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6. Fail OPPORTUNITIES: FACILITIES ~ "SUCH Based on the preceding scientific summaries, a number of needs and opportunities for U.S. neutron-scattering research are clear both in the short term and the long term. There is an immediate and critical need to develop state-of-the- art facilities in the United States to match and, where possible, extend the major have been made at research reactors in Western Europe in recent years and that have opened up entirely new areas of important scientific applications for neutron scattering. At the same time, it is essential to initiate without delay design studies for next-generation sources to assure long- term U.S. capabilities. The best U.S. reactor sources provide immediate potential, not only for greatly expanded, internationally competitive facilities for cold neutron research on materials but also instrumentation advances that for new high-intensity, high-resolution thermal neutron instruments. This would involve the application and further development of cold-neutron-source and guide-tube technology to allow high-efficiency transport of neutron beams to large guide halls and provide maximum flexibility for new instrument development. It is also essential to pursue advances in supermirrors, polarization techniques, focusing monochromators and collimators, and area detector systems to optimize the sensitivity of this new instrumentation. It should be noted that research and application in virtually all of these areas, along with time-focusing and correlation techniques 90
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91 for time-of-flight applications, are necessary for the successful development of a new generation of instruments at both steady- state and pulsed sources. Moreover, a healthy and fully competitive U.S. program in neutron-scattering research at existing sources can be achieved by an incremental funding increase that is a fraction of the current massive difference in operating funds and capital investment between the United States and Europe. The Japanese have already mapped out an ambitious program to bridge the even greater gap in neutron- scattering capabilities that they face relative to the Western Europeans. As pointed out in the scientific summaries, the development in the United States of new high-resolution, high-sensitivity instrumentation, and its utilization at both existing reactors and next-generation higher-flux sources, would provide major new scientific opportunities in all areas of neutron-scattering research, opportunities that are impossible to pursue by any other technique. The following provides both general and specific examples of these opportunities, which in some cases extend or summarize items discussed in Chapter 5. CONDENSED-MAr1ER PHYSICS The increased introduction of multidetector systems will greatly facilitate the search for diffuse scattering and weak satellite reflections in novel systems (e.g., charge- and spin-density waves). Moderate-resolution time-of-flight instruments of sufficiently high sensitivity at cold-source guides will permit studies of dynamics of physisorbed and intercalated atoms and molecules. Although it is anticipated
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92 that much of the spectroscopy at energies of 100 meV or higher will be performed on pulsed sources, the study of strongly dispersive high-energy excitations (e.g., high- energy spin waves, Stoner excitations) may often be done best at a steady-state source equipped with a hot source. The development of high-resolution spin-echo and backscattering spectrometers will open new regimes in the study of slow phenomena, such as relaxation effects in glasses and viscous fluids, spin glasses, and random magnets. Diffusion of hydrogen in metals, charge transport in ionic conductors, two-dimensional diffusion of intercalates in layered lattices, and rotational tunneling in molecular crystals are further examples of studies that will greatly profit from such instrumentation. With further technical advances in momentum focusing, these instruments would also be capable of high- resolution phonon linewidth studies, which would revolutionize our capabilities in addressing enharmonic effects, particularly with regard to electron-phonon interactions in superconductors. The use of spin-polarized neutron beams for scattering experiments has always been limited by low neutron fluxes. This is often because exotic materials unfortunately are usually available in very limited sample size in the vital initial phases of their characterization. An elegant class of experiments requiring analysis of the polarization of the scattered neutron beam has always been severely limited by low flux. The development of higher-flux sources and more efficient and versatile polarizers would permit this technique more nearly to approach its true potential. Among the important experiments in this area are studies of spin fluctuations in paramagnets, separation of transverse and longitudinal excitations in itinerant magnets, and the
