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Current Status of Neutron-Scattering Research and Facilities in the United States (1984)

Chapter: RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE

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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 53
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 54
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 55
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 56
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 57
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 58
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 59
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 60
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 61
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 62
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 82
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
Page 83
Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
×
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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Suggested Citation:"RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES: COMPARISONS WITH EUROPE." National Research Council. 1984. Current Status of Neutron-Scattering Research and Facilities in the United States. Washington, DC: The National Academies Press. doi: 10.17226/835.
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5. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE In this chapter we present selected summaries of important research areas and accomplishments during the past 6 years. These summaries have been coordinated by experts on the panel in the appropriate scientific disciplines involved and reviewed by the panel as a whole. The focus is generally on U.S. neutron-scattering work, except in cases where activity has been dominated by special capabilities abroad (e.g., high-resolution spectroscopy, broad areas of polymer and chemical research). Where the work involves new areas of scientific applications of neutrons that have evolved recently (e.g., polymer and materials science, biology, chemical spectroscopy), we have tried to provide more background to explain the special role of neutrons in these cases. An attempt has also been made to point out both new directions in science and examples of broad areas where instrumentation at U.S. neutron sources does not allow critical scientific opportunities to be pursued. CONDENSED-MATTER PHYSICS Physicists were the first scientists to exploit the unique properties of the neutron in studying condensed matter, and after 30 years of intense activity the level of interest and vitality of the field are undiminished. In the following RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 34

paragraphs we survey the contributions that neutron scattering has made to the various subfields. It is worth reflecting that most of the results discussed represent not only significant advances in our understanding of the physics of the materials but that neutron scattering provided unique information not available from other known techniques. Two examples, discussed more fully in subsequent sections, illustrate this particularly well. The first is the development of magnetic order in superconductors, where a series of key discoveries beginning in the late 1970s and involving a close interplay between imaginative materials synthesis and neutron-scattering studies have caused the simple dictum that “superconductivity and magnetism don’t mix” to be greatly revised. In this case although thermodynamic and magnetic measurements suggested the occurrence of unusual phenomena in those new materials, neutron scattering was essential to establish its nature. A second example is provided by ongoing recent studies of the effect of disorder on the collective behavior of condensed-matter systems. Major conceptual difficulties have arisen in recent years as to the behavior of a simple magnetic model system when placed in a magnetic field that varies randomly in direction from site to site. Although the model has attracted much interest as a prototype of real physical disorder, the production of such a random field on an atomic scale is experimentally not possible. However, as it turns out, the application of a uniform magnetic field to a dilute random antiferromagnet produces a random antiferromagnetic internal field and a physical situation that simulates the ideal theoretical model. Neutron-scattering measurements on such systems are now providing the critical RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 35

experimental tests of our understanding of simple random systems. Magnetic Systems The determination of magnetic structures was one of the first and most important applications of neutron diffraction. While many of the interesting magnetic structures of simple materials have now been determined, the ability to make these determinations on new materials will be essential for the indefinite future as long as new materials are developed. Most magnetic structures have been determined at medium-to low-flux sources using a two-axis diffractometer, but for very complex structures (Nd, PrAg, TmS), or for fine structural details (MnP), the polarization analysis technique can be extremely valuable. Use of this technique has been limited because there are only a few spectrometers in the world equipped for these measurements. Amorphous ferromagnets have enormous potential value in commercial applications, and neutron scattering has been used in a variety of ways to understand these materials on a microscopic basis. Diffuse scattering measurements have been used to obtain the radial distribution functions that describe the basic structure, inelastic neutron-scattering studies have given details of the spin dynamics, and small-angle scattering has been used to study the microstructure. Neutron experiments on these materials have been carried out worldwide. The advantage of using polarized neutrons has not yet been fully exploited in these studies. Another class of materials that has attracted great experimental and theoretical interest in recent years is that group of RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 36

rare-earth materials in which the rare-earth ion apparently has a fractional valence--the intermediate- or mixed-valence materials. These materials do not show magnetic order, and the neutron-scattering spectra show a broad quasi- elastic line with an energy width in the range from 10 to 100 meV. The shape of this line is consistent with a fast (10`13 sec) relaxation process of the rare-earth spins. Actinides frequently show many characteristics of the unstable 4f moment systems. In the rock salt uranium compounds, for example, experiments have shown that, even though the systems order magnetically, spin waves need not exist. A combination of x-ray photoemission spectroscopy (XPS) and neutron spectroscopy suggests that the unusual damping is caused by strong 5f-6d electron interactions. Both critical and inelastic magnetic scattering has contributed greatly to our understanding of the solid-state physics of the last row of the periodic table. In another kind of experiment, polarized neutrons have been used to measure the induced magnetic form factor in order to gain a better understanding of the behavior of the f electrons in these materials. In general, these form factors are not dramatically different from free-ion form factors for integral valence ions. Interesting exceptions are CeSn and CePd, where evidence of some 5d polarization is found at low temperatures. As an example of the subtlety of magnetic systems and of the continuing long-term need for state-of-the-art neutron-scattering facilities, it should be noted that new and exciting facts about the most studied of all magnetic materials, Cr, Fe,and Ni, are still being discovered through neutron-scattering experiments. Recent polarization analysis experiments of the diffuse scattering from Fe and Ni in the paramagnetic RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 37

phase have revealed short-range magnetic order extending well above Tc. In another type of polarized-beam experiment, the appearance of “forbidden” magnons in Ni well below Tc indicates a deviation in the local and bulk magnetization directions. Commensurate diffuse excitations have been observed in the incommensurate spin-density wave state of Cr metal. These excitations were completely unexpected and are not yet understood. All of these experiments give valuable insights into the fundamental nature of the magnetic behavior of these important metals. Practically all areas of magnetic research would benefit from more and improved neutron polarization analysis spectrometers. Higher source fluxes and improved polarizers are both important for the growth of this technique. Fortunately, recent advances in this country and Europe toward better polarizers have been made. Pulsed-neutron sources show promise for enabling inelastic magnetic scattering at high-energy transfers. For example, recent measurements have already extended the spin-wave spectrum in Fe to higher values than previously possible, and such sources promise new opportunities to locate the single-particle (Stoner) continuum. The lack of very-high-resolution instruments, such as a neutron-spin-echo spectrometer, has limited the U.S. research in important areas, such as the relaxation effects in spin glasses. Phase Transition From its inception, neutron scattering has made important contributions to the field of phase transitions and critical phenomena. Not surprisingly, this has continued to be the RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 38

case for the past 6 years. Phase transitions occur in a wide variety of systems so that the work discussed in this section will overlap with results presented in most of the other sections. There has been an interesting evolution in the class of materials and the types of problems being pursued. Initial work, especially in the period 1965 to 1975, concentrated on prototypical phase transitions in model systems. Examples include soft modes in ferroelectrics and in other systems exhibiting structural transitions such as SrTiO3, ordering in binary alloys such as CuZn, and magnetic transitions in simple magnets such as RbMnF3, K2NiF4, Fe, and Ni. These experiments were invaluable in elucidating the basic principles governing critical phenomena. As a result of these early studies and of important advances in theoretical understanding, it has proven possible to address much more complicated issues. Recent neutron work has concentrated on the phase-transition behavior of more exotic systems. Indeed a number of the more interesting experiments involve issues that were barely conceived of at the time of the previous NAS report. Examples include random field effects, spin-Peierls transitions, devil’s staircase phenomena in incommensurate systems, re-entrant superconductivity, re-entrant spin-glass behavior, and other effects originating from competing interactions. In each case neutron-scattering experiments have provided a key to understanding the basic physics underlying these phase transitions. It is also interesting to note that with a few important exceptions, the crucial experiments have involved conventional elastic and inelastic neutron-scattering (including SANS) techniques. The most serious limitation typically has been sample size or, equivalently, neutron flux. In RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 39

many cases major improvements in momentum or energy resolution, or both, currently available or under development in Europe, will be of increasing importance. In this brief report, it is impossible to survey all the beautiful experiments that have been performed in the last 6 years. We therefore limit ourselves to a few representative samples. The effects of randomness, especially in systems with competing interactions, are now being extensively explored. Particularly dramatic effects are observed in spin systems with random magnetic fields, that is, magnetic fields with zero average value but nonzero variance. Neutron experiments in both the United States and Europe have verified that a uniform field applied to a random antiferromagnet generates a random staggered magnetic field; this makes possible a systematic study of the phenomena. High-resolution experiments on both two- and three-dimensional systems have revealed that long-range magnetic order is not attained in the presence of a weak random field and that instead one sees with decreasing temperature a continuous evolution from the paramagnetic state to a frozen microdomain state. Such random field effects are undoubtedly important in a range of other physical systems, including random-anisotropy rare-earth amorphous alloys. Spin glasses have been the subject of intense experimental and theoretical study in recent years, and neutron techniques have contributed greatly to our understanding of these materials. In Europe, a combination of the neutron-spin- echo and polarization-analysis techniques have been used to measure the time- dependent spin-correlation function of Cu-Mn alloys over a time range from 10 `12 to 10` 9 sec. In the United States, elastic polarization analysis measurements on single RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 40

