In This Issue...
The First Non-Trivial Atom Circuit: Progress towards an Atom SQUID
Researchers from the National Institute of Standards and Technology (NIST) and the University of Maryland (UM) have created the first nontrivial “atom circuit,” a donut-shaped loop of ultracold gas atoms circulating in a current analogous to a ring of electrons in a superconducting wire. The circuit is “nontrivial” because it includes a circuit element—an adjustable barrier that controls the flow of atom current to specific allowed values. The newly published* work was done at the Joint Quantum Institute, a NIST/UM collaboration.
Ultracold gases, such as the Bose-Einstein condensate of sodium atoms in this experiment, are fluids that exhibit the unusual rules of the quantum world. Atomic quantum fluids show promise for constructing ultraprecise versions of sensors and other devices such as gyroscopes (which stabilize objects and aid in navigation). Superfluid helium circuits have already been used to detect rotation. Superconducting quantum interference devices (SQUIDs) use superconducting electrons in a loop to make highly sensitive measurements of magnetic fields. Researchers are striving to create an ultracold-gas version of a SQUID, which could detect rotation. Combined with ultracold atomic-gas analogs of other electronic devices and circuits, or “atomtronics” that have been envisioned, such as diodes and transistors, this work could set the stage for a new generation of ultracold-gas-based precision sensors.
To make their atom circuit, researchers created a long-lived persistent current—a frictionless flow of particles—in a Bose-Einstein condensate of sodium atoms held by an arrangement of lasers in a so-called optical trap that confines them to a toroidal, or donut, shape. Persistent flow—occurring for a record-high 40 seconds in this experiment—is a hallmark of superfluidity, the fluid analog of superconductivity.
The atom current does not circle the ring at just any velocity, but only at specified values, corresponding in this experiment to just a single quantum of angular momentum. A focused laser beam creates the circuit element—a barrier across one side of the ring. The barrier constitutes a tunable “weak link” that can turn off the current around the loop.
Superflow stops abruptly when the strength of the barrier is sufficiently high. Like water in a pinched garden hose, the atoms speed up in the vicinity of the barrier. But when the velocity reaches a critical value, the atoms encounter resistance to flow (viscosity) and the circulation stops, as there are no external forces to sustain it.
In atomic Bose-Einstein condensates, researchers have previously created Josephson junctions, a thin barrier separating two superfluid regions, in a single atomic trap. SQUIDs require a Josephson junction in a circuit. This present work represents the implementation of a complete atom circuit, containing a superfluid ring of current and a tunable weak link barrier. This is an important step toward realizing an atomic SQUID analog.
* A. Ramanathan, K. C. Wright, S. R. Muniz, M. Zelan, W. T. Hill III, C. J. Lobb, K. Helmerson, W. D. Phillips and G. K. Campbell. Superflow in a toroidal Bose-Einstein condensate: an atom circuit with a tunable weak link. Physical Review Letters. Published online March 28, 2011.
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Closer Look at Cell Membrane Shows Cholesterol 'Keeping Order'
Cell membranes form the “skin” of most every cell in your body, but the ability to view them up close and in motion cannot be rendered by many experimental techniques. A team of scientists working at the National Institute of Standards and Technology (NIST) and University of California, Irvine, recently developed a way to magnify them dramatically. Their work has helped illuminate the important role of cholesterol within this boundary between the cell and the outside world.
The multi-institutional team used tools at the NIST Center for Neutron Research (NCNR) to examine the membrane at more than 1,000 times the resolution offered by an optical microscope—the equivalent of magnifying the point of a needle to the size of a large building. This enabled an unprecedented look at the membrane, which—because it controls access to our cells—is a major target for many drugs.
“Drugs that affect pain sensation, heart rhythm, mood, appetite and memory all target proteins lodged in the cell membrane that function like little gates,” says Ella Mihailescu of the Institute for Bioscience and Biotechnology Research, a joint institute of NIST and the University of Maryland. “Because membranes and their proteins are important to medicine, we would like a better picture of how the membrane functions—and not just a better snapshot. We want to see it move, as it does constantly in real life.”