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93 characterization of the nature of the ordering and excitations of materials undergoing cooperative nuclear spin orientation at low temperatures. CHEMISTRY Most of what has been accomplished to date in neutron spectroscopy of molecules in condensed systems has only scratched the surface of what it is possible to do with neutrons in this area--this will clearly be one of the major growth areas of neutron-scattering research in the future. The development in the United States of high-resolution, low-energy spectroscopy instrumentation, which provides perhaps the most unique information with respect to other methods, is a particularly critical need. It would open up for the first time in this country detailed studies of tunneling phenomena, rotational processes, and diffusion in molecular solids and in molecules bound in homogenous and heterogeneous chemical media. At the same time, the U.S. pulsed-source effort combined with existing reactor instrumentation can provide the higher-energy vibrational spectroscopic capability that is needed. The special sensitivity of neutrons (e.g., to H atom motions in optically opaque media) and the ability to interpret spectroscopic intensities directly provide unique opportunities in a number of areas. A more unified approach in the use of quasi-elastic, low- energy rotational and vibrational spectroscopy will be needed to achieve a full understanding of molecular dynamics, interactions, and bonding in a number of systems (e.g., chemical adsorbents and catalysts, intercalated materials). It should
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94 be noted that much of future activity in neutron chemical spectroscopy will involve the use of difference spectra and isotopic substitution, where the signal from the species of interest is small compared with that from the surrounding media. This, combined with the fact that often the sample sizes available for spectroscopic studies of new novel compounds are very small, will require development of much more sensitive instrumentation and higher-intensity sources than are currently available. In the area of chemical crystallography there is a great opportunity by a combination of increased flux (from more intense sources) and the use of area detectors, which would provide up to a factor of 100 improvement in sensitivity, to expand the range of applications of neutron diffraction to new classes of materials, e.g., inorganic complexes and ceramics, which are only capable of growth as very small (<1 mm3) single crystals. As another example, such increases in data rates would also open up new applications of neutron diffraction for real time (~1 see) in situ studies of solid- state chemical changes occurring during the processing of bulk ceramics, including new kinds of "electronic" refractory materials. BIOLOGY Future opportunities for biological research fall in the areas of crystallography, solution studies, and molecular dynamics. Neutron crystallography at high resolution has been well developed in studies of small proteins but would greatly benefit from higher-flux sources to allow extension to larger
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9s proteins, which must be explored to probe fully the structure of biological systems. Low-resolution crystallography clearly requires improved instruments and higher-flux sources of subthermal neutrons. There are no low-resolution, cold- source-based diffractometers in the United States that are comparable with the instruments available in Europe, and almost all of the major contributions in this area (nucleosomes, purple membranes, and virus structure) have emerged from recent studies at the Ins titut Laue-Langevin. In this regard, the possibility of combining low-resolution crystallographic and solution scattering measurements using a single instrument should be explored. Solution studies have been and will continue to be important, owing to the unique structural information provided by the hydrogen-deuterium contrast. However, there is a critical need of improved instruments for operation at long o (2-15 A) wavelengths and higher fluxes. It is significant to note that, at wavelengths in the 5 ~ region, the D11 instrument in Grenoble provides an intensity more than an order of magnitude higher than any U.S. instrument. Such gains in flux or resolution or both would open up in the United States applications to a wide range of biological problems, which cannot now be approached. Finally, it should again be stressed that the exciting potential of neutron inelastic scattering in the study of low-energy relaxation and chain dynamics in biological assemblies can only be fulfilled if modern high-sensitivity cold-neutron time-of-flight and spin-echo instruments are developed in the United States.