crystals of Cu-Mn have demonstrated strong short-range magnetic correlations associated with short-range nuclear order. Exotic effects are also observed in alloys with competing ferromagnetic and antiferromagnetic interactions. With increasing concentration of antiferromagnetic bonds one observes an evolution from ferromagnetism to spin-glass behavior. For ferromagnets with concentrations near the crossover point one sees “re-entrant behavior,” that is, with decreasing temperature the system evolves from a paramagnet to a ferromagnet to a spin glass. The spin-wave frequency appears to soften at both the paramagnetic-ferromagnetic and ferromagnetic-spin glass transitions. In addition, unusual diffuse scattering is observed in the re-entrant phase, presumably originating from residual microdomains of the ferromagnetic network. These effects have been seen in such systems as FexCr1-x, FexNi1-x, PbA1, and FeMnPC. This is still an active and rather controversial area of research. It should be noted that the most precise experiments have been performed at Grenoble utilizing the small-angle scattering and high-energy resolution spectrometers. Systems with competing anisotropies rather than competing interactions have also been studied. Here again one observes new magnetic states whose basic structures and excitations could only be elucidated with neutrons. The study of phase transformations and other highly cooperative phenomena is perhaps the most challenging and subtle that condensed-matter physics has to offer. At the same time, fresh ways of thinking about such systems have greatly expanded our ability to understand such systems and suggested subjects for new studies. Such systems are very often magnetic because of their relative freedom from RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 41

complicating extraneous interactions. As in the past, neutrons will continue to provide an indispensible tool for such studies. New Materials and Phenomena The unique capabilities of neutron studies are readily apparent in the study of the coexistence of magnetic and superconducting long-range order, which was discovered (“engineered” is a better term) in the late 1970s. The phenomenon of superconductivity is quite tolerant of large concentrations of impurity atoms, so long as they are not magnetic in character, but is quickly destroyed by small concentrations of magnetic impurities. This peculiar fact was readily explained by the Bardeen, Cooper, and Schrieffer (BCS) theory, which showed how superconductivity can arise from a binding of pairs of electrons that travel in time-reversed orbits. Magnetic impurities that do not respect time-reversal symmetry destroy the pairing, whereas chemical impurities simply scatter the electrons into new time-reversed orbits leaving the superconductivity intact. The early studies of the magnetic suppression of superconductivity were carried out on simple binary alloys in which magnetic impurity atoms were substituted randomly into the lattice of the superconducting metal. Reasoning that a more stringent test of coexistence would result if the electrons responsible for the superconductivity could be effectively separated from those electrons responsible for the magnetism, new studies were undertaken on more complex ternary systems, in which the magnetic atoms are separated from the atoms responsible RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 42

for the superconductivity by a barrier of inert atoms, preventing strong interaction between the two. In the case of materials such as DyMo6S8, heat-capacity measurements showed the presence of a new kind of ordering occurring below the temperature of the onset of superconductivity. Neutron-diffraction studies were necessary to establish the nature of the ordering, which was found to be simple antiferromagnetism in which planes of atoms with magnetic moments up and down alternate. The superconductivity is not destroyed, proving that superconductivity and antiferromagnetism can simultaneously coexist at the same temperature. In materials such as ErRh4B4, neutron-scattering studies have shown that ferromagnetic arrangement of magnetic moments arises at temperatures below that at which the sample becomes superconducting and that in the process the sample regains its normal conductivity. Thus, superconductivity and ferromagnetism can exist in the same material but apparently cannot coexist at the same temperature. Further insight into the nature of the competition between magnetism and superconductivity has recently come from studies of smallangle neutron scattering, which reveal that even when the ferromagnetic state is marginally unstable, an entirely new type of order, taking the form of a long- wavelength oscillating magnetic disturbance can coexist with the superconducting state. These findings may have far-reaching implications for the directions of future research in superconductivity. Graphite is a prime example of a layered structure composed, in this case, of covalently bound sheets of carbon atoms with adjacent sheets held together more loosely by van der Waals forces. New compounds with unusual properties RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 43

can be prepared by inserting, for example, alkali metal atoms between the graphite sheets. Among the fascinating and incompletely understood aspects of these resulting graphite intercalation compounds (GIC) is the structure of the inserted atoms. Neutron-scattering studies have elucidated the nature of the stacking of adjacent metal layers and have shown that at high temperatures the ordering is liquidlike. Unsaturated materials, prepared with low-metal-vapor pressure, have metal atoms inserted between every nth graphite layer, where n is a simple integer. This phenomenon is known as staging. Recent neutron-scattering experiments under hydrostatic pressure have revealed a new fractional staging sequence, related to the n = 3 stage by particle-hole symmetry, whose existence helps to decide between competing theories of the staging phenomenon. A recent first study of the dynamic structure factor and diffusion processes in higher-stage metal-graphite compounds has only been possible by joint U.S.-ILL experiments using high-intensity and high-resolution spectrometers at the ILL. Models of one-dimensional (1-D) magnetic systems show in addition to linear spin-wave excitations, localized large-amplitude excitations that preserve their integrity as they move along a chain. It is expected that these excitations, called solitons, exist in real materials, and considerable effort has been made to observe them by neutron scattering. Both planar 1-D ferromagnets and antiferromagnets in external magnetic fields are expected to have soliton excitations, although the effects are more difficult to interpret in the ferromagnetic case, where neutron measurements in CsNiF are still controversial. Soliton effects have been successfully observed in the nearly classical S = 5/2 antiferromagnet, RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 44

TMMC, by a group at ILL. In CsCoC13 (S = 1/2), which is an easy-axis antiferromagnet, soliton effects are expected even in the absence of an external magnetic field, and they have been seen by neutron scattering. The longitudinal excitations can be thought of as Doppler-shifted quasi-elastic scattering from moving domain walls (solitons), whereas the transverse excitations involve the creation of domain wall pairs. A new class of materials that has attracted considerable interest in the past few years are incommensurate systems, in which two subsystems with mutually incompatible translational symmetry coexist. For example, if the instabilities associated with structural phase transformations involve competition between forces of various ranges, the resulting distortion waves may be incommensurate with the underlying lattice. This mechanism was first identified with charge- density wave (CDW) formation in metals, and there have been several recent studies of CDW transformations in quasi-1-D metals such as potassium cyanoplatinate (KCP) and TTF-TCNQ. A related transition, known as a spin- Peierls transition, may occur in S = 1/2 magnetic chains. The transition, which is predicted to be a simple dimerization for nearest-neighbor interactions, has in fact been observed in two different quasi-1-D magnets. Incommensurate phase transformations have also been discovered in insulators, a particularly clean example being potassium selenate (K2SeO4), where a low-frequency phonon with an incommensurate wave vector has been observed by neutron studies above the critical temperature. Elementary theory shows that this system has a two- component order parameter and should have critical properties identical to the XY ferromagnet. This has been RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 45

verified, in part, with neutron studies. The phenomenon of “locking in,” whereby an initially incommensurate lattice distortion becomes pinned at a commensurate value was first observed by neutron studies, and recently an example of electric- field-induced lock-in transformation has been found in the ferroelectric thiourea. Crystal Dynamics The frequencies and displacement patterns of the vibrational modes provide the most direct available insight concerning the strength and range of interatomic forces in solids. Optical and electron tunneling spectroscopies provide valuable but limited information concerning vibrational properties. Inelastic neutron scattering has proven the most reliable and complete tool for investigating phonon dispersion relations in solids. The techniques, using triple-axis spectrometers at moderately high-flux reactors are by now classical. They will continue to fulfill a vital need so long as the understanding of the forces in increasingly sophisticated new materials challenge condensed-matter scientists. The interatomic forces in metals are profoundly modified by screening from the conduction electrons. In the case of d-electron metals these screening effects can be seen as more or less sharp anomalous features in the phonon dispersion and provide exacting tests of state-of-the-art electronic-band-structure calculations. Such tests have been provided in recent years through detailed neutron-scattering studies of various transition metals, alloys, and transition metal carbides. Metals with flat or otherwise well-nested portions of Fermi surface can show giant Kohn anomalies, which are RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 46

