Book Review: RiMG v. 63
Reviews in Mineralogy and Geochemistry, Volume 63, Neutron Scattering in Earth Sciences edited by Hans-Rudolf Wenk, The Mineralogical Society of America, Washington D.C., 2006, 471 pp. US $40 (ISBN 978-0939950-75-1).
Advanced photon and neutron sources are large-facility materials analysis centers that have become available to earth scientists in the last few decades. Synchrotron x-rays are perhaps more familiar because small scale x-ray diffraction sources are commonly used for atomic structure determination of geological, and indeed all, material samples. Although neutron scattering presents many advantages and opportunities for the study of geological materials, it is still a little mysterious and the user community among earth scientists is still quite small.
This volume of RiMG (#63), edited by Hans-Rudi Wenk, a distinguished earth scientist as well as an advocate for the use of neutrons in geosciences, provides a sourcebook of background knowledge, facilities, and applications of neutron scattering in geosciences. Its aim, along with the symposium that accompanied it, is to have experts present and explain the broad variety of applications that neutron scattering has for geological materials so to improve the visibility –and viability – of this powerful technique for earth scientists. This volume covers a wide range of relevant applications, with good examples and extensive references – well worth it for anyone contemplating or involved in neutron scattering measurements.
The advantages that neutron scattering offers for the study of earth science materials stem from the fundamentally different way that neutrons interact with materials, compared to x-rays and electrons. The neutron responds to the strong nuclear force and does not feel the Coulomb forces from the electrons that dominate x-ray and electron scattering. The nuclear cross section is small and varies strongly between nuclei - explaining the deep penetration of neutrons into matter, and providing a contrast almost invisible to x-rays. The ability to observe hydrogen (and so water) is noted by several authors - a photograph in chapter 2 shows a neutron image of a rose taken through several cm of lead shielding. Diffraction from crystals produces symmetry, spacing and strain information as with x-rays but with nuclear details added. Neutron scattering is sensitive to magnetic structures.
Many of the authors point out that some of these measurements can now be done using the greatly enhanced intensity of synchrotron x-rays. The combined advantages, however, of nearly-transparent sample chambers, large sample size, sensitivity to the lightest elements and elemental/isotopic contrast produces unique, complementary information for material analysis. The current commissioning and buildup of neutron beamlines at the SNS facility presents an almost irresistible opportunity for mineral and rock characterization in the earth sciences.
Chapter 1 by John Parise starts with the neutron itself and the properties that make it uniquely suited for materials analysis, but the majority of the chapter covers the details of the scattering interaction – necessary to interpret the scattered/diffracted intensities. Table 3 is a useful summary of differences from x-ray scattering. Chapter 2 by Sven Vogel and Hans-Georg Priesmeyer, covers much of the “nuts and bolts” of the production, control and measurement of neutrons. They discuss reactor and pulsed sources and the forms of experiments that can be carried out on various beamlines and the detector technologies that are used for each.
Nancy Ross and Christina Hoffmann in chapter 3 bring in the details of applications in single-crystal diffraction with neutrons, both in the instruments available and the analysis of several examples. The main drawback is the need for a relatively large crystal. Chapter 4 by Robert Von Dreele, one of the authors of GSAS, presents the use of the Rietveld method for the analysis of neutron powder diffraction data. Nearly all neutron data analysis is done with one of the handful of programs available. He covers the contributions to peak profiles and spacings from a number of factors. Karsten Knorr and Wulf Depmeier follow this in chapter 5 with a series of powder diffraction examples, to determine mineral structures where light or neighboring elements appear. They note that perhaps the most noticeable drawback for neutrons is the limited number of sources available.
Chapter 6 by Richard Harrison introduces the magnetic scattering capabilities of neutrons, deriving the expressions needed to extract magnetic structure. There are several examples of studies in iron-bearing minerals and lots of excellent illustrations of the moment patterns in the various crystal lattices.
Chapters 7 and 8 present applications for the kinetic processes involved in material transformations, introducing time dependence as an experimental factor. In chapter 7 Simon Redfern looks at order-disorder transformations using controlled temperature changes and time dependent diffraction measurements.He presents examples of cation ordering in a number of minerals, particularly where the neighboring elements Mg/Al/Si are involved.
