I am sure that Bertram Brockhouse would join me, if he were present with us, in saying how immensely grateful we are to the Royal Swedish Academy of Sciences for affording us the high honor of being named Nobel Laureates in recent days. Being selected as such is a dream for any scientist who hopes that his work will prove useful to others. It is especially gratifying to me to have met at this symposium with so many of you who have contributed greatly to bringing this field of neutron scattering to its present state of usefulness and prominence.

The Chalk River Laboratories of AECL Research provides neutron beams for research with the NRU reactor. The NRU reactor has eight reactor loops for engineering test experiments, 30 isotope irradiation sites and beam tubes, six of which feed the neutron scattering instruments. The peak thermal flux is 3 x 1014n cm−2s−1. The neutron spectrometers are operated as national facilities for Canadian neutron scattering research. Since the research requirements for the Canadian nuclear industry are changing, and since the NRU reactor is unlikely to operate much beyond the year 2000, a new Irradiation Research Facility (IRF) is being considered for start-up in the first decade of the next century. An outline is given of this proposed new neutron source.

The National Institute of Standards and Technology (NIST) offers a wide range of neutron beam instruments to the U.S. community. Many of these are of recent construction, and are located in the Cold Neutron Research Facility, a large guide hall with fifteen experimental stations on seven neutron guides. A new liquid hydrogen cold source is being installed. Thermal neutron instruments in the reactor confinement are to be upgraded over the next few years.

The neutron and gamma-ray scattering facilities at the University of Missouri Research Reactor Center include six elastic and inelastic neutron scattering instruments currently in operation and three additional ones under construction. A gamma-ray diffraction and quasi-elastic spectrometer are also available. The Center has no formal user program, but encourages external collaborative work particularly that involves graduate student thesis research.

Since its beginnings in Oak Ridge and Argonne in the late 1940's, neutron scattering has been established as the premier tool to study matter in its various states. Since the thermal neutron wavelength is of the same order of magnitude as typical atomic spacings and because they have comparable energies to those of atomic excitations in solids, both structure and dynamics of matter can be studied via neutron scattering. The High Flux Isotope Reactor (HFIR) provides an intense source of neutrons with which to carry out these measurements. This paper summarizes the available neutron scattering facilities at the HFIR.

We describe the Small Angle Neutron Scattering spectrometer at McMaster University. We then discuss three examples of recent studies using this instrument: (1) the membrane-water interface in multilamellar vesicles; (2) magnetic ordering of Ni0.8Co0.2/Cu multilayers; and (3) precipitate sizes in titanium stabilized interstitial-free steels.

Small-angle neutron scattering (SANS) is increasingly used to obtain statistically-representative data on particle or pore sizes, number and volume fractions, morphology and total surface areas in technological materials. Until recently, however, it had not been possible to perform in-situ microstructural investigations during thermal treatment. This paper reports on a new high-temperature (up to 1700°C) SANS furnace for materials research. Two interchangeable inner furnaces were built for a single outer atmosphere chamber so that either an oxidizing, a reducing, or a neutral environment can be used. Results derived during sintering of a controlled-porosity silica gel will be presented. The new furnace has made it possible for the first time to measure total porous surface areas and the evolution of pore sizes in situ during densification without interruption. Such measurements are expected to lead to improved process models offering quantitative predictability of product microstructures from the processing history of real materials.

A recent small-angle neutron scattering (SANS) instrumentation development at the University of Missouri Research Reactor Center (MURR) using bent, double Si crystal diffraction is presented. Neutron rocking curves of flat single crystals are only a few seconds of arc with correspondingly low integrated intensities. The rocking curve can be significantly broadened if the crystals are bent. However, the phase-space acceptance and emergence windows of two bent crystals in parallel can be matched only if the reflecting planes have different d-spacing. We have used a combination of the Si (111) and (220) reflections in the work presented here. The peak intensity of the rocking curve with this configuration was approximately 2.5 x 105 neutrons per second over a 5 cm2 area of sample at 1.5 Å. The rocking curve wings fell sharply (approximately as Q"11), permitting accurate scattered intensity measurements down to a minimum Q value of 0.005 1/Å at a Q resolution of 0.003 1/Å. The technique also offers the possibility of variable resolution without too much difficulty; changes up to a factor of 2 can be made by adjusting the sample-to-analyzer crystal distance, while changes of a factor of 10 can be made by using different Si reflections. Since the incident neutron wavelength can be tuned to the peak of the leakage spectrum, this technique is ideal for low-flux reactors, which otherwise cannot be used for SANS research. The usefulness of one-dimensional SANS is demonstrated by measurements of Portland cement.

The new high-resolution neutron powder diffractometer BT-1 at the NIST reactor has proven to be a powerful and versatile instrument in its first year of operation. With 32 detectors arranged at 5° intervals and a 12° 2θ scan range, powder diffraction patterns can be collected to 167° 2θ. There is a choice of three monochromator take-off angles (75°, 90°, and 120°) so that the peak-width minimum can be matched to the rf-spacing range that is most important for each sample; all choices have a wavelength close to 1.54 Å. Data can be collected on sample sizes ranging from 200 mg to 30 g. Temperatures of 0.3 K to 1400 K are routinely available, and a magnetic field of 7T can be applied with a superconducting magnet. Typical data collection times range from 1-12 hours depending upon sample size and desired resolution. Examples are given of a variety of materials applications.