precursors of charge-density wave instabilities such as the Peierls transformation discussed above. Of the various quasi one- and two-dimensional systems studied extensively with neutrons, the 1-D metal (KCP) is the most spectacular. Phonon linewidth effects are a potential rich aspect of e-p interaction largely unexplored with neutrons owing to. lack of energy resolution. Ideas for high-resolution momentum focusing instruments exist but would require higher-flux sources to achieve their full potential. The presence of a weakly coupled incommensurate chargeor mass-density wave in an otherwise periodic structure gives rise to new gapless excitations, which correspond to relative translations of the density wave. Such modes, sometimes called phasons, differ from true acoustic modes, not only in the nature of the atomic displacements involved but also in the fact that they become overdamped as their wave vector goes to zero, making them difficult to study with optical probes. Such modes have now been seen in several incommensurate systems. In one of these, HgxAsF6, incommensurate mercury chains behave independently at high temperatures, and neutron studies show that at short wavelengths the dynamics are those of a 1-D harmonic fluid. Inelastic neutron- scattering studies have measured the dispersion of longitudinal elastic waves in graphite intercalated with various donor and acceptor atoms. These measured dispersion curves provide direct information on the magnitude and range of the interlayer forces. For example, the graphite-alkali atom coupling is greater than the normal graphite-graphite coupling in pristine graphite, whereas the reverse is true for the acceptor intercalant, FeCl3. Results also scale according to the intercalant areal density, suggesting that every intercalant RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 47

atom, but not every graphite atom, contributes to the interplanar stiffness. Although of great interest, from the viewpoint of 2-D physics, it has proven difficult to carry out similarly detailed studies of phonons propagating within the intercalant layers, because of sample size restrictions. This is one example of a limitation imposed by present reactor fluxes. Defect Systems To characterize completely the dynamics of hydrogen dissolved in metals requires measurements over a wide range of energy transfer. The diffusive motions result in quasi-elastic scattering, requiring instruments with energy resolution down to 1 µeV. This field of research has been active in Europe using the very high resolution available with neutron backscattering spectrometers. The low-energy vibrational aspects are coupled to the acoustic phonons of the host lattice and have been studied throughout the world using triple-axis spectrometers. The optic modes, or localized modes for dilute hydrides, have been measured by both triple-axis and time-of-flight techniques. The Japanese have demonstrated the value of a pulsed neutron source by measuring local vibrational spectra up to 800 meV. The large incoherent cross section of H makes it possible to perform scattering measurements on dilute solutions, and recent efforts in the United States have extended these spectroscopic studies to samples in the ~0.1 percent H regime where hydrogen embrittlement occurs. These improvements have also provided unique information on the local environment and bonding potentials of hydrogen trapped by impurities in metals. The study of diffusive motions is also important in fast RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 48

ion conductors. Here the time scale is somewhat shorter than is the case for hydrogen in metals so that triple-axis spectrometers can be employed. Such experiments have been performed both in the United States (on LiA1, for example), and in Europe (on SrC12, for example). The introduction of complex defects with internal degrees of freedom into a crystal lattice produces a variety of interesting effects. Resonant hybridization of the defect states and phonon modes of matching symmetry and energy have been clearly observed in neutron-scattering measurements of dilute CN` impurities in KC1. In the mixed-crystal system (KCN)x(KBr)1` x, the CN` ions freeze into a random distribution of orientations as the temperature is lowered, producing an orientational glass state analogous to spin glasses. This state was discovered through the observation of diffuse elastic neutron scattering. The greatest shortcoming of the U.S. efforts in the study of defect systems is the lack of very-high-resolution spectrometers for studying quasi-elastic scattering and tunneling phenomena. The backscattering instruments in Europe have given them unchallenged leadership in such fields as hydrogen diffusion. Surfaces and Overlayers At the time of the NAS report on Neutron Research on Condensed Matter in 1977, a few demonstration neutron experiments on surface phenomena had been reported. As we shall discuss in this section, this has now developed into a major area of research with active programs at many major facilities in the United States and Western Europe. It seems clear RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 49

that this field will continue to grow, and, indeed, certain of the problems (e.g., high-energy spectroscopy) are well matched to pulsed neutron sources, so that one can expect major activity at the new pulsed sources as they come online. Because neutron source intensities are relatively low, and further since neutrons interact weakly with matter, one might expect that surface signals would be undetectably low. This is indeed the case for virtually all single-crystal surface experiments. However, there is a variety of microcrystalline substrates, most notably exfoliated graphite, with surface areas as large as 40 m2 g` 1. Thus for a 2.5-cm3 sample the illuminated surface area may be as large as 300m2. In favorable cases, it is then possible to separate out scattering events that originate from physisorbed or chemisorbed overlayers from the bulk scattering. Elastic, quasi-elastic, and inelastic neutron-scattering studies of surface overlayers have now all been reported. The most detailed studies to date have been for rare-gas atoms, diatomic molecules, and hydrocarbons physisorbed on graphite. Some experiments in thick films and on co-existing bulk and film phases have also been reported. They involve primarily powder diffraction with an energy analyzer to eliminate inelastic events or else inelastic scattering using triple-axis or time-of- flight techniques. In most cases, the essential limitation has been absolute signal levels. Thus, an increase in neutron flux by an order of magnitude would have a fundamental impact on this field. Elastic-scattering experiments yield information on the overlayer structures and transitions. The first such studies were on atoms and diatomic molecules physisorbed on graphite; species studied with neutrons include H2, D2, RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 50

4He, 3He, Ar, and N2 on graphite. All of these exhibit triangular structures with a lattice constant either commensurate or incommensurate with respect to the graphite substrate. By changing the coverage, the temperature, or both, it has been possible to study both commensurate-incommensurate and melting transitions. The structures of the various phases have been accurately determined, but because of flux limitations, only qualitative information has been obtained on the details of the phase transitions. Systems exhibiting more complicated phases and phase transitions have also been studied with elastic neutron-diffraction techniques. These include O2, CF4, N2, C2D6 (ethane), and C5D12 (butane) on graphite as well as C5D12 (neopentane) on TiO2. Bilayers of O2 on graphite exhibit an antiferromagnetic transition at 12 K, and evidence for the concomitant superlattice peak has been obtained. For nonspherical molecules the structures may be quite complicated; the unit cell may be a low-symmetry parallelogram, and the molecules may be tilted with respect to the surface. Such structures have been solved for butane and N2O2 on graphite. The butane system also appears to exhibit quite interesting melting behavior. Very recently, diffraction studies have been reported for CD4 and Ar on NiC14. It has also been realized that these physisorbed systems may exhibit co-existing bulk and film phases with reversible transitions between the surface and bulk structures. Such effects have now been seen directly with neutrons for C2D4 on graphite. Of course, one of the salient advantages of neutrons over other scattering techniques is the ability to study dynamics in the range of typically 0.1 to 100 meV. So far, only a limited number of inelastic-scattering experiments RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 51

have been carried out on surface systems. Interesting results have been obtained in both the United States and Europe for hydrogen chemisorbed on metallic surfaces such as Raney Ni, platinum, and palladium blacks. These are all of fundamental interest in the field of catalysis; vibrational excitations are observed whose frequencies match well to the results of model calculations. Collective effects have also been studied using triple-axis techniques. For example, the phonon density of states for monolayer argon on graphite has been obtained; the in-plane results can be reasonably well described at low temperatures by a two- dimensional roton. Finally, a limited number of quasi-elastic experiments probing surface diffusion have been performed. Particularly interesting behavior has been observed for CH4 on graphite near melting. Clearly, this is a vigorous, rapidly expanding field in neutron scattering. We expect that an increasing number of experiments will be performed on high- surface-area substrates, especially those relevant to catalysis. Increased flux would greatly facilitate these studies. Liquids and Glasses Neutron-scattering techniques have provided the most detailed and complete experimental results on the structure and dynamics of isotropic systems in both the liquid and amorphous states. These systems present one of the outstanding challenges in statistical physics, since, in contrast to solids, there is no available first-order theory in terms of independent quasi-particles such as magnons and phonons. Indeed, progress has only been possibly by detailed comparison of the results RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 52

of neutron scattering with the results of molecular-dynamics simulations, a technique that has so far been more successful for liquids than for glasses or amorphous solids. Nevertheless, real progress has been made in recent years in a fundamental understanding of the statistical physics of these systems, particularly in the use of kinetic theories. An area in which the above considerations do not apply is that of the quantum fluids 3He and 4He, where neutron scattering has been used to study the detailed predictions of available theories. In the following, we describe some examples of results over the past 6 years that illustrate the power of the technique and the progress being made. Quantum Fluids Recent neutron inelastic scattering studies have revealed the elementary excitation spectrum of 3He for the first time, stimulating intense theoretical effort and greatly increased understanding of the underlying physics of Fermi liquids. In view of the extremely high neutron absorption cross section of 3He, these experiments represent an unrivaled tour de force of experimental techniques. Future research in this area will provide new insights into the dynamics and quasi-particle excitations in 3He, 4He, and mixtures of the two isotopes, leading to new theoretical advances in the understanding of quantum liquids. These studies will be greatly enhanced by the availability of intense beams of epithermal neutrons from pulsed spallation sources. An outstanding problem in quantum statistical mechanics has been the existence and properties of a Bose condensate in superfluid 4He and the nature of the momentum distribution RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 53

in both normal and superfluid He. Recent neutron-diffraction results have led to very precise estimates of the condensate fraction in superfluid 4He, while high momentum transfer experiments on normal and superfluid 4He using epithermal neutrons show promise of giving very detailed measurements of the momentum density distribution, but again increased fluxes are needed to provide the necessary resolution and sensitivity. Classical Liquids One of the outstanding results from neutron-scattering studies of classical liquids in recent years has been the measurement of the density dependence of both the static and dynamic structure factors in dense krypton gas. These results give direct information about the nature and magnitude of the three-particle correlations in this system for comparison with the predictions of statistical physics, and thus promise much better models for these many-body effects. Additional progress can be expected as higher-intensity time-of-flight instruments and pulsed sources become available for measurements of the structure and dynamics of simple fluids. Recent results from neutron-scattering studies of liquid crystals at reactors in Europe have identified the existence of hydrodynamic instabilities and time- dependent states and phase diagrams with multicritical properties in liquid crystals. Further progress in this area can be expected, especially in the area of novel magnetic fluids with liquid-crystal-like ordering. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 54

Glasses and Amorphous Solids The principal achievements in the fundamental understanding of this important class of materials has come from detailed measurements of the static structure factor by x-ray and neutron scattering, combined with the detailed information about local order derived from extended x-ray absorption fine structure (EXAFS). In particular, the use of isotopic substitution methods for neutron diffraction, combined with x-ray diffraction results, has led to direct determination of the partial structure factors for several glasses. This has provided a better understanding of the short-range structure and chemical ordering in a wide variety of glasses. However, much work remains to be done both on the intermediate-range structure and on the recrystallization process (using small- angle neutron scattering). Progress in this area requires exceptionally high-quality data extending over the widest possible range of momentum transfers, placing severe demands on instrumentation. The development of intense pulsed spallation sources will greatly enhance detailed analysis of interatomic arrangements in these systems. To date, little work has been done on the dynamic properties of glasses, at least partially because of the lack of adequate theoretical models. Nonetheless, important results have been obtained on reasonably simple metallic glasses (Ca10Mg30Zn30) and compared with computer simulation studies done using pair potentials derived from pseudopotential theory. These experiments have shown that the pair potentials used in the simulations give a good representation of the dynamics and have led to increased efforts to derive better potentials for semiconductors and insulators. New predictions RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 55

concerning phonon density of states in amorphous materials based on the fractal nature of self-similar structures need to be explored by neutron-scattering experiments. As these and other efforts continue, neutron scattering will unquestionably provide the ultimate test of most theoretical models, since it provides data over a uniquely wide range of energy and momentum. NEUTRON OPTICS A range of phenomena similar or analogous to those of classical optics is exhibited by slow neutrons. During the past 6 years considerable interest has been generated in this field from the point of view of fundamental physics. This has been due largely to the successful application of the perfect silicon-crystal interferometer to neutron experiments and to the increased availability of more intense sources of long-wavelength neutrons. Neutron optical ideas and techniques play an important role in all aspects of neutron-scattering technology, in particular in the extraction, filtering, and monochromatization of beams. Substantial advances have been made in recent years in focusing monochromators, collimation systems, multilayer mirrors, and beam guides. We will not attempt to enumerate these advances in the application of neutron optics to neutron scattering generally but will instead discuss the variety of interesting experiments that have made an impact on our understanding of basic quantum-mechanical phenomena. The Bonse-Hart perfect-crystal interferometer was first shown to work for neutrons by H. Rauch, W. Treimer, and RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 56

U. Bonse at a small reactor in Vienna, Austria, in 1974. It is topologically analogous to the Mach-Zehnder interferometer of classical optics. It consists of three identical perfect silicon-crystal slabs, cut perpendicular to a set of strongly reflecting lattice planes from a single monolithic crystal. The incident beam is coherently split and recombined by Bragg reflection. Alternative two- and three- crystal geometries have been employed successfully. In recent years, these devices have also been used at synchrotron radiation sources to measure the dispersion correction f `close to the x-ray absorption edges. These measurements of the real part of the forward-scattering amplitude directly reflect the EXAFS of the absorption spectrum. In the neutron case, these devices have allowed the observation of quantum-mechanical interference phenomena on a macroscopic scale. They are extremely sensitive to very small potentials acting on the neutrons. The current level of sensitivity approaches 10`12 eV. Interference effects induced by the gravitational field of the Earth have been observed. In fact, the much smaller phase shifts due to the Earth’s rotation (a quantum-mechanical analog of the famous Michelson-Gale-Pearson optical experiment) have also been observed. These experiments are the first tests of the principle of equivalence in the quantum limit. The first direct observation of the prediction that the operator for rotation through 2 rad causes a reversal of sign of the wave function for a fermion was also done with this device. A complete list of the experiments utilizing thermal neutron interferometry is now quite long. It includes the observation of the coherent superposition of spin states, study of the charge dependence of the four-body nuclear RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 57

interaction, search for quaternions in quantum mechanics, observation of the neutron analog of the Fizeau effect, search for nonlinear terms in the Schrödinger equation, measurement of the longitudinal coherence length of the neutron, and a search for a neutron Aharonov-Bohm effect. Interference in spin space, using the nuclear magnetic resonance-Ramsey technique, has recently been used to observe optical rotation via parity-violating weak interactions. While the perfect crystal interferometer utilizes interference by amplitude division, interferometers based on division of the wave front have also been successfully employed in recent years. In order to achieve a reasonable beam separation, these devices all utilize long-wavelength neutrons ( ` 20 Å) obtained from a cold source. The neutron Fizeau experiment was carried out using this type of interferometer. Many of the well-known optical phenomena have now been observed with neutrons, including two-slit interference (Young’s experiment), operation of Fresnel zone plate lenses, thin-film multiple interferences, and diffraction by a straight edge. Cold neutrons have also been essential in recent experiments on the neutron lifetime and continuing searches for the electric dipole moment (EDM) of the neutron. Novel methods for producing ultra-cold neutrons involving Doppler-shifted Bragg scattering and downscattering in liquid helium have been demonstrated successfully in the last several years. At present, fundamental physics utilizing neutron optical techniques is primarily carried out at four places: the University of Munich, the Institut Laue- Langevin in Grenoble, the University of Missouri, and the Massachusetts Institute of Technology. It is difficult to predict the future directions RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 58

that this field will take. However, there are various fundamental neutron-optics- related experiments currently in progress and others on the immediate horizon. It is likely that continuously higher-precision searches for a neutron EDM will be pursued using ultra-cold neutrons. The measurement of the energy dependence of neutron-scattering lengths using various interferometers will be pursued as these data become necessary for scattering experiments using epithermal neutrons from pulsed spallation sources. A Michelson-Morley experiment with neutrons should be done. Very small effects, such as the coupling of the neutron spin to the curvature of space-time (owing to the presence of the Earth), are in principle detectable with significantly larger interferometers, having linear dimensions of a meter or so. Neutron trajectory and effective mass experiments in dynamically diffracting crystals are in progress. Experiments related to fundamental temporal and spatial coherence questions and Wheeler’s delayed-choice questions have just begun. The next 5 years will probably be as exciting as the past 5 years. CHEMISTRY Crystallographic Research The years from 1977 through 1983 were active ones for structural studies using neutron diffraction, not only because of extensive work with single crystals but also because these years included vigorous growth in the application of high- resolution powder diffraction to structure refinement by means of the technique RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 59

in which the entire diffraction pattern is fitted to a detailed crystallographic model. This technique, which has become known as the Rietveld method, has revolutionized structure studies on both sides of the Atlantic for a wide variety of materials that are not available as single crystals. Moreover, the penetrating power of the neutron makes possible the study of crystal structure under extreme conditions of temperature and pressure. Materials that have received extensive attention include compounds that contain light atoms in the presence of heavy atoms, such as organometallic compounds and heteropoly complexes; ionic conductors; superconductors; both alloys and organic compounds; framework structures, such as zeolites; simple and moderately complex compounds in which neutron and x-ray data can be combined for studies of electron density; hydrogen-bonded compounds; magnetic compounds; and simple organic and inorganic compounds in which high-precision structure analyses can be correlated with physical properties or compared with ab initio calculations of expected molecular conformations. We discuss below particular examples of some of these types of structural study. Hydrides, Organometallic Compounds, and Heteropoly Complexes Because of its sensitivity to hydrogen and other light atoms in proximity to heavy metals, neutron diffraction has proved invaluable in studying the structures of transition-metal hydrides and metal complexes with organic ligands exhibiting C-H/metal interactions. Between 1977 and 1983 approximately 50 metal hydride structures have been investigated by single-crystal neutron techniques, providing examples of hydrogen in terminal, bridging, and interstitial environments. Many powder-diffraction studies of binary and ternary hydrides RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 60