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96 POLYMERS As summarized in Chapter 5, many fundamental questions that are also of technical importance in the polymer field could be answered if low-wave-vector (q) and very-high-resolution elastic and inelastic neutron-scattering instruments using high-intensity cold-neutron beams are made available in the United States. For example, in the area of polymer dynamics, quasi-elastic neutron-scattering instruments covering an extended q range (O.O1 A~1 < q < 2 A-1) could provide an integrated understanding of the high-frequency motions that determine the chemical and electrical properties of polymers and the low-frequency (long-wavelength) motions that dominate the mechanical and transport properties. Neutrons can have a unique and critical role here by allowing a definitive test of various theories of polymer dynamics and relaxation phenomena. The development of advanced high-resolution and high- intensity SAN S instruments will also allow time-resolved measurements of changes in the molecular conformation and microstructure in polymer systems. An example would be studies of polymer chains under external stress either by steady-state or oscillatory shear or extension of the sample, with SANS observations made along the stress direction or phase locked with the oscillatory motion. Other important time-resolved measurements would also be opened up by state- of-the-art SAN S instrumentation and higher intensity sources, including studies of polymer phase decomposition or chemical reactions. There is also a great need for higher cold-neutron intensities to allow, for example, highly sensitive difference measurements by SAN S to probe polymer-surfactant interactions
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97 related to tertiary oil recovery and other industrial applications. More-sensitive instruments would also be essential for the study of interracial behavior of polymer membranes that have potential for future materials separation and electronic applications. MATERIALS SCIENCE There are major opportunities in materials-science applications if higher fluxes, along with diffractometers and SANSinstruments using enhanced area detection and focusing collimation systems become available. The resulting 1 to 2 orders-of-magnitude increased sensitivity and resolution will the size range and level of microstructure] can be studied in bulk materials. be made in the study of nucleation ~ fraction of scatterers that can be detected is lowered. Major improvements in neutron also permit the kinetics of processes such as greatly extend features that Important advances can phenomena if the volume _ substantially facilities will precipitation, phase decomposition, coarsening, and damage accumulation to be followed in real time (~0.1 to 100 see). Moreover, complex stress states in metals and composites can be measured with greater resolution. If much larger intensities of very-long-wavelength neutrons become available, the recent extension of SANS diffraction theory to include refraction effects could open up small-angle scattering studies of many materials phenomena that are too large to be described adequately by diffraction alone. These advanced capabilities for microstructure research and evaluation will provide important fundamental information directly related to the
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98 processing, behavior, and reliability of advanced structural materials. NEUTRON OPTICS There is considerable motivation to develop much larger perfect crystal interferometers having dimensions of a meter or more, with independently oriented and positioned beam splitters. With these devices one could seriously pursue a neutron Cavendish experiment, higher-order gravitationally induced phase shifts, and a neutron version of the Michelson- Morley experiment. Research involving long-wavelength and ultra-cold neutrons, such as an improved electron dipole moment (EDM) search, will require the development of cold sources with large beams and high fluxes. Role of Pulsed Sources While many of the opportunities outlined above can also be addressed in a complementary way by spallation neutron sources (moss particularly inneutron-diffraction applications), these sources are new, and we are just beginning to learn how to use both their spectral and pulsed characteristics. The current favorable position of D.S. pulsed-neutron research provides an ideal opportunity to develop these characteristics over the next few years. We already know that pulsed sources are superbly matched to research in both high-resolution and low-resolution diffraction from powders, glasses, and liquids. They exceed reactor sources for applications requiring high-Q or extreme (e.g., high-pressure) environments. In
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99 addition, it is clear that time-of-flight spectroscopy above the thermal neutron range will rapidly become the province of these sources, as improved instruments and higher peak intensities are achieved. However, competitive application of pulsed sources to subthermal neutron-scattering research and to studies of the dynamics of ordered or single-crystal specimens will require the development of new-generation sources and instrumentation. The effective use of cold and "cool" moderators in pulsed sources is an important area, which must be explored further. For the present, the biggest challenge for spallation sources is how to exploit their rich epithermal spectrum. For example, recent measurements of spin waves up to 150 meV from iron using single crystals, and of electronic crystal- field transitions up to 250 meV in oxide systems, indicate the opportunities in certain applications (magnetic systems, metal hydrides) for the study of high-energy excitations. Similarly, recent incoherent-neutron-scattering studies of high-energy modes in hydrogen-bonded systems show the potential for important neutron spectroscopic applications where optical methods cannot provide needed information on the dynamics of chemical systems. Ultimately, the use of the pulsed structure of these sources by a sequenced application of a variety of stimuli to the sample could open up new applications of neutron scattering. In sum, pulsed sources open up an extended region, of (Q,~) space, and within the next few years we can expect new aspects of condensed matter to be found that lie in this regime. This has been the lesson of neutron scattering for the last 30 years, and we see no reason to change this optimistic view.
