摘要:

The present paper describes a newly built four‐circle diffractometer for X‐ray structure analysis with synchrotron radiation and particularly with the use of anomalous scattering due to a relatively heavy atom or atoms. This system is installed on beamline 10A in the Photon Factory (PF) in the National Laboratory for High Energy Physics, and its 2θarm is rotated in the plane perpendicular to the floor. In this system, a one‐crystal monochromator of the horizontal dispersion type is used. When the 111 reflection from a Si monochromator is used, the energy resolution is about 14 eV for X‐rays of about 10 keV. The four‐circle diffractometer, a monochromator and an alignment carriage for the four‐circle diffractometer can be controlled by a computer and, therefore, once the optical system has been adjusted for X‐rays of a certain wavelength then the diffraction intensity values from a single‐crystal sample can be automatically measured at any wavelength chosen. An NaI(Tl) scintillation counter is used for both the intensity measurements of the diffraction beam and the monitoring of the incident‐beam intensity. In order to make the counting rate high, 50 MHz counters are used for the above two intensity measurements. In this set‐up, about 1% counting loss occurs at a counting rate of 55000 s−1. With this diffractometer, diffraction intensities have been measured for a hemimorphite single‐crystal at two energy values on both sides of the ZnKabsorption edge. The variation of the intensities, after being normalized by the standard reflection and the monitor counts, was somewhat large in comparison with the data measured in the usual laboratory. From these data measured at two energy values, a so‐called two‐wavelength anomalous‐scattering difference Patterson function was calculated, where the Zn–Zn vector peaks were t

ISSN:1600-5767    

DOI:10.1107/S0021889884011419

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

The absorption factor for the scattering of X‐rays or neutrons in cylindrical samples is calculated by numerical integration for the case in which the absorption coefficients of the incident and scattered beams are not equal. An extensive table of values having an absolute accuracy of 10–4is given in a companion report [Sears (1983). Atomic Energy of Canada Limited, Report No. AECL‐8176]. In the present paper an asymptotic expression is derived for the absorption factor which can be used with an error of less than 10−3for most cases of interest in both neutron inelastic scattering and neutron diffraction in c

ISSN:1600-5767    

DOI:10.1107/S0021889884011420

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

The potential of synchrotron radiation sources (SRS) for the topography of polytypic structures has been assessed. Thus it has been found that the white radiation and low divergence characteristics of the SRS are uniquely suitable for accommodating the complications of mixed polytypes, heavily distorted crystals and edge‐reflection topography. Computer aids are particularly important for separating contributions from mixed polytype crystals. All these techniques are illustrated in this paper using silicon carbide as a prototype polytypic crysta

ISSN:1600-5767    

DOI:10.1107/S0021889884011432

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

Selective application of only parameter shifts with large eigenvalues permits singular or near‐singular least‐squares problems to be solved. This `eigenvalue filtering' process has been applied to the refinement of the unit‐cell, crystal‐orientation and reflecting‐range parameters needed to process oscillation films. Eigenvalue filtering permits automatic identification and determination of just those combinations of parameters that are most relevant for processing films from a particular crystal in a particular setting. The procedure may be carried out with alignment information from an individual data film or from multiple alignment films taken at different spindle angles. Eigenvalue filtering has been incorporated in a refinement program that minimizes discrepancies between observed and calculated fractions recorded for partially recorded reflections. This permits the reflecting range (combined mosaic spread, beam cross‐fire, and wavelength spread) to be refined along with the unit‐cell and orientation parameters. Observed fractions recorded may be obtained by visual estimation prior to film scanning, or the program may be used in a `post‐refinement' mode with data obtained from actual intensity measurements. The relatively unreliable information represented by the positions of spots on the film is not used except for indexing. The program handles crystals of any symmetry in any setting and the oscillation spindle may be inclined from normal beam geometry. The mis‐setting angles are defined in a crystal‐fixed coordinate system, making it easy to compare refinement results for data taken at diffe

ISSN:1600-5767    

DOI:10.1107/S0021889884011444

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A single‐stage computer procedure to calculate an electron density map suitable to detect errors in a tentative macromolecular model has been developed. In this procedure, an atom of the tentative model does not contribute to the phases used to calculate electron density values at or near its current position, that is within the region containing it and a neighborhood surrounding that region. In this way, the phases used to calculate electron density values within a region are not biased by the model atoms contained within that region or its neighborhood. The number of atoms which are omitted for a given region is maintained at a small fraction of the total structure so that the phases used to calculate electron density values may still be a good approximation to the phases of the complete structure. The procedure was used to improve the model of the Fab portion of the mouse galactan‐binding immunoglobulin J539 (IgA2, κ), which contains 431 resi

ISSN:1600-5767    

DOI:10.1107/S0021889884011456

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

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 con

ISSN:1600-5767    

DOI:10.1107/S0021889884011468

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

With a compact arrangement using single‐crystal collimator and analyzer (silicon 220 reflection) of channel‐cut type, angle‐resolved scattering (ARS) curves of X‐rays (CuKα1) for optically flat mirrors have been measured at various glancing angles,ω, of X‐rays to the mirror surface with an angular resolution of 4′′. Weak scattered intensity (10−3–10−5of the specularly reflected beam) is observed over an angular range of a few hundred arc s below and above the direction of specular reflection. Whenωis close to the critical angle for total reflection, the scattered intensity at the low‐angle tail of ARS curves is higher than that at the high‐angle tail. This asymmetric tail profile of ARS curves is explained by simply superposing intensities of specularly reflected beams from surface eleme

ISSN:1600-5767    

DOI:10.1107/S002188988401147X

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

This paper describes a method of indexing back‐reflection Laue patterns by microcomputer which has been successfully used for the cubic system. A program using this procedure has been shown to give possible indexing schemes within 12 min. A simple method of measuring the angle between the normal to the crystal surface and a nearby symmetry pole to within ~0.1°, which does not require a highly accurately machined or perfectly aligned camera, is also descr

ISSN:1600-5767    

DOI:10.1107/S0021889884011481

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

A method of processing X‐ray small‐angle data for a three‐phase system, based on the hypothesis formulated by Goodisman, Brumberger&Cupelo [J. Appl. Cryst.(1981),14, 305–308] has been extended to the determination of the polydispersity of one phase. For supported metal catalysts, the characterization of the metal phase is obtained from its correlation function evaluated from a balanced subtraction between those of the catalyst and of the support, measured in two separate experiments. A fairly good agreement with electron microscopy data is o

ISSN:1600-5767    

DOI:10.1107/S0021889884011493

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY

摘要:

The purpose of this paper is to establish an explicit correlation between the values and probable deviations of the observed intensities and the potential accuracy of the structure eventually determined using those intensities. This problem involves several steps: the choice in real space of a reference stochastic model defining the ideal state of ignorance; the determination in reciprocal space of the probability density corresponding to that model and of the probability law corresponding to the experiment; the determination of the information; the transfer of the information from reciprocal to real space; the interpretation of the information in terms of structural resolution. A rigorous treatment of this problem is hindered by the inadequate knowledge of the statistical correlations between different reflections; more realistically, it is convenient to address a simpler problem, corresponding to an ideal experiment in which the intensities relevant to each reflection are measured independently of all the other reflections. In this case the mathematical problems can easily be solved and a parameter introduced – called projection information – whose value can be determined. This information is the sum of two terms, one associated with the modulus, the other with the phase of each reflection. By resorting to a mathematical model it is possible to make use of the projection information to determine the value of a parameter akin to structure resolution. The final result of this work is an operational definition of resolution, based upon the stochastic properties of the experimental observations. This resolution can be used to assess and compare the intrinsic quality of different structure analyses and of different stages of one structure determination, before the structure is sol

ISSN:1600-5767    

DOI:10.1107/S002188988401150X

出版商:International Union of Crystallography    

年代:1984

数据来源: WILEY