have been carried out, including such potentially technologically important materials as FeTiHx and LaNiH5. In studies of C-H/metal interactions, neutron diffraction has provided information on models for C-H bond activation processes that are of fundamental importance in catalysis. In recent years there has been increased emphasis on the study of cluster compounds, which may provide models for the bonding of hydrogen atoms and small molecules to metal surfaces. Crucial studies have also been carried out on heavy-metal-based heteropoly complexes, compounds of extensive importance as reagents and catalysts, which contain hydrogen as integral parts of the heteropoly anions. Ionic Conductors and Ceramics Studies of ionic conductors such as ß``-alumina, AgI, Ag2S, Cu2S, and related ternary systems, as well as Na1-xZr2SixP3` x012 (NASICON) and Na3Sc2 (PO4)3, are of great interest and exploit the unique advantages of neutron diffraction, in combination with x-ray diffraction, for examining details of disorder and thermal motion. Another important class of ionic conductors contains lithium in a matrix formed by a refractory metal oxide. Neutron diffraction is the key to determining the critical role of Li and accurate oxygen cage geometries in such “electronic” ceramics (e.g., LiReO3, LiTaO4). Such new materials have great potential, for example, in the development of more efficient small batteries. Superconductors Neutron diffraction has played an essential role in providing a basis from which to understand the unusual properties of organic conductors and superconductors. For example, RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 61

in the partially oxidized tetracyanoplatinate complexes, the one-dimensional electronic conductivity has been studied as a function of metal-metal distance for a range of materials whose structures are known from neutron diffraction. More recent neutron-diffraction studies of organic, superconducting materials of the TMTSF family have explored the structure-property relationships that might exist between materials that exhibit superconductivity only at high pressure and materials that are superconducting on appropriate anion substitution. Neutron powder diffraction has been used to study a wide range of ternary superconducting alloys, in particular the Chevrel phase structures. Framework Structures Studies of zeolites are giving the first precise information on the locations of extra framework molecules and ions in these technologically important materials. For example, studies of zeolite rho with various compositions and over a wide range of temperatures have shown that this zeolite is noncentrosymmetric for most compositions. The sizes of the pores are sensitive to the degree of departure from centrosymmetry. These and other zeolite structure data are being used to test structural models of the reversible dehydration and ion-exchange properties of zeolite molecular sieves. Charge-Density Studies Techniques for combining high-resolution neutron and x-ray diffraction data to yield detailed information on electronic charge-density distributions in crystals have been extended from structures involving only atoms in the first row of RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 62

the periodic table to compounds containing heavier elements. These studies are yielding important information on chemical bonding, including the distribution of d electrons in transition-metal systems. New opportunities for understanding chemical bonding have been opened by the development of high-intensity polarized-beam diffractometers at the ILL, which are measuring the spatial extent of p and d electrons in complex chemical systems. Although these systems are not magnetically ordered in any sense, a strong magnetic field is used to induce a spin susceptibility. The results of such measurements, when combined with high-resolution neutron diffraction and x-ray results, can be compared with first-principle calculations of the atomic wave functions. Hydrogen-Bonded Compounds The study of the structures of hydrogenous solids has been one of the particular strengths of neutron diffraction ever since its early development in the 1950s, and interest in these compounds continues unabated. Particular attention has been directed toward the study of very short hydrogen bonds and the question of whether they are symmetrical. A number of cases have been found of very short hydrogen bonds that do not cross centers of symmetry, and these appear to be asymmetrical. If the crystal structure can be centrosymmetric, and if the distance between two oxygen atoms is shorter than about 2.45 Å, a symmetric O-H-O hydrogen bond is indistinguishable from an asymmetrical one with statistical disorder across the center. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 63

High-Precision Structure Studies The high precision currently available from neutron-diffraction structure analysis has made possible a variety of new types of studies--atomic positions may frequently be determined with a reproducibility of 0.001 Å or better. For example, investigations of a number of pyroelectric materials have shown that observed changes in polarization with temperature are consistent with the values calculated from the lattice constants and the nuclear positions. In another type of investigation, conformations of small molecules determined at temperatures about 20 K are being compared with those inferred from ab initio quantum- mechanical calculations. Such comparisons are providing insight into the distinction between effects intrinsic to the molecules and effects of forces in the solid. Molecular Fluids and Molten Salts In recent years, neutron diffraction work on “chemical” liquids has been carried out almost exclusively in Europe, where a sizable number of physicists and chemists, notably in the united Kingdom, have turned to liquid-state problems. In the united States, two small programs have concentrated on water and water-based solutions and on molten salts. It has now become clear that in order to unravel the structure of molecular fluids and molten salts, the partial structure functions descriptive of these systems must be resolved by multiple experiments with isotopically substituted samples. This requires elaborate sample preparation techniques and access to high-flux reactors or pulsed sources. The substitution method has been successfully applied to ions with isotopes having large differences in coherent-scattering lengths, RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 64

such as Ni, Nd, and Cl. The method can be extended to nuclides with small differences in scattering length such as 12C and 13C or 14N and 15N. However, the large-scale investigation of these interesting systems is at the edge of the capabilities of currently available neutron sources and would greatly benefit from an order of magnitude in intensity. Structural solution chemistry has already been revolutionized by the isotopic substitution method, which allows the unique determination of ionic hydration and of ion-ion pair distribution functions. The method will be extended to the study of molecular fluids. In order to determine molten salt structure completely, both cation and anion isotopes should be available. Salts of the type MX and MX2 have been systematically investigated, giving special attention to the role of ion size. Future work will focus on binary melts. Chemical Spectroscopy With neutron molecular Spectroscopy one may currently study the dynamics of molecules ranging from slow translational and rotational diffusion (10` 6 sec) to the highest-energy internal vibrations (10`14 sec) at wavevectors (0.01-10 Å` 1), which directly probe molecular or macromolecular dimensions, something no other single experimental technique is capable of. However, most of this range is inaccessible with instruments at present U.S. neutron sources, as we will discuss in more detail below. Consequently, activities in this field in the United States have been somewhat limited. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 65

Vibrational Spectroscopy Current U.S. research efforts in neutron molecular spectroscopy are concentrated on vibrational spectroscopy in the range 25-250 meV using recently developed specialized instrumentation at reactors and pulsed neutron sources. Unlike most other spectroscopic techniques, incoherent inelastic neutron scattering (IINS) is not governed by symmetry-based electromagnetic selection rules. Instead the amplitude of the vibrational mode and the nuclear-scattering cross section of the atom involved principally determine IINS intensities. These factors favor the motions of H-atoms, and, therefore, certain kinds of normal modes (e.g., methyl torsions), which are often inactive or weak in IR or Raman scattering, have very large intensities in IINS. Most (but not all) of the work in this field has, therefore, concerned hydrogen-containing molecules and has often played a crucial role in assignments of vibrational bands when used in conjunction with light scattering. For example, a series of experiments on compounds with very short hydrogen bonds using IINS measurements demonstrate an apparent change in the dynamics of the O-H-O bond as the O-O distance r becomes less than about 2.44 Å. This was accomplished by measuring the out-of-plane bend (OHO) as r decreases. It had been impossible to assign this mode in light-scattering experiments. Other cases for which IR/Raman spectroscopy is often difficult or impossible include metallic systems, some intercalated species, and inorganic complexes that fluoresce. Moreover, recent work at both U.S. and European reactors has shown that IINS provides a powerful, often unique tool for in-situ spectroscopy of chemisorbed species RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 66

on catalysts over a wide range of temperature and pressure. These studies have already provided direct information on the bonding states and interactions of H atoms on the surface of Ni and Pt particles and on adsorption and decomposition of organic molecular species on these catalysts. An important advantage of IINS in such studies is that the scattering can be described by relatively simple theory, so that spectral intensities can be used quantitatively in the interpretation of spectra. For example, in the spectroscopic work on catalysts, both vibrational peaks and intensities were used to determine the geometry and force constants of surface-bound species. A series of important experiments has also been performed (in Europe) on metal cluster compounds as analog systems of chemisorbed materials. These data are extremely valuable as an aid in interpreting neutron or optical spectra of species bound on (or in) a real catalyst. Low-Energy Spectroscopy Low-energy neutron spectroscopy is currently done almost exclusively in Europe, where a series of specialized spectrometers has been developed over the past 5 or 10 years, primarily using cold neutrons, which have expanded the spectral range of neutron scattering by up to 5 orders of magnitude (down to 10 ` 9 ev). These include superior, highly sensitive cold-neutron time-of-flight spectrometers, as well as ultra-high-resolution backreflection and spin-echo spectrometers. This capability, combined with the unique wave-vector regime available with neutrons, has opened up entirely new and exciting areas of applications of neutron scattering, not only in chemical spectroscopy but, as reflected in other RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 67

