scholarly journals The effect of the diffusion time and pulse gradient duration ratio on the diffraction pattern and the structural information estimated from q-space diffusion MR: Experiments and simulations

2008 ◽  
Vol 194 (2) ◽  
pp. 230-236 ◽  
Author(s):  
Amnon Bar-Shir ◽  
Liat Avram ◽  
Evren Özarslan ◽  
Peter J. Basser ◽  
Yoram Cohen
Keyword(s):  
Diffraction Pattern ◽  
Structural Information ◽  
Diffusion Time ◽  
Diffusion Mr ◽  
Pulse Gradient

Measurement and Reduction of Radiation Damage in Frozen Hydrated Crystalline Specimens

1978 ◽  
Vol 36 (3) ◽  
pp. 70-77 ◽  
Author(s):  
R.M. Glaeser ◽  
S.B. Hayward
Keyword(s):  
Diffraction Pattern ◽  
Structural Information ◽  
Structural Disorder ◽  
Structural Features ◽  
Biological Macromolecules ◽  
Diffraction Intensity ◽  
Object Structure ◽  
Specimen Material ◽  
Unit Cells ◽  
Identical Unit

Highly ordered or crystalline biological macromolecules become severely damaged and structurally disordered after a brief electron exposure. Evidence that damage and structural disorder are occurring is clearly given by the fading and eventual disappearance of the specimen's electron diffraction pattern. The fading and disappearance of sharp diffraction spots implies a corresponding disappearance of periodic structural features in the specimen. By the same token, there is a oneto- one correspondence between the disappearance of the crystalline diffraction pattern and the disappearance of reproducible structural information that can be observed in the images of identical unit cells of the object structure. The electron exposures that result in a significant decrease in the diffraction intensity will depend somewhat upon the resolution (Bragg spacing) involved, and can vary considerably with the chemical makeup and composition of the specimen material.

Constraint-induced direct phasing method

2017 ◽  
Vol 73 (1) ◽  
pp. 54-60
Author(s):  
Hongliang Xu
Keyword(s):  
Single Crystal ◽  
Diffraction Pattern ◽  
Powder Diffraction ◽  
Diffraction Data ◽  
Structural Information ◽  
Initial Structure ◽  
X Rays ◽  
Structural Refinement ◽  
Low Resolution ◽  
Crystallization Experiments

The best and most detailed structural information is obtained when the diffraction pattern of a single crystal a few tenths of a millimetre in each dimension is analyzed, but growing high-quality crystals of this size is often difficult, sometimes impossible. However, many crystallization experiments that do not yield single crystals do yield showers of randomly oriented microcrystals that can be exposed to X-rays simultaneously to produce a powder diffraction pattern. Although single-crystal diffraction data consist of discrete spots or X-ray reflections, the diffraction of microcrystals in a powder forms rings so that the reflections overlap. Thus, the analysis is more challenging due to unavoidable errors in the structure-factor amplitudes and the low-resolution data available for structure determination. This paper introduces a constraint-induced phasing method that (i) improves structure solutions measured by success rate, quality of solutions and various figures of merit, and (ii) extends low-resolution powder diffraction data to atomic resolution by adding unmeasured reflections. Application results have shown clearly that the constraint-induced phasing method is an effective way to produce initial structure models that are suitable for further structural refinement and completion.

Measurement and reduction of damage in frozen hydrated crystalline specimens

1978 ◽  
Vol 36 (2) ◽  
pp. 10-11
Author(s):  
Robert M. Glaeser ◽  
Steven B. Hayward
Keyword(s):  
High Resolution ◽  
Electron Diffraction ◽  
Diffraction Pattern ◽  
Electron Diffraction Pattern ◽  
Cell Membranes ◽  
Structural Information ◽  
Protein Crystals ◽  
Biological Macromolecules ◽  
Diffraction Intensity ◽  
Specimen Material

Highly ordered or crystalline biological macromolecules become severely damaged and disordered after a brief electron exposure, as may be seen by observing the fading and loss of the specimen's electron diffraction pattern. Loss of the diffraction pattern intensity has, in turn, a one-to-one relationship with a loss of the possibility to see structural information in the image. The actual electron exposure that results in a significant decrease in the diffraction intensity will depend first of all upon the resolution (Bragg spacing) involved, and in some cases upon the chemical make-up and composition of the specimen material. For high resolution features (in the range 3Å to 5Å resolution) of specimens such as protein crystals and cell membranes, the structure can become damaged and disordered after an exposure of about 1 electron/Å2 or less. Roughly speaking, this exposure is about 104 times lower than that which is required to produce a statistically defined image at high resolution.

Meridional Circulation, Turbulence, and Diffusion Processes in Stars

1976 ◽  
Vol 32 ◽  
pp. 109-116 ◽  
Author(s):  
S. Vauclair
Keyword(s):  
Convection Zone ◽  
Diffusion Processes ◽  
Meridional Circulation ◽  
Diffusion Time ◽  
Work In Progress ◽  
First Results ◽  
Stellar Envelopes ◽  
And Diffusion ◽  
Turbulence And Diffusion ◽  
Hr Diagram

This paper gives the first results of a work in progress, in collaboration with G. Michaud and G. Vauclair. It is a first attempt to compute the effects of meridional circulation and turbulence on diffusion processes in stellar envelopes. Computations have been made for a 2 Mʘstar, which lies in the Am - δ Scuti region of the HR diagram.Let us recall that in Am stars diffusion cannot occur between the two outer convection zones, contrary to what was assumed by Watson (1970, 1971) and Smith (1971), since they are linked by overshooting (Latour, 1972; Toomre et al., 1975). But diffusion may occur at the bottom of the second convection zone. According to Vauclair et al. (1974), the second convection zone, due to He II ionization, disappears after a time equal to the helium diffusion time, and then diffusion may happen at the bottom of the first convection zone, so that the arguments by Watson and Smith are preserved.

Analysis of electron-diffraction intensity profiles using a photodiode-array detection system

1983 ◽  
Vol 41 ◽  
pp. 322-323
Author(s):  
J. B. Warren
Keyword(s):  
Electron Diffraction ◽  
Diffraction Pattern ◽  
Detection System ◽  
High Energy ◽  
Photographic Emulsion ◽  
Diffraction Intensity ◽  
Silicon Photodiode ◽  
Photodiode Array Detection ◽  
Analog To Digital ◽  
Photodiode Array

Electron diffraction intensity profiles have been used extensively in studies of polycrystalline and amorphous thin films. In previous work, diffraction intensity profiles were quantitized either by mechanically scanning the photographic emulsion with a densitometer or by using deflection coils to scan the diffraction pattern over a stationary detector. Such methods tend to be slow, and the intensities must still be converted from analog to digital form for quantitative analysis. The Instrumentation Division at Brookhaven has designed and constructed a electron diffractometer, based on a silicon photodiode array, that overcomes these disadvantages. The instrument is compact (Fig. 1), can be used with any unmodified electron microscope, and acquires the data in a form immediately accessible by microcomputer.Major components include a RETICON 1024 element photodiode array for the de tector, an Analog Devices MAS-1202 analog digital converter and a Digital Equipment LSI 11/2 microcomputer. The photodiode array cannot detect high energy electrons without damage so an f/1.4 lens is used to focus the phosphor screen image of the diffraction pattern on to the photodiode array.

Light Optical Diffraction Studies of Macromolecular Ultrastructure

1970 ◽  
Vol 28 ◽  
pp. 258-259
Author(s):  
Glen B. Haydon
Keyword(s):  
High Resolution ◽  
Diffraction Analysis ◽  
Diffraction Pattern ◽  
Electron Micrographs ◽  
Optical Diffraction ◽  
Electron Micrograph ◽  
Its Analysis ◽  
Resolution Electron ◽  
Diffraction Patterns

Analysis of light optical diffraction patterns produced by electron micrographs can easily lead to much nonsense. Such diffraction patterns are referred to as optical transforms and are compared with transforms produced by a variety of mathematical manipulations. In the use of light optical diffraction patterns to study periodicities in macromolecular ultrastructures, a number of potential pitfalls have been rediscovered. The limitations apply to the formation of the electron micrograph as well as its analysis.(1) The high resolution electron micrograph is itself a complex diffraction pattern resulting from the specimen, its stain, and its supporting substrate. Cowley and Moodie (Proc. Phys. Soc. B, LXX 497, 1957) demonstrated changing image patterns with changes in focus. Similar defocus images have been subjected to further light optical diffraction analysis.