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100 Concluding Remarks One of the clear conclusions that emerges from the recent rapid advances in neutron-scattering instrumentation and sources abroad, and from the more modest developments the United States over the past few years, is that there is a much broader community, covering many disciplines, that needs and will respond to new and modernized capabilities in neutron-scattering research. Thus, it seems clear that the provision of a new generation of neutron instruments outlined above would more than double the existing neutron- scattering user community, particularly if instrument development is combined with incremental personnel resources to allow a more effective effort for the assistance of users. The role of workshops for the user community and effective user policies and procedures for neutron facilities will also be essential. In fact, it is our view that the increasing importance of neutron-scattering facilities to a broad range of disciplines and users requires the active participation of representatives from these diverse fields in the planning of new instrumentation and sources. Moreover, in order to assure that future neutron sources meet the total needs of U.S. science and technology, it is essential that the university, industrial, defense, and federal laboratory communities have a direct role in establishing the appropriate balance of capabilities to be included in such new sources. It seems most appropriate that an independent, broadly based advisory group should be established by the National Academy of Sciences to provide guidance to the government on the technical characteristics, user policies, and siting of future major neutron centers.
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101 Our examination of existing U.S. neutron sources suggests that a program to allow the United States to achieve an internationally competitive position with other industrialized nations in neutron-scattering research would require an increase from all funding sources of an average of ~615 million/year (in fiscal year 1983 dollars) capital expenditure over a S-year period. This would allow, e.g., the timely development or modernization of ~30 critical instruments, including associated cold-source and experimental-hall construction, to meet the new multidisciplinary science opportunities outlined above. A gradual rise in personnel and experimental support to a total increase of ~312 million at the end of this development phase would be required to allow the science to be done and provide incremental resources for the assistance of hundreds of additional users. Such an investment can be compared with the ~6300 million capital investment in Western Europe during the past decade and the current >650 million difference in scientific operating expenses between the United States and Europe. Finally, we would address both the need and the opportunity to plan for a new generation of neutron sources for the mid-199Os and beyond. While current U.S. steady-state sources are and will remain competitive for at least the next decade in innate intensity (if not flexibility), these sources will be between 20 and 25 years old in 1990. We must consider ways in which their capabilities can be replaced with even greater capabilities to meetfuture scientific needs. Currently, new designs are under consideration for an advanced research reactor featuring increases in power density and total power, which would produce a steady-state flux of about 5 ~ 1015 neutrons/cm2-sec. With improved beam-tube design in such
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102 a new reactor, it would be possible to increase neutrons at the sample position for many experiments by an order of magnitude over present generation reactors. Moreover, one advantage of a vigorous testing of pulsed sources and related instrumentation is that new accelerator advances may allow the achievement within the next 15 years of pulsed sources with peak thermal fluxes of ~1017 neutrons/cm2-sec and average fluxes above 1014 neutrons/cm2-sec. For example, design studies have recently been initiated for a next-generation pulsed source based on a fixed-field (FFAG) proton accelerator. If successful, could ultimately achieve these flux characteristics at a lower capital and operating cost than that projected using current accelerator designs. Thus, there is an immediate opportunity to carry out systematic planning and design for new sources that will clearly be needed by the mid-199Os. Considering the long lead time for the construction and instrumentation of these sources, it is essential that support be provided so that such design efforts may be implemented quickly. alternating-gradient such a source
Representative terms from entire chapter: