By Neal Singer
A Sandia researcher has developed a simulation program designed to track the illicit trade in fissile and nonfissile radiological material well enough to predict who is building the next nuclear weapon and where they are doing it.
“By using a cluster analysis algorithm coded into a program,” says David York (6763), “I evaluated those traffic patterns and routes in which thefts, seizures, and destinations of materials were reported. Data from these examinations were enough to allow me to retrospectively depict the A. Q. Khan network before it was uncovered."
Khan is a Pakistani scientist linked to the illicit proliferation of nuclear technical knowledge. Cluster analyses link data of common place, time, or material. Testing a computer simulation on a known past event is one means of establishing the program’s validity.
In the Khan analysis, David generated an analysis of networked routes indicative of a nuclear trafficking scheme between countries. In several verified incidents, inspectors seized uranium enriched to 80 percent, as well as dual-use items indicative of small-scale development of crude nuclear devices.
In the study, David collected and collated data from 800 open-source incidents from 1992 to the present, along with the movement of dual-use items like beryllium and zirconium. He plotted the incidents on a geographic information system (GIS) software platform. He came up with a network of countries and routes between countries indicative of an illicit nuclear and radiological trafficking scheme.
“The number of incidents and the quantity and quality of material seized is disturbing,” David says, “particularly because this may represent a small percentage of the actual amount of material being trafficked.”
The situation may be worse than it appears because much information about nuclear material traffic is classified, David says, to prevent embarrassment to countries through which a nuclear weapon or the materials to fabricate a weapon may have passed.
David presented his results in October at the International Safeguards Conference sponsored by the United Nations’ International Atomic Energy Agency (IAEA) in Vienna, Austria. He has also been invited to present his methods and conclusions to the European Union’s Illicit Trafficking Working Group at the June meeting of the IAEA.
How does the method work? “One begins by conducting cluster analyses on the GIS platform for material or activity similar to the incident in question. This gives the analyst an idea of corridors used by potential smugglers. It also indicates where the material might have come from and where it is,” says David. “If the trafficker has only a certain amount of time to reach a destination, and you have that information, one can ask what is the shortest route from point A to point B, or find major highways needed to accommodate a large shipment.”
For the tool to be effective, “Enough information must be collected under a cooperative international framework,” David says. “Then info must be analyzed to separate patterns from noise, essentially creating intelligence.”
Nation-states that reuse nuclear fuel through reprocessing can create and ship dangerous
materials that previously were confined to the more industrialized world.
“We’re trying to develop a market niche for this kind of tracking program,” says Sandia Manager Gary Rochau (6763), “and I think we’re ahead of everyone’s headlights.”
The method can be used to track other materials, such as drugs. “We have a lot of interest from a lot of agencies,” says Gary.
Trafficking may be engaged in by amateur smugglers trying to feed their families in a post-Soviet era. It may also be practiced by organized crime that finds a lucrative market in moving illicit materials, and by terrorists interested in the potential devastation and psychological effects of the use of nuclear materials.
David developed the program as part of his master’s thesis while a student intern at Sandia. -- Neal Singer
It might not be science fiction much longer.
Sandia researchers are developing the next generation of screening devices that will identify hazardous and toxic materials even if concealed by clothing and packaging materials.
Working in the underutilized terahertz (THz) portion of the electromagnetic spectrum that lies between microwaves and infrared, a team of Labs scientists is harnessing Sandia’s strengths in a variety of technical areas with the goal of building a highly integrated miniaturized terahertz transmitter-receiver (transceiver) that could make a number of applications possible.
The project, the Terahertz Microelectronics Transceiver Grand Challenge, is in its second of three years of funding through Sandia’s internal Laboratory Directed Research and Development program.
“The technology being developed in the Grand Challenge can be used to scan for items such as concealed weapons or materials, explosives, and weapons of mass destruction,” says Mike Wanke (1725), principal investigator. “In addition, we believe it will find applications in advanced communication systems and high-resolution radars. However, the infrastructure needed to move the terahertz technology from the laboratory to the field is unavailable right now. We want to develop that infrastructure and invent the necessary technologies.”