A specular and diffuse neutron reflectometer was designed at the Massachusetts Institute of Technology nuclear reactor, MITR-II, and the description of the facility is presented in this paper. This reflectometer uses a horizontal sample geometry so that both solid and liquid samples can be measured. To minimize sample or beam motion and reduce measurement time, two neutron beams are extracted from the same beam port and are incident simultaneously on the same sample surface at grazing incident angles of 0.2 and 1.5 degrees, respectively. The reflected neutrons are detected by a linear position sensitive detector and energy-analyzed by the time-of-flight method. The reflection flight path is enclosed in an evacuated chamber to reduce background counts. The reflectometer performance is estimated as 0.004-0.33 Å-1 dynamic range, 5% Q resolution, 1-10-7 measurable specular reflectivity range, and a 4 orders of magnitude range in diffuse reflectivity counts. Diffuse reflectivity can be measured as a two-dimensional function of both the wavelength and the in-plane diffuse reflection angle. The instrument can be used to investigate a wide range of surface systems of condensed and soft-condensed matters.

We discuss the design of a new backscattering spectrometer to be installed at the Cold Neutron Research Facility at the National Institute of Standards and Technology. Si (111) crystals cover both monochromator and analyzer which are spherically bent to a radius of curvature of ~ 2 m to focus the incident and scatterered neutron beams. The bending increases the intrinsic lattice gradient of Si beyond its Darwin limit, resulting in an energy resolution of ~ 0.75 μeV FWHM. The monochromator is Doppler-driven, allowing users access to a dynamic range of ±60 μeV. The elastic Q-range covers 0.15 to 1.8 Å-1. The most novel aspect of this design lies in the incorporation of a phase-space-transform chopper. This device rotates at 4700 rpm while neutrons are Bragg-diffracted from sets of pyrolytic graphite crystals affixed to its periphery. The process enhances the neutron flux at the backscattering energy of 2.08 meV, but at the expense of a larger horizontal divergence. Computer simulations indicate a resultant flux increase of order 3 should be obtained.

A highly versatile multiple disk chopper neutron time-of-flight spectrometer is being installed at the Cold Neutron Research Facility of the National Institute of Standards and Technology. This new instrument will fill an important gap in the portfolio of neutron inelastic scattering spectrometers in North America. It will be used for a wide variety of experiments such as studies of magnetic and vibrational excitations, tunneling spectroscopy, and quasielastic neutron scattering investigations of local and translational diffusion. The instrument uses disk choppers to monochromate and pulse the incident beam, and the energy changes of scattered neutrons are determined from their times-of-flight to a large array of detectors. The disks and the guide have been designed to make the instrument readily adaptable to the specific performance requirements of experimenters. We present important aspects of the design, as well as estimated values of the flux at the sample and the energy resolution for elastic scattering. The instrument should be operational in 1996.

Time resolved phase transition and strain experiments have been performed on the millisecond time scale using a Bragg-edge transmission technique that has been developed at the Los Alamos National Laboratory. The precision with which lattice parameters can be determined from edge positions is sufficient to perform high-resolution strain measurements in uniaxial stress.

Diffraction peak shapes generated from high purity, low strain uranium silicide powder have been used to deconvolute diffraction peaks from radiation damaged material. The deconvolution was based on the assumption that defect scattering near a strained Bragg peak could be represented as the summation of a series of discrete peaks with the same convoluted peak shape measured for unirradiated material in the same range of d-spacing. Discrete peaks were taken to have the same density as the measured time of flight data so that a deconvoluted scattering intensity was generated at each experimental point. This deconvolution technique accounts in a natural manner for peak asymmetries arising from the time structure of the neutron pulse shape and instrument broadening without arbitrary analytical functions and fitting parameters.

An introduction to the methodology of neutron reflectometry is. given in which the fundamental aspects regarding the actual performance of specular reflection measurements and subsequent analysis of the data are described. The application of this technique to the determination of interfacial structure or composition in thin film and multilayer materials of interest in the fields of magnetism, superconductivity, polymer science, electrochemistry, and biology is illustrated by specific examples. The microscopic information provided by neutron reflectivity which complements that obtained by other probes is emphasized, in particular information which is obtainable because of the inherent isotopic (most notably in the case of hydrogenous materials) or magnetic moment (both magnitude and orientation) sensitivity of the neutron.

The phase problem that troubles x-ray diffraction is also a problem for x-ray and neutron reflectivity; however, there are a range of small angles just beyond the critical angle for which the reflectivity contains information on the phase of the surface scattering amplitude. In principle this phase information can be used to determine the asymmetry of the normal gradient of the scattering amplitude density. We discuss here practical conditions under which this symmetry can, or cannot, be extracted from reflectivity data.