Optical microscopes offer limited resolution, while the more powerful electron microscopes require freezing samples before they can be magnified. But by using neutron diffraction, which does not require frozen subjects, the team not only observed the membrane more closely and in motion, but they also gained insight into the long-known phenomenon of the membrane growing thicker and stiffer in the presence of cholesterol.
These lipid chains form a two-layer skin with the “heads” of the lipids facing outward toward the cell’s exterior and interior and the “tails” intermingling on the inside of the cellular membrane. Cholesterol is known to be important for managing disorder in membranes. The team saw for the first time that when cholesterol is present, these tails line up in a tight formation, looking like a narrow stripe from which the lipid chains stretch outward—and producing the order that had been previously anticipated, but never shown directly. But without cholesterol, the tails go a bit wild, flapping around energetically and in some cases even pushing up toward their chains’ heads.
Mihailescu says the findings hint that cholesterol may have profound consequences for the membrane’s gatekeeper proteins, which are very sensitive to their environment. “The membrane and its proteins interact constantly, so we’re curious to learn more,” she says. “With this unique magnification technique, we can explore the cell membrane more effectively than ever possible, and we are now establishing a research program with the University of Maryland to do so in greater detail.”
* M. Mihailescu, R. G. Vaswani, E. Jardon-Valadez, F. Castro-Roman, J. A. Freites, D. L. Worcester, A. R. Chamberlin, D. J. Tobias and S. H. White. Acyl-chain methyl distributions of liquid-ordered and -disordered membranes. Biophysical Journal, March 2011, Vol. 100, pp. 1455-62, DOI: 10.1016/j.bpj.2011.01.035.
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A Measurement First: NIST 'Noise Thermometry' System Measures Boltzmann Constant
Researchers at the National Institute of Standards and Technology (NIST) have for the first time used an apparatus that relies on the “noise” of jiggling electrons to make highly accurate measurements of the Boltzmann constant, an important value for many scientific calculations. The technique is simpler and more compact than other methods for measuring the constant and could advance international efforts to revamp the world’s scientific measurement system.
The Boltzmann constant* relates energy to temperature for individual particles such as atoms. The accepted value of this constant is based mainly on a 1988 NIST measurement performed using acoustic gas thermometry, with a relative standard uncertainty of less than 2 parts per million (ppm). The technique is highly accurate but the experiment is complex and difficult to perform. To assure that the Boltzmann constant can be determined accurately around the world, scientists have been trying to develop different methods that can reproduce this value with comparable uncertainty.
The latest NIST experiment used an electronic technique called Johnson noise thermometry (JNT) to measure the Boltzmann constant with an uncertainty of 12 ppm.** The results are consistent with the currently recommended value for this constant. NIST researchers aim to make additional JNT measurements with improved uncertainties of 5 ppm or less, a level of precision that would help update crucial underpinnings of science, including the definition of the Kelvin, the international unit of temperature.
The international metrology community is expected to soon fix the value of the Boltzmann constant, which would then redefine the Kelvin as part of a larger effort to link all units to fundamental constants.*** This approach would be the most stable and universal way to define measurement units, in contrast to traditional measurement unit standards based on physical objects or substances. The Kelvin is now defined in terms of the triple-point temperature of water (273.16 K, or about 0 degrees C and 32 degrees F), or the temperature and pressure at which water’s solid, liquid and vapor forms coexist in balance. This value may vary slightly depending on chemical impurities.
The NIST JNT system measures very small electrical noise in resistors, a common electronic component, when they are cooled to the water triple point temperature. This “Johnson noise” is created by the random motion of electrons, and the signals they generate are directly proportional to temperature. The electronic devices measuring the noise power are calibrated with electrical signals synthesized by a superconducting voltage source based on fundamental principles of quantum mechanics. This unique feature enables the JNT system to match electrical power and thermal-noise power at the triple point of water, and assures that copies of the system will produce identical results. NIST researchers recently improved the apparatus to reduce the statistical uncertainty, systematic errors and electromagnetic interference. Additional improvements in the electronics are expected to further reduce measurement uncertainties.
The new measurements were made in collaboration with guest researchers from the Politecnico di Torino, Italy; the National Institute of Metrology, China; the University of Twente, The Netherlands; the National Metrology Institute of Japan, Tsukuba, Japan; and the Measurement Standards Laboratory, New Zealand.
