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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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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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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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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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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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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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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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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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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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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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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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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:
neutron beams