sections of this report, in condensed-matter physics, polymer science, and biology. In chemical applications, low-energy neutron spectroscopy is an extremely sensitive tool for probing the intermolecular potentials and the details of rotation and diffusion of molecular species in condensed systems. For example, neutrons can be used to measure rotational ground-state splittings (tunneling) by spin-flip scattering. These transitions are between different nuclear spin states and, therefore, forbidden in light-scattering studies. Such measurements most often require ultra-high resolution (European instruments for this purpose have achieved 0.5-µeV resolution), but some work has been attempted at U.S. reactors on triple-axis spectrometers at relatively low resolution (~50 µeV). Perhaps the most notable such example is a study of NH3 rotations in Ni (NH3)6I2, where both the form of the orientational potential and (by the application of hydrostatic pressure) its approximate dependence on intermolecular distance could be determined. Similar work was carried out on nitromethane and methane but could only be completed by making use of the appropriate instruments at the ILL. Thus the entire field of rotational tunneling spectroscopy, including appropriate theoretical advances, has been developed in Europe since the mid-1970s. Perhaps the most impressive example is a series of experiments and calculations on tunneling states in methane as well as its deuterated forms. In the low-temperature phase III of CD4, e.g., eight transitions were resolved in the range 0-8 µeV on the IN-10 spectrometer at the ILL. This work has led to a detailed understanding of quantum-mechanical rotations and intermolecular potentials in the solid methanes. In an extension of this work to RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 68

methane absorbed on graphite, it was possible to determine the relative importance of molecule-molecule and molecule-substrate forces by comparing tunneling levels for various types of rotations of the molecules on the surface. We should like to stress that this type of low-energy spectroscopy, when combined with IINS vibrational spectroscopy, provides a powerful and comprehensive probe for studying molecule-surface interactions. The same general comments apply to the study of rotational and diffusive molecular motions with relatively long time constants (10`10 sec) by quasi-elastic neutron scattering (QNS). This is a powerful method for studying such motions in a wide variety of systems, ranging from hydrogen diffusion in metals to relaxation processes in polymers. Because of the ability to measure QNS at different momentum transfers, one can determine the details of reorientation or diffusion mechanisms on an interatomic or molecular scale. The application of these techniques and the development of advanced high-resolution neutron-scattering instruments is expanding rapidly in Europe. In fact, QNS has become a heavily utilized probe for investigation of molecular motion and diffusion in homogeneous as well as heterogeneous systems, such as zeolites, clays, clathrates, and other chemical adsorbents and intercalated materials. Measurements have been extended down to less than 1 atomic percent of molecular species. BIOLOGY Most of what we know about biological structures comes from the observation of radiation scattered by objects. Visible RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 69

and infrared light, electrons, and x rays have provided structural information ranging from the organization of the atoms in macromolecules to the arrangement of cells in organisms. With the development of high-intensity sources and improved instruments, it has recently become possible to use neutrons as an additional form of radiation with which to study the structure and dynamics of macromolecules and molecular assemblies. Neutrons have different scattering properties compared with the other forms of radiation that have been used and therefore can reveal different aspects of structure and, in fact, have capabilities that are uniquely useful. Isotope effects, particularly the difference between hydrogen and deuterium, can be exploited by selective labeling of the molecules of interest or of the solvent. An additional advantage is that there is no radiation damage to the specimen. Finally, neutron inelastic scattering provides the possibility of probing low-energy states of motion in biomolecular structures. The principal biological areas in which neutrons have been applied during the past decade have been in high- and low-resolution macromolecular crystallography, in studies of molecular assemblies and molecules in solution, and in the study of some partially ordered systems. In all cases the difference between the scattering of hydrogen and the scattering of deuterium has been used as an important tool. In studies of protein crystals, for example, the location of hydrogen atoms (which are not observed in x-ray crystallographic experiments) has been studied, and their replacement by deuterium has led to ideas concerning structural dynamics. Moreover, crystallographic studies at low resolution RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 70

have exploited enhanced contrast between protein and nucleic acid components of macromolecules. Specific deuteration of parts of macromolecules or macromolecular assemblies has been used to extend the range of information that can be obtained from solution scattering measurements. Similarly, studies of biological membranes and related lipid systems have provided critical information beyond that obtainable by x-ray diffraction. In the following, several examples of biological studies are discussed. Protein Crystallography High-resolution protein crystallography with neutrons was first attempted in Europe in the early 1960s but became truly feasible only in the mid-1970s. Currently the major facilities capable of efficient data collection exist in the United States, and a comparable facility is close to completion at ILL. Low- resolution studies of single crystals of such assemblies as nucleosomes and viruses have been attempted only at ILL and have never been performed in the United States, primarily for lack of suitable instrumentation. Studies of protein crystals using neutron diffraction at a resolution of 2 Å or better have focused on the localization of hydrogen atom positions and on the exchange of hydrogen for deuterium. By documenting the position of hydrogens, it is possible to understand more completely the chemistry of proteins, which often involves hydrogen atoms in critical roles. Examples of such studies are myoglobin, trypsin, ribonuclease, lysozyme, and crambin. Studies of trypsin and myoglobin led to the elucidation of the location of individual hydrogen atom crucial to the activity of each RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 71

of these proteins, while unexpected results concerning the details of hydrogen bonding between the enzyme and a substrate analog emerged from the analysis of ribonuclease. A second line of crystallographic studies has been to follow the exchange of hydrogen and deuterium, using the exchange patterns to derive ideas concerning the distribution of static and dynamic regions of the structure. Examples in which important information has been gained are ribonuclease, trypsin, and myoglobin. Certain structural regions have been found to be relatively stable, at least so far as the dynamics of a molecule in a crystal are concerned. Continuation of these studies is anticipated and may shed important light on the dynamics of proteins and, possibly, other macromolecules. Of particular importance may be the comparisons with the dynamic results obtained by NMR. A third issue that has concerned the practitioners of neutron crystallography has been the interaction of macromolecules with the solvent environment. Using neutrons, it is possible to document the occurrence of bound water molecules with much greater assurance than is the case with x rays, and ideas about solvation of proteins have emerged from such studies. It is clear that high-resolution neutron crystallography has been an important and productive development as applied to biological macromolecules. It is expected that contributions from this approach will continue in the future and that a number of important biological issues may be addressed in the areas of protein structure, enzyme mechanisms, protein dynamics, and solvent interactions. Low-resolution (15 Å or lower) studies in Europe utilizing contrast variation have been performed on a number of assemblies RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 72

consisting of proteins and nucleic acids. Such studies are capable of delineating precise boundaries between the components and are useful if high-resolution data cannot be collected. The instrumentation is provided by modification of modern small-angle instruments, which must be equipped with large area detectors and have access to cold neutron beams (~10 Å). Solution Scattering This technique has been found to be of great importance in studies of biomolecules consisting of components that could be selectively masked by the use of appropriate H2O/D2O solvents. Instruments capable of necessary measurements are now available on a number of reactors in this country and in Europe, although the best European instruments have higher flux and are capable of lower wave-vector measurements, which permits examination of larger structures. A number of different assemblies have been studied in solution using neutron scattering. Significant findings concerning nucleosomes, viruses, lipoproteins, ribosomes, and other systems have been derived from studies in both Europe and the United States. We will now discuss an example of such studies. The Ribosome More than 10 years ago, it was proposed that the organization of the proteins in a ribosome could be determined using neutron scattering from ribosomes in solution. The basic idea was to place two deuterated proteins in an otherwise hydrogenated structure. Neutron scattering from solutions RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 73

of such labeled particles could then be used to obtain a measurement of the distance separating the centers of the two proteins in question. By a process of triangulation, a three-dimensional map of the positions of proteins in a complex structure can be generated. Such a map will be extremely useful in understanding functional relationships of different parts of the ribosome structure. In 1974, the first measurement was carried out. Since then, a large number of pairwise measurements have been made, and the positions of 15 of the 21 proteins of the small subunit of the E. coli ribosome have now been established. Once a map is obtained, the positions of other macromolecular ligands can also be investigated, as can issues concerning conformational changes that may accompany different states of activity. Partially Ordered Systems Partially ordered systems, particularly those involving membranes or muscles, for example, can be investigated using the same instrumentation as the solution measurements, although more specialized instruments for low-resolution crystallography have been and are being developed. Membranes, such as those of the myelin sheath, sarcoplasmic reticulum, retinal rod, and halobacterium, have been studied. Furthermore, details of lipid bilayer structure have been documented using deuterium labeling of specific sites. An example is bacteriorhodopsin, a small protein found in crystalline patches in the plasma membrane of a microorganism. The protein is responsible for converting incident light energy into energy stored in an electrochemical gradient RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 74

