Dudley Creagh

The resolution of discrepancies in tables of photon attenuation coefficients

Abstract This paper examines the extent to which theoretical and experimental tabulations of the photon attenuation coefficients are consistent with one another. It sets out the criteria which must apply if accurate measurements are to be made and suggests the manner in which the theoretical data is best combined to give realistic comparisons with experimental data.

The Australian diffractometer at the Photon Factory

Outlined are design features of a versatile high‐resolution two‐axis diffractometer that is being constructed for operation at the Photon Factory as an Australian national facility. The instrument features optional use of multiple‐imaging plates on a translating cassette to allow rapid recording of an almost complete range of data covering both the high‐angle and small‐angle scattering regime or alternatively the use of electronic detectors. The instrument will be capable of operation in various modes including the following: (i) high‐resolution powder diffraction with single‐channel counter and crystal analyzer, (ii) high‐resolution, high‐speed powder diffraction in the Debye–Scherrer mode with imaging plates as recording medium, either stationary or translating (for time‐dependent studies), (iii) small‐angle x‐ray scattering with imaging plates as recording medium, (iv) protein crystallography in screenless Weissenberg mode, and (v) two‐ or three‐axis single‐crystal diffractometry. The salient features ...

The use of radiation for the study of material of cultural heritage significance

Abstract For the indigenous people of Northern Australia the expression of their experience of life, their “dreaming”, is in the form of painting, usually on the bark stripped from trees growing in their tribal lands. These are often works of great beauty and the major collecting institutions in Australia and elsewhere have significant holdings of Aboriginal bark paintings. A wide range of analytical techniques (optical microscopy, FTIR microscopy, Raman microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy, and synchrotron radiation X-ray diffraction) has been used in a project to determine how best to conserve Aboriginal bark paintings.

Chapter 1 Synchrotron Radiation and its Use in Art, Archaeometry, and Cultural Heritage Studies

Synchrotron radiation has become an increasingly important tool for research in the fields of art, archaeometry, and the conservation of objects of cultural heritage significance. Scientists using conventional laboratory techniques are finding that the fundamental characteristics of synchrotron radiation — high brightness, low divergence, and highly linear polarization — can be used to give information not readily available in the laboratory context. In the authors experience, experiments do not translate directly from the laboratory to the synchrotron radiation laboratory: there are subtle differences in the use of what seem to be similar experimental apparatus. To achieve the best results, the research scientist must be able to discuss his or her research aims meaningfully with beamline scientists. And to be able to do this, the research scientist must have an understanding of the properties of synchrotron radiation, and also the various techniques that are available at synchrotrons but are unavailable in the laboratory. The chapter includes a discussion of synchrotron radiation and its properties, monochromators, detectors, and techniques such as infrared (IR) microscopy; soft X-ray spectroscopy; X-ray diffraction; micro-X-ray diffraction and X-ray fluorescence analysis; X-ray absorption spectroscopy (XAS), including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES), and X-ray tomography. The underlying principles of these techniques are discussed here. Later in this book, authors will address these techniques in more detail.

On the design of a high‐speed combined high‐resolution powder diffractometer and small‐angle scattering system with time‐resolution capability based on the use of imaging plates and CCCC monochromators

Design features of a proposed high‐resolution, high‐speed powder diffractometer for operation at a synchrotron source are described. A key feature of the design is the use of imaging plates to record an almost complete range of data including both the high‐angle and the small‐angle scattering regime. The x‐ray optics involve the use of a condensing‐collimating channel‐cut monochromator (CCCC) to achieve narrow parallel beams with cross sections of the order of 50 μm in the plane of diffraction, so that geometrical aberrations are very low and the beam cross section is well matched to the spatial resolution of the imaging plate detector. Angular resolution in 2θ of the diffractometer is easily variable, and designed to range from 0.01° upwards. Optional translation of the detector perpendicular to the incident‐beam direction makes it possible to obtain time‐resolved data. Applications of the instrument include high‐resolution powder data collection for Rietveld analysis, studies of dynamic phenomena such a...

An eight-position capillary sample spinning stage for the diffractometer at BL20B at the Photon Factory

An eight-position capillary sample spinning stage has been developed for use in conjunction with the versatile vacuum diffractometer (BIGDIFF) at BL20B at the Photon Factory. BIGDIFF is often used in its powder diffraction mode using powders mounted in capillaries and up to eight imaging plates to record the diffraction pattern from the sample. Using the multiple spinning stage a number of diffraction patterns can be recorded on the imaging plates if the imaging-plate cassette is moved behind the Weissenberg screen to a new position after exposure of the sample to the beam. Not only is this system more efficient in terms of time saved in the pumping-down process, but also it has the advantage of allowing the diffraction patterns of standards to be recorded, thereby calibrating both the angle scale of the diffractometer and the intensity scales of the imaging plates absolutely.

On the use of photoelectric attenuation measurements for the determination of the anomalous dispersion correction ƒ′ for X-rays

Abstract Suggestions are made about the way photo-electric attenuation measurements may best be used to estimate the X-ray anomalous dispersion correction ƒ′.

X-ray and NMR investigations of cobalt

Abstract X-ray diffraction and zero-field NMR spectra were studied for powdered and bulk polycrystalline and single-crystal specimens of cobalt. The nature of the h.c.p.-f.c.c. phase transition was studied. It is shown that the two stacking fault NMR lines are both due to deformation faults in h.c.p. regions.

Tables of X-ray absorption corrections and dispersion corrections: the new versus the old

Abstract This paper compares the data on X-ray absorption coefficients calculated by Creagh and Hubbell and tabulated in International Tables for Crystallography, vol. C, ed. A.J.C. Wilson (1990) section 4.2.4 [1] with empirical (Saloman, Hubbell and Scofield, At. Data and Nucl. Data Tables 38 (1988) 1, [6]) and semi-empirical (Hubbell, McMaster, Kerr Del Grande and Mallett, in: International Tables for Crystallography, vol. IV, eds. Ibers and Hamilton (Kynoch, Birmingham, 1974) [2]) tabulations as well as the renormalized relativistic Dirac-Hartree-Fock calculations of Scofield [6]. It also makes comparisons of the real part of the dispersion correction ƒ′(ω, 0) and tabulated in ref. [1], with theoretical data sets (Cromer and Liberman, J. Chem. Phys. 53 (1970) 1891, and Acta Crystallogr. A37 (1981) 267 [4,5]; Wang, Phys. Rev. A34 (1986) 636 [85]; Kissel, in: Workshop Report on New Dimensions in X-ray Scattering, CONF-870459 (Livermore, 1987) p. 9 [86]) and data collected using a variety of experimental techniques. In both cases the data tabulated in ref. [1] is shown to give improved self-consistency and agreement with experiment.

Domain wall and “domain” 59Co NMR in hexagonal cobalt

Abstract The ‘domain’ NMR signal for hcp cobalt is shown to be from nuclei at edges of domain walls with a spin-lattice relaxation time larger than that at the wall centres by a factor of 13 ± 3.