Electron Diffraction Analysis of Microtwin Structures in Regrown (III) Silicon

1982 ◽  
Vol 40 ◽  
pp. 460-461
Author(s):  
P. Ling ◽  
R. Gronsky ◽  
J. Washburn
Keyword(s):  
Electron Diffraction ◽  
Ion Implantation ◽  
Diffraction Analysis ◽  
Diffraction Pattern ◽  
Direct Consequence ◽  
The Other ◽  
Electron Diffraction Analysis ◽  
Defect Microstructures ◽  
Ion Implanted

The defect microstructures of Si arising from ion implantation and subsequent regrowth for a (111) substrate have been found to be dominated by microtwins. Figure 1(a) is a typical diffraction pattern of annealed ion-implanted (111) Si showing two groups of extra diffraction spots; one at positions (m, n integers), the other at adjacent positions between <000> and <220>. The object of the present paper is to show that these extra reflections are a direct consequence of the microtwins in the material.

Video-Enhanced Microscopy--Better Visibility of Plant Cell Structures

1982 ◽  
Vol 40 ◽  
pp. 182-185
Author(s):  
N.S. Allen ◽  
R.D. Allen
Keyword(s):  
Diffraction Pattern ◽  
Theoretical Basis ◽  
Numerical Aperture ◽  
Gain Control ◽  
Control Unit ◽  
Camera Control ◽  
Variable Gain ◽  
Contrast Method ◽  
Fine Detail ◽  
Cell Structures

Various methods of video-enhanced microscopy combine TV cameras with light microscopes creating images with improved resolution, contrast and visibility of fine detail, which can be recorded rapidly and relatively inexpensively. The AVEC (Allen Video-enhanced Contrast) method avoids polarizing rectifiers, since the microscope is operated at retardations of λ/9- λ/4, where no anomaly is seen in the Airy diffraction pattern. The iris diaphram is opened fully to match the numerical aperture of the condenser to that of the objective. Under these conditions, no image can be realized either by eye or photographically. Yet the image becomes visible using the Hamamatsu C-1000-01 binary camera, if the camera control unit is equipped with variable gain control and an offset knob (which sets a clamp voltage of a D.C. restoration circuit). The theoretical basis for these improvements has been described.

Convergent Beam Electron Diffraction

1978 ◽  
Vol 36 (3) ◽  
pp. 304-315
Author(s):  
G. Lehmpfuhl
Keyword(s):  
Electron Diffraction ◽  
Diffraction Pattern ◽  
Crystal Surface ◽  
Diffraction Spot ◽  
Crystal Thickness ◽  
Large Area ◽  
Electron Microscopic ◽  
Additional Information ◽  
Microscopic Investigations

Introduction In electron microscopic investigations of crystalline specimens the direct observation of the electron diffraction pattern gives additional information about the specimen. The quality of this information depends on the quality of the crystals or the crystal area contributing to the diffraction pattern. By selected area diffraction in a conventional electron microscope, specimen areas as small as 1 µ in diameter can be investigated. It is well known that crystal areas of that size which must be thin enough (in the order of 1000 Å) for electron microscopic investigations are normally somewhat distorted by bending, or they are not homogeneous. Furthermore, the crystal surface is not well defined over such a large area. These are facts which cause reduction of information in the diffraction pattern. The intensity of a diffraction spot, for example, depends on the crystal thickness. If the thickness is not uniform over the investigated area, one observes an averaged intensity, so that the intensity distribution in the diffraction pattern cannot be used for an analysis unless additional information is available.

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