HFIR's Cold Neutrons for New Materials Insights

At the turn of this century, ORNL's High Flux Isotope Reactor (HFIR) was disassembled and its beryllium reflector was replaced. HFIR resumed full-power operations (85 megawatts) in December 2001 following a 14-month outage.

ORNL’s High Flux Isotope Reactor (HFIR) has been a tool for materials researchers for nearly four decades. Far from over the hill, the HFIR, which is in the midst of an upgrade program, figures highly in the Laboratory’s strategy for becoming a world leader in neutron science. The research reactor took a yearlong maintenance break in 2001 for the replacement of its beryllium reflector (which reflects neutrons back into the reactor core). The sojourn provided ORNL and the Department of Energy’s Basic Energy Sciences program an opportunity to improve the reactor’s neutron-scattering capabilities.

One of the premier features of the upgrade is the addition, scheduled for 2003, of equipment that literally chills the neutrons produced by the research reactor. As the neutrons pass through this “cold source,” they will rebound through an environment of super-cold liquid hydrogen, which will reduce their thermal energies and slow them so they have a longer wavelength.

The Solid State Division’s Herb Mook, an ORNL corporate fellow and recipient of ORNL’s distinguished scientist award for 2001, says the rich supply of long-wavelength neutrons will make HFIR a valuable tool for the study of larger, more complex atomic and molecular structures.

A view of HFIR's reactor fuel core as it emits blue Cerenkov radiation.

“Thermal (hot) neutrons have wavelengths of a few angstroms, which is the size of the crystal structure for materials like iron or nickel,” Mook says. “Cold neutrons have wavelengths 10 to 20 times longer. Those longer wavelengths will allow us to study more complex materials, such as living cell structures and polymer blends.” The chilled neutrons will pass through a beam guide—Mook compares its function to an optical fiber—to newly constructed support facilities equipped with several state-of-the-art instruments for the neutron analysis of materials. They include two small-angle neutron scattering (SANS) diffractometers. “SANS will allow us to look at very large-scale structures, such as polymers, which are long chains of molecules,” says Mook. “Polymer blends make up a huge amount of the materials we use every day.”

Also likely for further studies with SANS are complex spherical molecular structures called micelles, which hold the promise of becoming tiny chemical processing plants if scientists can learn more about their structure and function. If researchers are successful, these micelles could offer a “greener” alternative to processes that use chlorofluorocarbons, which damage the earth’s protective ozone layer. The friendlier processes would use pressurized carbon dioxide (CO₂), a much more environmentally benign gas, to produce plastics, polystyrene, and other ubiquitous modern materials. Mook notes one practical application of a CO₂ process already in use: a new method for dry cleaning, which traditionally has used harmful solvents. More sophisticated uses are in the future, and Mook believes neutron analysis will help researchers learn how to develop greener processes using micelles.

DOE’s Office of Basic Energy Sciences is funding a second SANS instrument that will be the cornerstone for the Center for Structural and Molecular Biology. The use of neutrons for the studies of biological materials promises to become a growth industry, owing largely to the new facilities at ORNL. “Neutrons have not been exploited to their full extent in the past for biological studies,” Mook says. “The cold neutrons, with their longer wavelengths, will allow us to look at the sizes and shapes of these more complex biological molecules.”

HFIR’s SANS instruments will complement a similar instrument at DOE’s Spallation Neutron Source at ORNL. Whereas the SNS instrument’s strength will be its versatility—the ability to analyze at many different resolutions and scales—HFIR’s SANS instruments will offer high throughput, high resolutions, and the longest-length scales to researchers.

The upgraded HFIR will have 15 state-of-the-art neutron-scattering instruments that will be among the world's best. Full descriptions of these instruments are available at http://neutrons.ornl.gov. (Illustration enhanced by Renee Manning)

HFIR’s instrument hall will also house two new triple-axis spectrometers that will focus on the studies of dynamics—the ways that atoms and molecules move, which affect their properties. “There is a force between atoms that holds the structure together,” says Mook. “If you hit atoms with a neutron, they vibrate. An analysis of the vibrational pattern provides information on how the atoms are bound together.” Mook believes that knowledge gained about atomic structure and magnetism will lead to even greater leaps in technologies such as data storage.

Other additions to HFIR will be a powder diffractometer, a dedicated residual stress diffractometer, and the U.S.-Japan wide-angle neutron diffractometer. Once the upgrade is completed, HFIR will be positioned to serve the neutron science community for many more years. Pressure-vessel test results assure that HFIR can operate for 35 more years.—B.C.

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Related Web sites

• High Flux Isotope Reactor (HFIR)
• ORNL's Solid State Division

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