A remarkable suite of instruments harnesses the power of the SNS.
Despite its status as one of the world's largest scientific facilities, the true value of the Spallation Neutron Source is not defined by the scale of the buildings and equipment required to generate proton pulses with unprecedented power. For those conducting research at the SNS, a suite of specialized instruments is the secret to providing scientists a unique breadth of opportunity to expand their understanding of the structure and functioning of materials, under a range of real-world conditions. Ken Herwig notes that the long-term development plan for the SNS dictates that the facility seems to always be in the process of adding instruments, some of which are larger than entire buildings that housed earlier generations of equipment.
"We have instruments that have been in the user program several years," Herwig says." We also have instruments that have recently been added to the program, some that are being commissioned, and instruments that are planned or under construction but will not be in the program for another four or five years."
The SNS target station has a total of 19 instruments in various stages of development on the 24 available beamlines. Tentative plans exist for some of the remaining slots, with preliminary concepts for the 25 additional beamlines at the facility's proposed second target station.
The original three
(BaSIS) and two reflectometers, were completed as a part of the original SNS construction.
The BaSiS instrument is designed to examine how materials move at the atomic scale over a relatively long range of times—picoseconds to nanoseconds. The instrument also can analyze liquid and solid samples.
One of the most ubiquitous, and most studied, materials on the earth is water. Accordingly, more than half of the publications generated by BaSiS are devoted to the study of water. Much of the recent scientific interest centers around how water behaves when present in much more limited quantities. The very thin layers of water that cover the surface of hydrophilic materials under ambient conditions are one example. Water in these systems may be one or a few molecules thick. The presence of water exerts great influence on the surface properties of the materials. Recent results include the observation of multiple processes occurring in individual water layers.
Across the experimental hall, the Magnetism Reflectometer is used to investigate the surface layers of materials and has the unique capability of studying magnetism in very thin films or multilayers.
The research has applications in the development of materials used for data storage devices, like DVDs and camera memory cards and computer disk drives.
"One buzzword these days is 'spintronics'," Herwig says, "using the spin state of a material's electrons to store or transfer information. With a layered material, the instrument is able to probe the magnetic coupling from one layer to another and determine how well the material might function."
The SNS's third original instrument is the Liquids Reflectometer, designed to look at horizontal surfaces for studies ranging from polymer interfaces to synthetic cell-membranes to liquid surfaces. Researchers have employed the Liquid Reflectometer to help develop a thin polymer film that behaves differently depending upon the acidity, or pH, of its environment.
"If researchers can make a pH-sensitive thin film and wrap the film in a drug, they can control where the drug is released in the body," Herwig says. "The researchers created a layered sample with markers that were visible to neutrons between the layers. Then, as the pH of the sample's environment became more acidic, they could observe the film dissolving layer by layer."
The next generation
Since the SNS came on line in 2006, instruments have been added at a rate of about two each year. The Wide Angular-Range Chopper spectrometer was designed to explore the movement of energy waves, called phonons, through hard materials.
The spectrometer's most distinctive capability is measuring phonons moving through powders as well as through single crystals. The process has applications in the analysis of superconducting materials, like the recently discovered iron arsenide family of superconductors. Researchers hope to understand how the behavior of phonons changes as a function of temperature and to determine whether phonons play a role in the superconductivity of these materials.
As a complement to this research, the Spallation Neutrons and Pressure diffractometer (SNAP) enables SNS researchers to study a variety of powdered and single-crystal samples under extreme conditions of pressure and temperature. The increased neutron flux of the SNS, combined with SNAP's ability to generate extreme pressure using specialized sample environment equipment, has broadened the pressure range accessible to neutron researchers.
An ambitious goal for SNAP is to reach 100 GPa, a pressure approaching that of the earth's core-mantle boundary. To obtain this extraordinary pressure, sample volumes must be very small, requiring the neutron beam to be concentrated to a similarly small area. To accommodate the small sample, ORNL researchers have designed a mirror that can focus the beam to the exceptionally small width of about 75 microns.
"At 100 GPa," Herwig says, "SNAP will reveal phase transitions in materials under high pressure and temperature, particularly the structure of minerals deep within the Earth." Other state-of-the-art instruments are in the queue. The Powder Diffractometer, or POWGEN, will come online in 2009 and is expected to be the SNS's workhorse in a wide range of structural studies.
POWGEN will enable researchers to predict how materials will undergo structural transitions as they respond to temperature, pressure and magnetic fields. Understanding these changes in the presence of gases is especially important for the study of catalysts and hydrogen storage materials.
Herwig expects POWGEN to be particularly popular for studies of energy-related materials such as cathodes and anodes in batteries and solid oxide membranes for fuel cell applications.
A second instrument scheduled to come on line in 2009 is TOPAZ, a single-crystal diffractometer. Herwig says the instrument can generate substantially more information from analyzing a single crystal of a material than currently can be produced from a powdered sample. The advantage comes from the researcher's ability with a single crystal—unlike a powder—to know how the material is aligned in the neutron beam when taking a measurement. TOPAZ holds promise for a number of potential applications, including analyzing the structure of small- to medium-sized molecules with pharmaceutical appliocations. One of this instrument's particular skills is finding a material's hydrogen atoms. This unique characteristic is often critical when researchers are required to locate molecular binding sites or to distinguish one molecule from another.
Producing the science
As intended, the remarkable array of instruments available to scientists at the SNS is channeling the raw power of the facility's neutron flux into the specialized applications needed to illuminate new frontiers in the fields of materials and biological science. The instruments support research that falls into two categories. One is curiosity-driven, without immediate direct application but vital to the creative energy of the scientific process. The other equally important research category is often use-inspired and focused on specific issues such as energy production or storage.
The instruments are producing huge streams of data that in turn have resulted in hundreds of scientific papers. While the majority of these papers are still in the peer review process, they hold the promise of opening new horizons throughout the field of materials research. The expectations for the SNS are high, befitting one of the Department of Energy's largest and most modern scientific facilities.
The self-imposed expectations are equally high for the SNS staff. They are aware of their role at a unique moment in history, asked to attack some of humankind's most critical scientific challenges and equipped with the most advanced instruments available to the scientific community. Thus far, they appear willing and able to meet the challenge.
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