* The currently accepted value of the Boltzmann Constant is 1.380 6504 x 10-23 joules/kelvin.
** S.P. Benz, A. Pollarolo, J. Qu, H. Rogalla, C. Urano, W.L. Tew, P.D. Dresselhaus and D.R. White. An electronic measurement of the Boltzmann Constant. Metrologia. Published online March 30, 2011.
*** See the Oct. 26, 2010, NIST Tech Beat article, “‘Sí’ on the New SI: NIST Backs Proposal for a Revamped System of Measurement Units,” at http://www.nist.gov/public_affairs/tech-beat/tb20101026.cfm#SI.
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Microreactors: Small Scale Chemistry Could Lead to Big Improvements for Biodegradable Polymers
Using a small block of aluminum with a tiny groove carved in it, a team of researchers from the National Institute of Standards and Technology (NIST) and the Polytechnic Institute of New York University is developing an improved “green chemistry” method for making biodegradable polymers. Their recently published work* is a prime example of the value of microfluidics, a technology more commonly associated with inkjet printers and medical diagnostics, to process modeling and development for industrial chemistry.
“We basically developed a microreactor that lets us monitor continuous polymerization using enzymes,” explains NIST materials scientist Kathryn Beers. “These enzymes are an alternate green technology for making these types of polymers—we looked at a polyester—but the processes aren’t really industrially competitive yet,” she says. Data from the microreactor, a sort of zig-zag channel about a millimeter deep crammed with hundreds of tiny beads, shows how the process could be made much more efficient. The team believes it to be the first example of the observation of polymerization with a solid-supported enzyme in a microreactor.
The group studied the synthesis of PCL,** a biodegradable polyester used in applications ranging from medical devices to disposable tableware. PCL, Beers explains, most commonly is synthesized using an organic tin-based catalyst to stitch the base chemical rings together into the long polymer chains. The catalyst is highly toxic, however, and has to be disposed of.
Modern biochemistry has found a more environmentally friendly substitute in an enzyme produced by the yeast strain Candida antartica, Beers says, but standard batch processes—in which the raw material is dumped into a vat, along with tiny beads that carry the enzyme, and stirred—is too inefficient to be commercially competitive. It also has problems with enzyme residue contaminating and degrading the product.
By contrast, Beers explains, the microreactor is a continuous flow process. The feedstock chemical flows through the narrow channel, around the enzyme-coated beads, and, polymerized, out the other end. The arrangement allows precise control of temperature and reaction time, so that detailed data on the chemical kinetics of the process can be recorded to develop an accurate model to scale the process.
“The small-scale flow reactor allows us to monitor polymerization and look at the performance recyclability and recovery of these enzymes,” Beers says. “With this process engineering approach, we’ve shown that continuous flow really benefits these reactors. Not only does it dramatically accelerate the rate of reaction, but it improves your ability to recover the enzyme and reduce contamination of the product.” A forthcoming follow-up paper, she says, will present a full kinetic model of the reaction that could serve as the basis for designing an industrial scale process.
While this study focused on a specific type of enzyme-assisted polymer reactions, the authors observe, “it is evident that similar microreactor-based platforms can readily be extended to other systems; for example, high-throughput screening of new enzymes and to processes where continuous flow mode is preferred.”
* S. Kundu, A. S. Bhangale, W. E. Wallace, K. M. Flynn, C. M. Guttman, R. A. Gross and K. L. Beers. Continuous flow enzyme-catalyzed polymerization in a microreactor. J. Am. Chem. Soc. dx.doi.org/10.1021/ja111346c.
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NIST, ASTM Land a One-Two Punch to Fight Explosives Terrorism
Trace-explosives detectors (TEDs) are an increasingly common sight at airports and on loading docks, and emergency response personnel carry them to evaluate suspicious packages. A new test material developed by the National Institute of Standards and Technology (NIST) in cooperation with ASTM International enables users of these products to evaluate their performance and reliability.
