) Unbounded Total Density N(?,x).- b) Total Density Bounded to N(x) = No.- 8.3.2 Description of Atomic Relocation.- 8.3.3 Balance Equation for Atomic Relocation.- a) The Diffusion Approximation.- 8.4 Solutions to the Specific Mixing Models.- 8.4.1 Thermal Mixing and Thermal Diffusion.- 8.4.2 Recoil Mixing.- a) Cascade Mixing, Diffusion Approaches.- b) Cascade Mixing, Forthright Solutions.- 8.5 Summary and Outlook.- 8.6 List of Symbols.- References.- Additional References with Titles.

Synchrotron X‐ray powder diffraction

Abstract: The application of synchrotron X-ray radiation to powder diffraction is described. A perfect Si double-crystal monochromator at the Cornell High Energy Synchrotron Source (CHESS) was used in conjunction with Si, Ge, LiF and Al2O3 analyzers to investigate resolution and intensity characteristics at selected wavelengths between 1.07 and 1.54 A. The results obtained with Ge at 1.54 A gave a resolution Δd/d of 5 × 10−4 at 2θ = 30°, falling to 2 × 10−4 at 2θ = 140°. Analysis of the peak shapes is described in detail, with particular emphasis on the asymmetry observed at angles below 50° due to axial divergence effects. With a simple correction to allow for these, the peak shapes are found to be well represented by a convolution of Gaussian and Lorentzian components, the respective peak widths of these being related to the intrinsic resolution and sample broadening effects. It is pointed out that the use of a crystal analyzer should eliminate shifts in the Bragg-peak positions owing to the displacement-type aberrations which occur with conventional focusing-type diffractometers. Except for a constant zero error, the mean discrepancy in observed and calculated peak positions between 0 and 90° for reference samples CeO2, Al2O3 and NiO is found to be only about 0.003°. Finally, some general remarks are made about the application of the Rietveld profile technique to structural analysis from synchrotron powder diffraction data.

Three-dimensional structure of bovine pancreatic DNase I at 2.5 A resolution

Abstract: The three-dimensional structure of bovine pancreatic deoxyribonuclease I (DNase I) has been determined at 2.5 A resolution by X-ray diffraction from single crystals. An atomic model was fitted into the electron density using a graphics display system. DNase I is an alpha, beta-protein with two 6-stranded beta-pleated sheets packed against each other forming the core of a 'sandwich'-type structure. The two predominantly anti-parallel beta-sheets are flanked by three longer alpha-helices and extensive loop regions. The carbohydrate side chain attached to Asn 18 is protruding by approximately 15 A from the otherwise compact molecule of approximate dimensions 45 A X 40 A. The binding site of CA2+-deoxythymidine-3',5'-biphosphate (Ca-pdTp) has been determined by difference Fourier techniques confirming biochemical results that the active centre is close to His 131. Ca-pdTp binds at the surface of the enzyme between the two beta-pleated sheets and seems to interact with several charged amino acid side chains. Active site geometry and folding pattern of DNase I are quite different from staphylococcal nuclease, the only other Ca2+-dependent deoxyribonuclease whose structure is known at high resolution. The electron density map indicates that two Ca2+ ions are bound to the enzyme under crystallization conditions.

Atomic structure of [011] and [001] near-coincident tilt boundaries in germanium and silicon

Abstract: High resolution electron microscopy (HREM) has been used to investigate the structure of [001] and [011] pure tilt grain boundaries, both near-coincident and coincident, in crystals of Ge and Si. The resolution limit for the detection of the rigid body translation using HREM has been established, and compared with the α-fringe technique. Several structural units for primary and secondary relaxations have been found; it is shown that the core extension of a secondary relaxation covers one primary period. Likewise, a detailed comparison between simulated and experimental images for a σ = 9 grain boundary is presented. Some atomic sites in the period of the grain boundary appear to be preferentially attacked by impurities.

Beam interactions, contrast and resolution in the SEM

Abstract: SUMMARY Backscattered and secondary electrons are both used in the SEM for imaging purposes. The backscattered signal is the result of high angle elastic scattering events, while the secondary signal is the result of knock-on inelastic collisions. The characteristic differences between images in the two modes arise from the details of the relevant interactions in the two cases. In order to examine this in a quantitative manner Monte Carlo electron trajectory simulation techniques have been used. Calculations of the ultimate resolution and depth of information of the secondary and backscattered images are presented, together with simulations of the edge brightness effect in high resolution secondary images and an analysis of the microanalytical application of atom number contrast in the backscattered mode.

Imaging Properties of Modified Fresnel Zone Plates

Abstract: The imaging properties of two types of modified Fresnel zone plate are calculated and compared with those of the equivalent normal zone plate, with particular reference to their use in soft X-ray microscopy. The modifications considered are (1) central obstruction of the zone plates, which improves the resolving power but also leads to loss in efficiency and to a higher fraction of the energy in the outer parts of the diffraction pattern; and (2) a central zone plate with first-order focal length ƒ surrounded by outer zones with third-order focal length ƒ. This gives improved resolution and higher focused energy, but a smaller depth of focus and increased background.

