Since beginning operations in 1966, ORNL's High Flux Isotope Reactor, known among the research community as "HFIR," has provided a uniquely powerful and versatile resource tool for visiting scientists from industry, academia and other national laboratories. The research reactor, now operating at 85 megawatts, not only generates the most intense neutron flux of any research reactor in the world, but also is home to a broad selection of instrumentation used to explore the structure and dynamics of materials.
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In 2009, more than 250 guest scientists conducted research at HFIR in areas ranging from physics to materials science to biology. Although the total was the largest in HFIR's history, the neutron scattering instruments in the reactor's user program consistently receive three times more user requests than can be accommodated.
Part of the reason for nearly a half-century of success is HFIR's ability to produce continuous and intense neutron beams. World-class neutron scattering research requires a high neutron flux, which HFIR provides at a quality and predictability equal to any reactor in the world. An intense source of neutrons is necessary for precise structural measurements because neutrons interact weakly with matter. As a result, a large number of neutrons must scatter to create an accurate picture of a sample's structure. "There are competing facilities in Europe," says Neutron Scattering Science Division Chief Scientist Stephen Nagler, "but Oak Ridge is as good as any of them at what we do. We can measure things at HFIR that are not measurable at any other reactor in the world."
Unique capabilities
Nagler gestures at a chart detailing current and proposed neutron instruments at ORNL and notes that the Laboratory is in a unique position with regard to neutron sources. "We have the Spallation Neutron Source, which is the world's most intense accelerator-based pulsed neutron source. Researchers also have access to HFIR which, although originally built primarily to produce isotopes, has four horizontal beam tubes that are used for continuous neutron beam experiments."
Each of HFIR's beam tubes can support more than one instrument. Three of the tubes deliver "thermal" neutrons with energies comparable to the vibration of atoms at room temperature. The fourth tube includes a "cold" source that generates lower-energy neutrons. Neutrons of different energies have a greater capability to "see" some features of materials, providing a considerable advantage for users able to conduct research at a single facility that can produce both thermal and cold neutrons. Bolstered by a recent upgrade, HFIR has an array of instruments operating on the reactor's cold and thermal beam tubes.
SANS instruments – HFIR's cold source feeds neutrons to two small-angle neutron scattering
(SANS) instruments. The instruments are normally used to look at nanoscale objects, such as proteins or polymers. One of the instruments is a general purpose SANS. The other, dubbed BioSANS, is dedicated to studying biomolecules. The instruments attract an increasingly broad cross-section of users investigating materials as diverse as biologically important proteins, polymers, metal alloys and high-temperature superconductors.
IMAGINE Single Crystal Diffractometer – The soon-to-be-installed IMAGINE Single-Crystal Diffractometer is expected to provide structural information on single crystals of a range of chemical and organic materials, yielding a hundredfold increase in performance compared with traditional diffractometers. The instrument is expected to be of particular interest to users in the pharmaceutical community.
HFIR's "cold side" also hosts a station used for neutron imaging and neutron radiography. These techniques can be used to measure phenomena involving the movement of liquids, such as the diffusion of water through soil or the motion of fluids in batteries. HFIR staff members are developing additional instruments for the "cold" beamline.
HFIR's "thermal neutron side" is home to seven instruments that measure the structure and dynamics of materials.
Powder Diffractometer – Powder diffractometry is a standard technique for examining the structure of crystalline materials, such as metals, minerals and ceramics, under a range of conditions. The advantage of neutron powder diffractometry over similar analytical techniques, such as X-ray diffractometry, is that while X-rays have trouble identifying light molecules, neutrons are sensitive to both light and heavy atoms. As a result of this advantage, HFIR's powder diffractometer is often the instrument of choice for researchers who need to analyze the structure of a newly created material.
Wide-Angle Neutron Diffractometer – Known as WAND, the instrument is also used in diffraction studies. One use is rapid data acquisition that provides an overview of several different types of structural measurements of a material in a short timeframe.
Nagler recalls one of the more interesting and unusual experiments conducted on the WAND, which involved a group of researchers wanting to study the structure of ferroelectric ice in deep space. Ferroelectric ice has been linked to various phenomena, including planetary formation.
"The researchers expected the ice to form under certain conditions at very low temperatures," Nagler says. "When they conducted the experiment, they found that some ice did indeed form. By measuring the structure of the ice, the group determined that certain astrophysical phenomena can be explained by the presence of ferroelectric ice in space. In this case, neutron scattering data that is normally used to measure very small things was used to investigate phenomena on a galactic scale."
Four-Circle Diffractometer – Unlike HFIR's other diffractometers, the Four-Circle Diffractometer is used for the studies of "single crystals," such as those in metal alloys and ceramic materials. The materials are those in which the arrangement of atoms, ions or molecules is repeated throughout their entire volume. Scientists use the wide-angle and powder diffractometers to study "polycrystals," samples that comprised a large number of single crystals that are randomly oriented throughout the sample.
Neutron Residual Stress Mapping Facility – HFIR's remaining diffraction instrument is the Neutron Residual Stress Mapping Facility. The instrument is managed by ORNL's Materials Science and Technology Division and is used to measure stress, often in large-scale materials such as jet engines, automobiles and bridges. Researchers use data from these measurements to determine the effectiveness of stress relief methods and to check failure analysis, design and life predictions provided by computer
Triple-axis Spectrometers – HFIR's toolbox contains three different types of triple-axis spectrometers. Generally speaking, these instruments enable scientists to study motions of atoms and their associated magnetic properties. Each of HFIR's three triple-axis spectrometers has a different emphasis. The HB3 spectrometer is a general-purpose spectrometer. The HB1 instrument is similar but can be set up to use polarized neutron beams that have only one magnetic state. HB1A, operated by both ORNL and Iowa's Ames Laboratory, differs from the others in that neutrons in the beam striking the sample have a fixed energy, while the other two instruments provide researchers with the ability to adjust the energy of the neutrons.
Recent experiments on the various triple-axis spectrometers have included studies of multiferroic materials that can be simultaneously magnetic and ferroelectric. Researchers anticipate that the materials could have applications in high-speed computation and communication. The spectrometers have been used to characterize some of the newly discovered class of iron-based superconductors described in Article 12, "Focusing on the Science."
A unique user experience
Years of experience with the HFIR user community have convinced Stephen Nagler that one of the main advantages of running a user facility is that researchers from all over the world bring good ideas with them. "HFIR staff members certainly have their own good ideas," he says, "but we benefit by interacting with other scientists. The interaction has definitely driven our research program in new directions. Every researcher brings individual insights. This collaboration generates a tremendous amount of creative energy for our research program."
Postdoctoral researcher Clarina De la Cruz notes that the symbiotic atmosphere at HFIR also has a beneficial effect on the research process itself. "I have worked at many of the neutron scattering facilities in the U.S., but HFIR is different. At HFIR, the researchers and staff take care of you. They work with you. They don't just want you to come in and do your work. They encourage you and want you to be successful. It's a small thing perhaps, but it's something I haven't experienced at other facilities."
Source: Oak Ridge National Laboratory