The new testing material, NIST Standard Reference Material (SRM) 2906, Trace Explosives Calibration Solutions, was designed to meet the specifications of ASTM E 2520-07, Standard Practice for Verifying Minimum Acceptable Performance of Trace Explosive Detectors. ASTM is one of the leading industrial organizations for the development of voluntary consensus standards.
The NIST reference material contains calibration solutions of three high explosives: RDX (an ingredient in Composition C-4), PETN, and TNT. Under the test protocol, users sequentially apply a single drop of explosive solution and a solvent blank to swipes, the solvents are allowed to evaporate, and the instrument is tested. A simple ‘yes-no’ alarm checklist is used to determine TED performance.
SRM 2906 includes four ampoules of each of the three explosives and a blank along with a dropper bottle for each. NIST researchers formulated the concentrations of these solutions to be near, but above, the detection limit of commercial swipe-type detectors, which are commonly based on ion mobility spectrometry. When tested with the solutions, properly functioning TEDs should provide an alarm response.
This SRM fully satisfies the need for independent test materials with low uncertainties in concentrations necessary for reliable TED evaluation. Equipment vendors may use the SRM to improve and optimize their designs and demonstrate to their customers how well their machines function. Buyers may use the SRM to make sound procurement decisions. The combination of a validated standard practice and SRM will provide TED users with a reliable means of verifying initial and continuing field performance of their equipment, contributing to the fight against explosives terrorism.
Development of SRM 2906 was sponsored by the Science and Technology Directorate of the U.S. Department of Homeland Security. For more information on SRM 2906, see https://www-s.nist.gov/srmors/view_detail.cfm?srm=2906. For more information on ASTM E 2520-07, see www.astm.org/Standards/E2520.htm.
SRMs are among the most widely distributed and used NIST products. The agency prepares, analyzes and distributes more than 1,000 different carefully characterized materials that are used throughout the world to check the accuracy of instruments and test procedures used in manufacturing, clinical chemistry, environmental monitoring, electronics, criminal forensics and dozens of other fields. For more information, see NIST’s SRM website www.nist.gov/srm/.
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Getting the Point: Real-Time Monitoring of Atomic-Microscope Probes Adjusts for Wear
Scientists at the National Institute of Standards and Technology (NIST) have developed a way to measure the wear and degradation of the microscopic probes used to study nanoscale structures in situ and as it’s happening. Their technique can both dramatically speed up and improve the accuracy of the most precise and delicate nanoscale measurements done with atomic force microscopy (AFM).
If you’re trying to measure the contours of a surface with a ruler that’s crumbling away as you work, then you at least need to know how fast and to what extent it is being worn away during the measurement.
This has been the challenge for researchers and manufacturers trying to create images of the surfaces of nanomaterials and nanostructures. Taking a photo is impossible at such small scales, so researchers use atomic force microscopes. Think of a device like a phonograph needle being used, on a nanoscale, to measure the peaks and valleys as it’s dragged back and forth across a surface. These devices are used extensively in nanoscale imaging to measure the contours of nanostructures, but the AFM tips are so small that they tend to wear down as they traverse the surface being measured.
Today, most researchers stop the measurement to “take a picture” of the tip with an electron microscope, a time-consuming method prone to inaccuracies.
NIST materials engineer Jason Killgore has developed a method for measuring in real time the extent to which AFM tips wear down. Killgore measures the resonant frequency of the AFM sensor tip, a natural vibration rate like that of a tuning fork, while the instrument is in use. Because changes to the size and shape of the tip affect its resonant frequency, he is able to measure the size of the AFM’s tip as it works—in increments of a tenth of a nanometer, essentially atomic scale resolution. The technique, called contact resonance force microscopy, is described in a paper recently published in the journal Small.*
The potential impact of this development is considerable. Thousands of AFMs are in use at universities, manufacturing plants and research and development facilities around the world. Improving their ability to measure and image nanosized devices will improve the quality and effectiveness of those devices. Another benefit is that developing new measurement tips—and studying the properties of new materials used in those tips—will be much easier and faster, given the immediate feedback about wear rates.
* J. P. Killgore, R. H. Geiss and D. C. Hurley. Continuous measurement of AFM tip wear by contact resonance force microscopy. Small. Published March 15, 2011.