across the membrane of the organism, which it does by pumping protons from the cytoplasm to the outside in response to light. Understanding its structure may lead to improved ideas concerning the nature of membrane proteins and their relationship to lipid bilayers, as well as leading toward an understanding of the energy transduction process itself. The neutron-scattering approach has been to use specific deuteration of the molecule by providing deuterated amino acids in the growth medium of the organism. In this way, individual classes of amino acids in bacteriorhodopsin can be deuterated biosynthetically. Purple membranes are then isolated from the organism, and their diffraction of neutrons is measured. Deuteration of different amino acids leads to large intensity changes, which can then be used in a model- building approach to test choices for the organization of the structure. This work is now in an advanced stage, and two of the seven helices have been assigned. Furthermore, it has emerged from this and other work that the helices are oriented so as to place polar groups toward the inside of the protein and nonpolar groups toward the outside where they may make contact with the nonpolar region of the liquid bilayer. Thus, this membrane protein is “inside-out” when compared with the normal organization of soluble proteins. It is to be expected that further study will lead to more refined techniques of labeling and measurement and to corresponding improvements in the views of biological structures derived from partially ordered structures. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 75

Inelastic Scattering The application of inelastic scattering to biological studies has only just begun. After a number of efforts in Europe, a start has been made in developing both the theoretical framework and the measurement techniques that will be needed for biological studies. An example of the work carried out so far is a study of the enzyme hexokinase, which changes its conformation when it binds one of its substrates, glucose. In a series of measurements in D2O solvent systems, French scientists have succeeded in demonstrating a change in the dynamics of the hexokinase molecule when glucose is bound. Such dynamic changes may accompany many instances of enzyme-substrate interactions and could be useful in probing the dynamic interdependence of multicomponent systems as well. The development in the United States of suitable instrumentation for low- energy (0-10 meV) neutron spectroscopy, competitive with the best European facilities, would permit an evolution of this potentially exciting area of biological study. Conclusion The applications of neutrons in biology have led to important new insights concerning membrane proteins, enzymes, the ribosome, nucleosomes, the organization of lipid bilayers, virus structure, nucleic acid-protein interactions, and many other areas. It is clear that, while the techniques may appear exotic and specialized to biologists, essential and unique information is gained. There is every expectation RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 76

that such gains will continue to grow in the future, as improved instrumentation and new sources are developed. POLYMER AND COLLOID SCIENCE The availability of SANS facilities has led to revolutionary advances in polymer science over the past decade. The substitution of deuterium for hydrogen makes it possible to extract information about the global molecular geometry of polymer molecules in the presence of high concentrations of other polymer molecules of the same chemical structure but differing isotopic composition. There are no other experimental methods for making such measurements. Results already achieved have stimulated significant theoretical advances, which, in turn, have given direction to many new experiments. These determinations of molecular geometry are derived from the coherent, elastic component of the scattering envelope. Since polymer molecules are large, it follows that experiments at low q are of greatest significance. Quasi-elastic scattering has played a lesser role in the study of polymers. This is partially due to the uncertainty in the theory used for interpretation and partially due to the inaccessibility of suitable instrumentation. The “best” spectrometer of this sort, the spin-echo machine at ILL does not go to sufficiently low q to be completely satisfactory. The problems that can be solved uniquely by quasi-elastic scattering are manifold, and the results are a natural extension of information obtained by other methods. Inelastic neutron-scattering measurements of the RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 77

higher-energy frequency dynamics of polymers can also be quite valuable. A complete analysis requires large deuterated single-crystal samples, which are usually not available, but determination of large-amplitude modes (e.g., torsions) and density of vibrational states by incoherent scattering yields information complementary to that obtained by optical spectroscopies. Since most of the research in the recent past on high polymers has been conducted for the purpose of the study of molecular geometry, we concentrate primarily on coherent elastic low-angle neutron scattering. The application of quasi-elastic scattering of polymers is at an earlier stage of development and will be discussed more briefly. Areas of Special Interest Polymer molecules are long-chain structures that are largely coiled, with molecular weights varying roughly between 104 and 5 × 106 and having a characteristic linear dimension, the centroidal radius of gyration (R), lying between 25 Å and 800 Å. In most cases, data obtained in a range between 0.3 ` qR ` 5 are adequate, which means that for the large molecules a minimum q (momentum transfer) of 4 × 10` 4 Å` 1 is desirable, and for smaller molecules the largest q required would be 0.2 Å` 1. It is difficult to do experiments at low values of q at sufficiently high flux, and this has caused researchers to avoid polymer systems in which molecular weights are greater than 300,000. In many cases, with polymeric electrolytes, for example, it is desirable to work at low polymer concentration, less than 1 percent by weight. The errors owing to low signal/noise RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 78

ratios are considerable in systems of such low concentration, and the time for a single experimental run becomes unreasonably long. The dimensions of particles in colloids are comparable with those in polymers, contrast is established by substitution of deuterium for hydrogen, and therefore the SANS requirements are of the same kind. One difference is that colloidal particles are compact, while polymer molecules are normally somewhat extended. As a result, the scattered signal from colloids is more intense, and experimental limitations tend to be less stringent. Quasi-elastic neutron scattering is used for examination of local motions such as methyl group rotation, polymer diffusion, and global dynamics of polymer molecules. The study of global dynamics is most demanding, and this aspect of polymer motion cannot be probed easily by other methods. Generally, the interest is in low-frequency motions in which long sections of the polymer molecule move in concert. The current instruments provide data at low frequency (down to 106 Hz) but are signal limited to q’s greater than 0.02 Å` 1, and this is not yet low enough to characterize chain dynamics fully. The limitation on q should be addressed in the development of new higher-q resolution spin-echo spectrometers. Some Results from Neutron Scattering The overall chain conformation of a polymer molecule in the bulk had been assumed alternatively to be (a) a random coil with no measurable excluded volume repulsive effects or (b) an ordered structure with neighboring chains strongly influencing mutual conformational arrangements. In the RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 79

absence of SANS, it was not possible to distinguish between these alternatives. SANS measurements on a number of systems strongly support assumption (a), a conclusion that is now accepted universally. While the bulk state is important, it is only one line on the temperature-concentration (T-C) map of a polymer system. Most polymers are synthesized or processed under varying T-C conditions. SANS has made it possible to study the molecular behavior at these conditions. Although most measurements were made in Europe, scientists from the united States have also made major contributions in this area. Moreover, some of the modern theoretical predictions (renormalization group calculations and scaling theory, for example) have been critically compared with conventional theories (mean-field theory and perturbation theory, for example). Advancement in this area so far has mainly been due to comparison with the SANS results. Dynamical studies in this area would be greatly advanced if high-flux, small- angle spin-echo instrumentation and backreflection spectrometers became available to U.S. scientists. The verification of the kinetic theory of rubber elasticity, a theory based on analysis of chain statistics, has been dependent on measurements of network swelling and stress-strain behavior. SANS measurements make it possible to study the deformation of the molecular chains directly. Indeed, it has already been shown from SANS that chain deformation is substantially less than predicted. While this result has been criticized as arising from improperly prepared materials, it has provoked new efforts to improve the theory. Certainly, further measurements of this kind can be expected to stimulate new interest in the molecular theory of elastomers. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 80

Block polymer molecules contain within a single polymer chain at least two different chemical subchains. In some interesting cases, these subchains separate into two microphases owing to mutual incompatibility of different parts of many molecules. These microphases are compact, each unit containing parts of many molecules. This is a colloidal-type structure of characteristic dimension 100 Å or so. Much attention has been given to these block copolymers owing to both their theoretical interest and to their significant practical applicability. They can be and have been studied conveniently by small-angle neutron scattering in which one of the blocks is labeled by deuterium. In a few cases where this has been done, it has been possible to decide which of several theoretical analyses is preferable. Molecular conformation in binary systems and kinetics during phase decomposition is an important area in terms of future polymeric materials. Although it is still in its infant stage, it is already clear that neutron scattering will be one of the most important tools in this area. A high-flux, low-q SANS instrument with time-resolved measurement capability will be especially valuable. This is because the kinetics of microphase decomposition will be the major factor that controls the morphology and performance of the material. Thus, it will be of great scientific and technological interest to study the molecular details of phase decomposition kinetics. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 81