Werner Kuhs and Thomas Hansen in chapter 8 discuss time-dependent processes in general and some experimental factors. They provide examples of the kinetics of phase transformations in water ices and clathrate and gas hydrates at controlled temperature and pressure. Chapter 9 by John Parise describes neutron scattering from samples at high pressure. The experimental aspects of cells which are relatively transparent to neutrons and capable of maintaining pressures up to several GPa on a relatively large sample are discussed. Experimental examples with minerals and clathrates are presented as well as a tour of the technology available at the SNAP beamline at SNS. In chapter 10 Chun-Keung Loong introduces the complications of inelastic processes where the neutron scatters with a change in energy due to collisions with protons or an exchange with lattice excitations. The information derived, displayed in a number of examples, involves changes in phonon spectra and mapping of crystal field levels in rare-earth phosphates and observation of the diffusion of water in clays.
Chapters 11, 12 and 13 describe the use of neutron scattering for disordered materials – glasses, liquids and materials where no long range order exists. Thomas Proffen introduces the pair distribution function (PDF), essentially the number distribution of pairs of scatterers as a function of separation, in chapter 11. He covers the process of taking data and backgrounds to produce the total scattering function which is analyzed with one of a number of available programs (there’s a list) into the PDF. There is model dependence in the analysis, which needs some care for reliable results. His examples include measurements of bond length changes, variable-scale domain structure formation and quartz and quartz-glass structures. In chapter 12, Martin Wilding and Chris Benmore discuss the thermodynamics of liquids and the phase transformations into ordered and disordered solids, and the relationship of their properties to the structural information available from the PDF. There is a collection of studies on geologically relevant materials, water, amorphous ices, various forms of silicate glasses and oxide liquids. Chapter 13 by David Cole, Kenneth Herwig, Eugene Mamontov and John Larese discusses the application to fluids in the geochemical and geophysical environment. Water, aqueous solutions and other fluids (could be gases) play a large role in the chemistry of minerals and rocks. Local structure in fluids and the behavior of confined or surface layered fluids can be explored by PDF techniques and inelastic /quasielastic (QENS) scattering techniques. QENS, mentioned briefly in Ch. 10, has a useful primer here. For studies of structures much larger than the atomic scale – up to micron sizes - a specialized technique is available. Andrzej Radlinski discusses small-angle neutron scattering (SANS) in chapter 14. Using thin films in a transmission mode, monochromatic neutrons, and measurements of the scattered intensities close to the beam direction – usually within a few degrees, information on microstructure, such as porosity or roughness may be extracted. The author covers the theoretical basis, particularly the fractal nature of some microstructures, and examples of applications to porosity determination in hydrocarbon and water-bearing rocks and clays.
Hans-Rudi Wenk presents in chapter 15 the measurement of texture – a preferred crystallographic orientation in the grains of a polycrystalline material. The aim is a 3-D map of the preferred orientations within a macroscopic sample. The penetrating power and large sample size available to neutron scattering are employed here. Examples include the effects of uniaxial stress on samples and the reconstruction of geologic deformation histories in core samples.
The measurement of internal strains and the corresponding stresses are the subject of chapter 16 by Mark Daymond. These stresses are critical to the deformation and ultimate strength of materials, and operate at a range of scales in polycrystals. The mapping of these stresses, and the way they vary under load is done by diffraction from a defined internal volume. Many facilities have specific beamlines to apply uniaxial stresses to samples during these measurements. Chapter 17 by Bjoern Winkler covers applications of imaging with neutrons – creating 2-D and 3-D images of the internal structure of an object. The images are essentially maps of the absorption of the neutron intensity along a given path. Multiple 2-D images can be assembled as in conventional tomography to a 3-D image. The difference in absorption cross-sections for different nuclei provides the contrast in the image – hydrogen/water are “darker” than most materials producing maps of hydrogen-rich volumes inside other materials. The advances in 2-D neutron imaging detectors are discussed and applications to time-dependent processes – neutron “movies”.
Dr. Timothy W. Darling
Physics Dept. MS 220
University of Nevada
Reno, NV 89557 USA