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Ground Broken for New Green Technology and Fire Safety Facilities
On March 25, 2011, the National Institute of Standards and Technology (NIST) held a groundbreaking ceremony at its Gaithersburg, Md., campus for three new facilities funded by the American Recovery and Reinvestment Act. The Net-Zero Energy Residential Test Facility, the expanded National Fire Research Laboratory, and the installation of more than 2,500 new solar energy modules to supply electricity to the NIST campus will all help to advance the state of the art in green and fire-safe building practices.
Resembling a typical suburban Maryland single-family home, the Net-Zero Energy Residential Test Facility is designed to produce as much energy as it consumes over the course of a year and will serve as a testbed for new home-scale energy technologies. The 2,700-square-foot (251-square-meter), two-story structure will use energy-saving appliances and design, as well as solar panels, to minimize the amount of energy it pulls from the grid and to generate at least an equal amount of energy. During a yearlong demonstration of the house’s capabilities, appliances, lights, and kitchen and bathroom fixtures will be computer controlled to simulate a family of four living in the fully furnished home.
The National Fire Research Laboratory will be expanded with a 21,400-square-foot (1,988-square-meter) laboratory space that will provide a unique capability for testing full-scale structures—up to two stories in height—as well as subassemblies and systems under realistic fire conditions.
A hydraulic loading system will help simulate the weight of an occupied building and its contents. Smoke and hot gases will be captured and treated to meet strict environmental requirements and to measure fire characteristics.
The laboratory will be managed and operated as a collaborative facility through a public-private partnership between NIST, industry, academia, and other government agencies.
The new photovoltaic system will represent a dramatic increase in NIST’s commitment to implementing renewable energy sources. When complete, the new solar energy system will feed directly into the existing electrical grid, generating more than 700 MWh of electricity annually—enough to power 67 homes. The system will also provide data that will be used to develop models to better predict energy output of photovoltaic modules and arrays.
A field toward the southern end of campus will be home to about 1,150 modules; approximately 1,000 modules will serve double-duty as canopies over a parking lot; modules installed on the roof of the campus’ Administration Building will expand an existing solar system array’s output threefold; and a small array on another building will power two charging stations for battery-powered maintenance vehicles.
The ceremony featured remarks by U.S. Representative Chris Van Hollen (D-MD-8); Nancy Sutley, chair of the White House Council on Environmental Quality; Henry Kelly, Acting Assistant Secretary and Principal Deputy Assistant Secretary for the Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy; former NIST Director Arden Bement, David A. Ross Distinguished Professor of Nuclear Engineering and Director of the Global Policy Research Institute, Purdue University; and other government officials.
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Charge It: Neutral Atoms Made to Act Like Electrically Charged Particles
Completing the story they started by creating synthetic magnetic fields,* scientists from the Joint Quantum Institute (JQI), a collaboration of the National Institute of Standards and Technology (NIST) and the University of Maryland, have now made atoms act as if they were charged particles accelerated by electric fields.
Reported in the journal Nature Physics,** these synthetic electric fields make each atom in a gas act, individually, as if it were a charged particle, but collectively they remain neutral, uncharged particles. This dual personality will help researchers simulate and study fundamental electrical phenomena and may lead to a deeper understanding of exotic phenomena involving charged particles such as superconductivity, the flow of electricity without resistance, or the quantum Hall effect, used by NIST to create a standard of electrical resistance.
Some aspects of electricity are difficult to study because, although oppositely charged particles are attracted to one another, similarly charged particles are repelled by one another. To get around this, NIST physicist Ian Spielman and his colleagues realized that they could make atoms, which are typically electrically neutral, act as if they are charged particles in an electric field—extending their earlier method for making neutral atoms act like charged particles in a magnetic field.
The researchers create their synthetic electric field in an ultracold gas of several hundred thousand rubidium atoms. Using lasers, the team alters the atoms’ energy-momentum relationship. This had the effect of transferring a bit of the lasers’ momentum to the atoms, causing them to move. The force on each atom is physically identical—and mathematically equivalent—to what a charged particle would feel in an electric field.