Instrumentation Needs Small-angle neutron scattering of high polymers and of colloids has become an essential research tool. More than 100 experimental studies have been published, the majority using neutrons from the D11 SANS facility on a cold- neutron guide at the ILL. The largest concentration of polymer and colloid work in the United States has been done during the past three years at the NSF facility at Oak Ridge, but important contributions have been made by researchers at the National Bureau of Standards, the University of Missouri, and Brookhaven National Laboratory. In Europe, SANS experiments have also been performed at Jülich, Saclay, and Harwell. Most modern instruments have two-dimensional detection, in which case scattering-intensity problems are less with isotropic materials than with oriented samples such as fibers, stretched rubber, or stretched plastics. This is a consequence of the fact that intensities for anisotropic materials are measured over a narrow azimuthal range, and much longer experiments are needed. It is significant that almost all results on anisotropic scattering have been reported from ILL, where fluxes are an order of magnitude greater than elsewhere. Similarly, studies on polyelectrolytes are most interesting at low concentration (one percent by weight or less), and reliable results from such weak scatterers require higher fluxes than are currently available. The needs for the future are clear. Higher neutron fluxes and an ability to attain a lower range in q are needed. With existing U.S. reactors, this could be obtained by use of a cold source and a monochromator of the velocity selector type. A guide hall adjoining the reactor building would RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 82

be needed for optimal resolution and flexibility. Recent work in Japan has demonstrated that useful research on some problems can be done at modest-flux pulsed sources equipped with a cold source. The possibilities for using the pulse structure of such sources in time-dependent studies will provide new opportunities when much higher-flux sources are developed. Future needs for quasi-elastic scattering of polymers are focused on the problem of chain dynamics. The most promising spectrometer for this work is the “spin-echo” instrument, which permits a much better energy discrimination in the low-energy range than is otherwise obtainable. Thus far only a small number of experiments on polymers have been performed on the only spectrometer of this kind in use. However, the uniqueness and importance of the dynamical information that can be obtained from such measurements makes the construction of such facilities in the United States critical for research in polymer and biological systems. State-of-the-art cold-neutron time-of-flight and backreflection spectrometers would also make major contributions to future studies of polymer dynamics. MATERIALS SCIENCE AND ENGINEERING Neutron scattering offers a unique method of studying the response of materials to external variables such as stress, strain, temperature, and processing variables. The penetrating power of a neutron beam permits the sampling of a large volume of specimens, so that the investigation of bulk properties is possible. The energy of the neutrons is sufficiently RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 83

low that the testing procedure itself does not introduce any additional damage, as may occur with electron microscopy. The sensitivity of SANS to heterogeneities in the size range from a few nanometers to about a micrometer has made it possible to follow, often in detail, microstructural changes in metals and ceramics resulting from deformation, irradiation, or processing. The appearance or dissolution of carbides, precipitates, dispersoids, voids, and microcracks, for example, can be detected, and frequently with sufficient sensitivity and precision that the kinetics of the process can be ascertained and compared with theoretical models. In many cases, the information yielded by neutron scattering on material behavior can be obtained in such detail and with such accuracy by no other method currently available. Studies of Microstructural Changes Produced by Temperature and Deformation It was demonstrated several years ago in Europe that SANS has the potential to monitor thermal processing of complex alloys. Little work has been done to date in this area in the United States. Recently, a SANS study has been carried out to investigate the effect of austenitizing and aging conditions on the size and density of precipitates in a precipitation-hardening high-strength low- alloy steel. The results were correlated with mechanical behavior. A study such as this can be used not only to optimize processing variables but also to determine the allowable leeway in these variables before serious degradation of mechanical properties results. SANS has been used to follow microstructural changes produced by prolonged exposure to high temperatures RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 84

in a ferritic stainless steel developed for use in power-generation applications. Extended service causes microstructural alterations, which affect the mechanical properties of the steel such that it may no longer meet design requirements. Scattering experiments have given detailed information on the size and number of carbides ultimately produced at various temperatures. Deformation greatly hastens the microstructural changes associated with aging. Such changes were clearly picked up by SANS in steel samples fatigued for only a few hours. It may be concluded that service-induced microstructural changes can be detected and even analyzed in detail by SANS, even in complex alloys. In Europe a number of investigations have been carried out in deformation- induced changes in microstructure. Advantage has been taken of the high-flux densities available in order to obtain small-angle scattering patterns over sufficiently short time intervals that the changes can be followed in situ. It should be noted that the scattering features of interest in metallurgical and ceramic studies frequently are rather large (some tens of nanometers). Their density is likely to be low. These are conditions that necessitate high-flux densities and measurement at very low values of the scattering vector. Such requirements cannot always be satisfied at U.S. facilities. Use of SANS in the Detection and Analysis of Damage Several recent experiments have pointed up the value of neutron scattering in the study of damage that appears in the form of cracks or voids. The ability of SANS to provide statistical information on the number, size, and shape of RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 85

microcracks has been demonstrated in an elegant investigation of the ceramic YCrO3, which undergoes extensive microcracking when it passes through a phase transformation at about 1100°C. It was found that the small-angle scattering cross sections from the cracked YCrO3 could be fitted well by the form of the scattering expected from an ensemble of randomly oriented thin disks. By combining the SANS results with data on the elastic constants of the YCrO3, it was possible to determine the number density, average size, and shape of the cracks. Grain-boundary cavitation is a phenomenon found in many metals and ceramics subjected to deformation at elevated temperatures. It was virtually impossible to examine the details of this process until the advent of SANS made available statistical information on the kinetics of void nucleation and growth. It has been found that most of the voids produced by fully reversed cycling are surprisingly small (about 35 nm). At this size they should not be stable against surface energy forces. Void volume fractions of less than 10` 6 can be measured and cavitation picked up by SANS after only 15 sec of fatiguing. No incubation time appears necessary for void nucleation. Since fatigue produces large numbers of small voids it is just possible to carry out SANS studies in facilities currently available in the United States (although higher-flux and lower-q limits would improve the results). However, the void nucleation rate in creep is low, whereas the growth rate is high. Therefore, successful creep cavitation studies by SANS demand the high- flux and low-q capabilities of the D-11 instrument at ILL and are not now feasible in the United States. Recent results on crept Cu from ILL RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 86

measurements show that, in contrast to the case of fatigue, no voids can be detected with sizes below the predicted value of the smallest stable void. As in the case of cyclic loading, no incubation time appears to be required for void nucleation. The measured void size distributions that evolve as creep proceeds have been modeled with remarkable accuracy on the basis of one of the well- known theories of void growth. The theory generally believed to be applicable for these experimental conditions was found to give poor agreement with the SANS results. This is the first instance in which such a precise comparison between theory and measurement of grain-boundary cavitation has been possible. Measurement of Texture and Residual Stresses by Neutron Diffraction X-ray diffraction techniques are well developed for the measurement of both texture and residual stresses. However, the limited penetrating power of x rays has restricted the determination of these quantities to surface layers. Measurement of bulk properties has required destruction of the sample. The x-ray methodology can be applied to neutron diffraction to permit the nondestructive evaluation of texture and residual stresses as a function of depth from the surface. This use of neutron diffraction has great potential. Several investigators have demonstrated that neutron diffraction can indeed be used to determine both texture and residual stresses throughout a component. Test pieces with known stress fields have been examined and satisfactory agreement found between measured and calculated values. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 87

The method now has been applied to the determination of stresses in a number of situations, e.g., in depleted uranium and in composites in cemented carbides. In the latter case, evidence was found of large hydrostatic components of stress between carbide and binder, which arise because of significant differences in their coefficients of thermal expansion. The stress is relaxed by creep or fatigue. Recent measurements have shown that pulsed-neutron time-of-flight neutron diffraction also can be used to investigate residual stresses. Grain interaction stresses in a deformed polycrystalline alloy were determined and found to be in accord with model calculations. (X-ray techniques are not suitable for measuring nonuniform stresses that have a wavelength on the order of the grain size.) Investigations of Phase Decomposition SANS has proved to be a valuable tool in the study of precipitation phenomena, particularly spinodal decomposition. Information on the kinetics of decomposition has been obtained by following changes as aging proceeds in the maximum scattering cross section and the corresponding value of the scattering vector. Several investigators are studying spinodal decomposition in the FeCr system. A coexistence curve determined from scattering patterns was found to be consistent with a curve deduced from reversion experiments but considerably different from calculated values. The high flux at ILL has permitted in situ observations to be made of the growth (or dissolution) of metastable precipitates. From the SANS data it was possible to determine whether zone formation at a given aging temperature took RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 88

place by nucleation and growth or by spinodal decomposition. In another recently reported in situ experiment at ILL, solute partitioning occurring during unmixing of a ternary alloy was studied. Three isotopes of one of the constituents were used in order to obtain independent scattering contrasts. RECENT NEUTRON-SCATTERING RESEARCH IN THE UNITED STATES; COMPARISONS WITH EUROPE 89

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