Mike says over the past three years, “the terahertz situation has begun to change dramatically, primarily due to the revolutionary development of terahertz quantum cascade lasers.”
These tiny lasers are semiconductor sources of terahertz radiation capable of output powers in excess of 10 mW. Previously, such powers could only be obtained by molecular gas lasers occupying cubic meters and weighing more than 100 kg, or free electron lasers weighing tons and occupying entire buildings.
Quantum cascade laser-based systems can be less than the size of a baseball and powered from a 9V battery. Sandia has been a leader in developing this new technology and in collaboration with MIT is responsible for several world performance records for the lasers. Also, the Labs and its partners are the only US institutions that have demonstrated the ability to grow the unique semiconductor crystals such that they can be turned into operating terahertz quantum cascade lasers. The crystals are grown by Sandia research scientist John Reno (1132), an expert in molecular beam epitaxy, a method of laying down layers of materials with atomic thicknesses onto substrates.
Sandia researchers spent the first year of the Grand Challenge using Sandia’s unique strengths in integrated microelectronics and device physics to develop components that are now being combined to create an integrated THz microelectronic transceiver, a core enabling element.
The team is currently developing the receiver, doing systems tests, and exploring packaging requirements. At the end of three years, the researchers expect to have an actual working prototype capable of detecting the materials and chemicals by reading distinctive molecular spectral “signatures.”
“Most materials and chemicals have their own unique terahertz spectral signatures,” Mike says. “A terahertz transceiver system would be able to measure, for example, the signature of a gas and determine what it is.”
“Atmospheric scientists and radio astronomers have spent years developing terahertz spectral signature databases to identify chemicals in nebula and planetary atmospheres,” says Greg Hebner (1128), program manager. “Even though the current devices are washing machine-sized, they are located in a few observatories, and one is even flying on a satellite. To address specific national security problems, we are working on reducing the size, weight, and power requirement as well as expanding the existing spectral databases.”
In addition to monitoring for concealed hazardous materials, Mike believes a terahertz system can be used to monitor the air for toxic materials. Using air sampling technology developed at Sandia and other locations, hazardous vapors can be pre-concentrated. Shining light from the quantum cascade laser through the concentrated sample provides a direct identification of the vapor. This technology can be used in conjunction with existing mass spectrometer-based systems to reduce false identifications.
“We are very optimistic about working in the terahertz electromagnetic spectrum,” Mike says. “This is an unexplored area and a lot of science can come out of it. We are just beginning to scratch the surface of what THz can do to improve national security.” -- Chris Burroughs
By Nancy Garcia
Gold is shiny, diamonds are transparent, and iron is magnetic. Have you ever wondered why that is?
Electronic structure determines many material properties, including electrical, optical, and magnetic. Sandia relies extensively on using and controlling such properties, for everything from assuring weapons reliability to creating devices from nanomaterials.
Predicting a material’s properties by first calculating its electronic structure would cut down experimental time and might lead researchers to uncover new materials with unexpected benefits.
However, commonly used simulations are inaccurate, especially for materials like silicon, whose strongly correlated electrons influence each other over a distance and make simple calculations difficult.
Now a Sandia team may have a solution that offers huge potential. Sergey Faleev (8756) and colleagues applied theoretical innovations and novel algorithms to make a hard-to-use theoretical approach from 1965 amenable to computation. The team’s approach may open the door to discovering new phases of matter, creating new materials, or optimizing performance of compounds and devices such as alloys and solar cells.
Their paper, “Quasiparticle Self-Consistent GW Theory,” appeared in the June 9, 2006, issue of Physical Review Letters. GW refers to Lars Hedin’s 1965 theory that elegantly predicts electronic energy for ground and excited states of materials. “G” stands for the Greens function — used to derive potential and kinetic energy — and “W” is the screened Coulomb interaction, which represents electrostatic force acting on the electrons. “Quasiparticles” are a concept used to describe particle-like behavior in a complex system of interacting particles. Self-consistent means the particle’s motion and effective field, which determine each other, are iteratively solved, coming closer and closer to a solution until the result stops changing.