So while the neutral atoms each experience the force of this synthetic electric field individually, they do not repel each other as would true charged particles in an ordinary electric field. This is analogous to an experienced group of dancers all following the moves of their instructor without getting in each other’s way.
According to Spielman, this work may enable scientists to study the Hall effect, a phenomenon where an electromagnetic field can cause charged particles traveling through a conductor to experience a sideways force, which has of yet been unobserved in cold-atom systems. The work may also facilitate measurements of the atomic equivalents of electrical quantities such as resistance and inductance. For neutral atoms in synthetic electric fields, inductance is a measure of the energy that is stored as a result of the atoms’ motion, and resistance is a measure of the dissipation, or energy loss, in the system. Measuring these quantities could provide insights into the properties of charged particles in analogous systems, including superconductors.
* See “JQI Researchers Create ’Synthetic Magnetic Fields’ for Neutral Atoms,” Dec. 15, 2009, at www.nist.gov/pml/div684/synthetic_121509.cfm.
** Y-J. Lin, R. L. Compton, K. Jiménez-García, W. D. Phillips, J. V. Porto and I. B. Spielman, A synthetic electric force acting on neutral atoms, Nature Physics. Published online March 20, 2011.
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New Study Maps Out Steps to Strengthen U.S. Resilience to Earthquakes
A new National Research Council (NRC) report presents a 20-year road map that outlines steps for increasing U.S. resilience to earthquakes, including a major earthquake that could strike heavily populated areas.
Prepared by a NRC-convened committee of experts, the report, National Earthquake Resilience—Research, Implementation, and Outreach, describes 18 categories of focused activities necessary for accomplishing the strategic plan adopted by the federal government’s National Earthquake Hazards Reduction Program (NEHRP). Established by Congress in 1977 with the aim of reducing the impacts of future earthquakes, NEHRP is led by the National Institute of Standards and Technology (NIST) and includes the Federal Emergency Management Agency, the National Science Foundation and the U.S. Geological Survey. NIST funded the study.
The committee defined an earthquake-resilient nation as “one in which its communities, through mitigation and predisaster preparation, develop the adaptive capacity to maintain important community functions and recover quickly when major disasters occur.”
Most of the report was mostly written prior to the March 11, 2011, earthquake in Japan, but the committee noted that the Japanese disaster is a reminder of the devastation that can result from earthquakes, even in a country acknowledged as a leader in implementing earthquake-resilience measures.
To read the NRC press release, go to: http://www8.nationalacademies.org/onpinews/newsitem.aspx?RecordID=13092
To access the full report, go to: http://www.nap.edu/catalog.php?record_id=13092
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Celebrating Superconductivity: NIST Debuts Online Museum of Quantum Voltage Standards
On April 8, 2011, the scientific community will celebrate the centennial of the discovery of superconductivity—the ability of certain materials to conduct electricity without resistance when cooled below a specific temperature. Quantum voltage standards are among the successful practical applications of superconductivity, so to mark the anniversary, the National Institute of Standards and Technology (NIST) has created an online museum highlighting important accomplishments and historical images from the voltage standards program.
Superconductivity was first discovered on April 8, 1911, by the Dutch physicist Heike Kamerlingh Onnes. Over four decades, NIST has developed a series of voltage standards based on superconducting Josephson junctions. The standards are used worldwide by industry, government and military laboratories to calibrate voltmeters—common instruments for applications ranging from the electric power grid to consumer electronics to advanced military equipment. The museum is available at http://www.nist.gov/pml/history-volt/.
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NIST's Ron Ross is Named to InformationWeek Government Top 50 List
Ron Ross, a National Institute of Standards and Technology (NIST) Fellow, has been named to InformationWeek Government’s CIO 50, which identifies 2010’s top information technology decision-makers in government. Ross is project lead of the Federal Information Security Management Act (FISMA) Implementation Project and plays a key role in setting cybersecurity requirements for federal agencies and providing guidance on meeting those requirements. Ross was chosen for “establishing guidelines that emphasize risk management and ‘continuous monitoring’ over basic compliance.” Ross was also recognized for his leadership on a joint task force to develop a unified security framework for defense, civilian and intelligence agencies.
Edited on March 31, 2011, to correct headline.
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