NEUTRON SCATTERING RESEARCH: BORN IN OAK RIDGE
The use of neutron scattering to obtain valuable information on the properties of materials was pioneered in Oak Ridge. Ernest Wollan installed a modified two-axis X-ray diffractometer at a beam port of the Graphite Reactor in November 1945, and he was joined in this work several months later by Clifford Shull.
The research by these two scientists and their associates laid the foundation for widespread application of neutron scattering techniques throughout the world and for the preeminent position of these techniques in many areas of scientific research. Their first two-axis neutron diffractometer is now on display at the Smithsonian Institution in Washington, D.C.
Wollan and Shull, both of the Laboratory's Physics Division, were well prepared for their pioneering efforts in this new field of research. Both had strong backgrounds in X-ray physics and X-ray diffraction techniques, and they quickly recognized the potential of neutron scattering for nuclear and solid-state studies. Within a few years, they and their associates had established neutron scattering as a very valuable, quantitative experimental technique; demonstrated the importance of neutron scattering in determining the positions of hydrogen atoms in materials; observed the existence of ferromagnetism and antiferromagnetism; reported fundamental results on ferromagnetic materials; and measured the nuclear scattering amplitudes for more than 60 elements and isotopes.
The determination of hydrogen positions in materials was of such interest to ORNL crystallographers that a separate program was established in the Chemistry Division under Henri Levy to study hydrogen bonding in crystals. Levy and Selmer Peterson were pioneers in developing the neutron scattering technique for detailed structural analysis of single crystals. Bill Busing, Harold Smith, Ray Ellison, Dan Danford, George Brown, Carroll Johnson, Paul Agron, Bill Thiessen, and Al Narten joined the Chemistry Division program at later dates. As the program moved to more advanced reactors, the experiments shifted to more complicated materials and to more sophisticated equipment. Automatically controlled instruments to orient samples and to position detectors were designed and built, and significant information was obtained on atomic structure and bonding in a large number of materials. The first minicomputer-controlled diffractometer was used in this research, and computer programs were developed for data analysis, which were rapidly adopted by crystallographers throughout the world.
In the Physics Division, Wollan and Shull were joined by Wallace Koehler in 1949 and Mike Wilkinson in 1950, and this group helped to explain a variety of phenomena in magnetic materials. Shull left ORNL for the Massachusetts Institute of Technology in 1955, and shortly afterward Joe Cable and Ray Child joined the program. In the late 1950s and early 1960s, this group discovered the rich array of exotic magnetic structures in the rare earth metals and alloys--one of the most significant and exciting achievements in the history of neutron scattering. Ralph Moon joined the group in 1963, shortly before Wollan retired and the group was transferred into the Solid State Division.
When the Oak Ridge Research Reactor (ORR) began operating in 1958, ORNL had a brief edge over other research facilities in neutron source intensity. In addition to the programs in crystallography and magnetism, in 1962 Mike Wilkinson and Harold Smith, both then in the Solid State Division, initiated a program of inelastic scattering to investigate the dynamic properties of atoms in solids. They were soon joined by Robert Nicklow and Herbert Mook. The group constructed a triple-axis spectrometer at the ORR, which was based on an instrument developed at the Chalk River Laboratory in Canada.
With the startup of the High Flux Isotope Reactor (HFIR) in 1966, ORNL once again had the most intense neutron source for research in the world. This high intensity, together with state-of-the-art instrumentation at the four HFIR beam ports, allowed experiments to be performed that had not been possible previously. Eventually, each of the beam ports was equipped with at least two instruments, and some of them at the time of installation were unique in the world.
In the late 1970s and early 1980s, strong efforts were made by ORNL and DOE to permit scientists from other organizations to use the HFIR facilities for both cooperative and proprietary research. The National Center for Small-Angle Scattering Research, which was strictly for the benefit of users, was established under an interagency agreement between DOE and the National Science Foundation (NSF); it included the design, construction, and operation of a sophisticated small-angle neutron scattering (SANS) facility at the HFIR using NSF funds. Wallace Koehler and Robert Hendricks were leaders in the project, and Koehler became the first director of the national center. George Wignall, who later became the national center's director, and John Hayter, who is scientific director of the Advanced Neutron Source project, came to ORNL to use the SANS facility for the study of polymers and colloids. In addition to the national center, a more formal user's program was initiated for all ORNL neutron scattering facilities, a special cooperative program was started with scientists from Ames Laboratory, and DOE established a cooperative neutron scattering program at ORNL with Japanese scientists as part of the U.S.-Japan Agreement on Cooperation in Research and Development in Science and Technology. The programs hosted about 150 users a year until late 1986.
In November 1986, the HFIR was shut down for more than three years because of concerns about safety and management of the reactor. The user programs were suspended until the summer of 1990, but interest in performing research on the HFIR instruments has increased rapidly since then. Unfortunately, the neutron scattering instruments are old, and the best facilities for research are found in Western Europe. The Advanced Neutron Source, which is planned for ORNL, would provide the highest-intensity neutron beams in the world, furnish new state-of-the-art instruments, accommodate over 1000 users annually, and undoubtedly return leadership in this very important field to the United States. The neutron is a unique and remarkable probe for studying materials, and neutron scattering research provides information that is essential in development of new and better materials for many technologies. The Advanced Neutron Source would permit these important investigations to continue well into the next century.
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