“Our code has no approximation except GW itself,” Sergey says. “It’s considered to be the most accurate of all GW implementations to date.”
“It works well for everything in the periodic table,” adds coauthor Mark van Schilfgaarde, a former Sandian now at Arizona State University. The paper reports results for diverse materials whose properties cannot be consistently predicted by any other theory. The 32 examples include alkali metals, semiconductors, wide band-gap insulators, transition metals, transition metal oxides, magnetic insulators, and rare earth compounds.
“Everything in solids is held together by electrostatic forces,” says van Schilfgaarde. “You can think of this as a huge dance with an astronomically large number of particles, 1023, that is essentially impossible to solve. The raw interactions among the particles are remarkably complex.
“Hedin replaced the raw interactions with ‘dressing’ the particle with a screened interaction,” van Schilfgaarde continues, “so the effective charge is much smaller. It becomes much more tractable but the equations become more complicated — you have an infinite number of an infinite number of terms. The hope is that the higher-order terms die out quickly.”
The researchers’ use of GW makes the expansion much more rapidly convergent.
“We’re pretty confident we got the approach right,” he says. He now would like another group to independently verify this way of framing the task.
Promise and challenges ahead
In 2006, Sergey was invited to present this work to pulsed power and materials researchers at Sandia/New Mexico. They use a molecular dynamics code, VASP (Vienna Ab-initio Simulation Package) to model, for example, equations of state in high-energy-density matter. These equations of state depend on quantities like electrical conductivity. Calculating this requires detailed knowledge of the electronic structure — a perfect application for Sergey’s work. The researchers hope to describe optical spectra, calculate total energy, and account for more than 10 atoms in a unit cell — at 100 times the current speed. These are goals Sergey and collaborators are pursuing through Laboratory Directed Research and Development funding.
Accelerating the code would facilitate modeling in other research areas at Sandia, such as simulating titanium dioxide used in surface science, or aiding research into carbon nanotubes that might be used in electronic or optical devices.
“To calculate absorption or optical spectra is a huge problem,” Sergey says with anticipation. “To make it faster is a huge problem. To make it more accurate is a huge problem. To incorporate VASP is a huge problem.”
Van Schilfgaarde agrees. “It’s quite an accomplishment to do it at all. It takes someone who is very strong in math, and a clever programmer. We spent easily five to six man-years between us to make it work.
“If we can get the approach right, we can have a theory that’s universally accurate for anything we want; that’s really pretty neat, just requiring knowledge of where the atoms are.”
The quest for precision
The 1998 Nobel Prize in chemistry was awarded for the widely popular density functional theory (DFT). A simplification often used at its core called the local density approximation (LDA) approximates small fluctuations in electron density by using an effective potential in place of many-body interactions. LDA is very good at predicting certain properties of materials, but has serious limitations. For example, in insulators and semiconductors it is well-known to underestimate band gaps (which an electron cannot cross) by
1 eV or more. Despite its limitations, this approach has garnered 30,000 physics citations in the last five years. (For Sandia connections to DFT Nobel laureate Walter Kohn, who trained some Labs researchers, see Lab News articles from the Oct. 23, 1998, and Aug. 23, 2002, issues.)
The work done at Sandia goes beyond this approach, taking advantage of the expertise of the three researchers. Van Schilfgaarde had previously written a large DFT code. Sergey studied electron interactions in graduate school. Takao Kotani of Osaka University asked about making a GW code from van Schilfgaarde’s large DFT code. He was invited to visit Sandia in the 2000-2001 academic year, the same year Sergey was hired. Sergey eliminated the dependence of GW on LDA, making it 10 times as accurate as DFT. (Small band-gap discrepancies remain, on the order of 0.1 eV, and can be attributed to neglecting higher-order terms.)Van Schilfgaarde believes the theory’s advantage would be to offer true insight into material behavior. “It’s kind of like adding night-vision goggles to soldiers working in the dark,” he says. “Probably in 10 years,” adds Sergey, “everyone will use this.”
-- Nancy Garcia