Geometry Study of a High Resolution PET Detection System Using Small Detectors

Abstract: The current trend in positron emission tomography (PET) towards very high spatial resolution tomographs raises the problem of detecting high energy gamma-rays with a high spatial accuracy while preserving the overall sensitivity of the camera It is the purpose of this paper to investigate the effects of some geometrical parameters on the detection efficiency and intrinsic spatial resolution of a circular array of narrow closely packed detectors Two approaches were employed: the first one uses the Monte-Carlo technique to simulate the interactions of gamma-rays in the detectors; the second one is based on the linear attenuation on a beam of gamma-rays impinging on a detector array Aperture functions and modulation transfer functions were obtained for various configurations of the detectors and septas Expressions for the resolution and the spectral signal-to-noise ratio were derived It is shown that minor modifications to the geometry can improve both the efficiency and intrinsic resolution of the detector array

Optimization of the experimental resolution for small‐angle scattering

Abstract: The instrumental optimization conditions for most small-angle scattering experiments in which the data are azimuthally symmetric require that the scattered flight path be equal to the incident flight path. This is in contrast to a recent analysis which shows that under some conditions the incident and scattered flight paths are in a ratio of two to one. The equal flight-path condition is also valid for experiments measuring sharp (Bragg-like) peaks, or where the intensity is required at specific scattering vectors, as in low-angle diffraction of ordered or semiordered systems. The implications of the optimization conditions on the resolution and count rates at the detector are discussed for both types of experiment, and the dependence of the resolution on the spectrometer geometry is considered.

Characteristics of Small Barium Fluoride (BaF2) Scintillator for High Intrinsic Resolution Time-of-Flight Positron Emission Tomography

Abstract: The implementation of BaF2 for high spatial and TOF resolution PET cameras has been investigated. Intrinsic characteristics relating to the exclusive use of its fast 220 nm component, such as self-absorption, reflection material, optical coupling, energy resolution, timing resolution, number of fast photons, photofractrion, coincidence efficiency, and photomultiplier are presented. Extrinsic characteristics such as intrinsic spatial resolution variation with radius, the need for tungsten partition and the optimization of detector ring radius and tungsten partition thickness were investigated. Schemes of using multiple scintillators on a single PMT due to the absence of small PMT with small transit time spread and economic reasons are also presented.

Improvement of protein phases by coarse model modification

Abstract: A procedure is suggested for the refinement of a set of protein phases and for its extension to a higher resolution, which is a development of the approach of Agarwal & Isaacs [Proc. Natl Acad. Sci. USA, (1977), 74, 2835-2839]. A new set of phases is obtained by combining the starting phases with those calculated from a stereochemically non-conditioned coarse 'atomic' model which is automatically constructed and subjected to a least-squares refinement in reciprocal space. The method has been tested with actinidin data generated from atomic coordinates. Starting from the phases calculated to 3 A resolution and the amplitudes calculated to 2 A resolution a new set of phases was obtained with a mean error of 31° for 12 713 non-centrosymmetric reflections in the range to 2 A. The refinement of the phases to 3 A, resolution for γ-crystallin IIIb from calf lens and its extension to 2.7 A resolution resulted in a noticeable improvement in the electron density map.

Ferritin Structure and Function

Abstract: The molecular structure of horse spleen apoferritin has been solved using X-ray crystallographic techniques and the model refined at 2.8A resolution.

A position-sensitive detector for neutron powder diffraction

Abstract: A linear position-sensitive detector has been constructed with an array of three proportional counters and installed on the neutron powder diffractometer at the University of Missouri Research Reactor. The event position is determined by the charge division method using digital arithmetic. Consideration has been given to the corrections necessary to account for the differences in detection efficiency, solid angle, dead time and resolution for different points along the detector, caused by the finite thickness and tangential orientation of the detectors. An important feature of the instrument is an oscillating radial collimator with high transmission placed between the sample and detector which enables the system to have low background and a high signal-to-noise ratio. By considering the angular rate of accumulating data, the peak intensity per channel and the resolution of a Bragg diffraction peak, the enchancement factor for the position-sensitive detector is estimated to be about 50 compared to a single detector with a step scan. This allows experiments to be performed with better statistics, better resolution and smaller samples, and enables marginal experiments to be worthwhile. A number of structures have been refined with this detector using Rietveld analysis including a new rare-earth-transition-metal phase whose structure has been solved from the powder diffraction data.

A Unified Treatment of Resolution Strategies for Logic Programs.

Abstract: ADDRESS: P.O. Box 407, Magdalena, New Mexico 87825-0407 PHONE: 505 854 2253 (home), 505 835 5396 (work) PERSONAL: Date of birth: December 24, 1944 Place of birth: Bordeaux, France Nationality: US citizen Marital status: Married, Catherine Marcoussis, 12/2/68 Children: One daughter, Sarah Suzanne, b. Canada 4/1/71 EDUCATION: Ph.D., Computer Science, 1973, Purdue University. D.E.A., Computer Science 1969, Universit Paris, France. M.Sc., Mathematics, 1968, Universit de Paris, France.

Alpha‐particle detection with superconductor detectors

Abstract: The Sn‐SnOx‐Sn tunnel junction of a crossed‐film type, cooled down to 0.32 K, is demonstrated as a good energy resolution detector for α particles. Three junctions (A, B: 300×400 μm2×3000 A and C: 200×300 μm2×3000 A) have been used to detect 5.3‐MeV α particles from 210Po. The present work suggests that reduction of the leakage current enables us to utilize superconducting tunnel junctions with large volumes. Such a device could provide a new means for detection of unclear radiations.