The Berkeley Science Review: Articles

Plutonium or Produce?
Distinguishing radioactivity from harmless imports (view PDF)
by Michael Calvert

California is home to three of the four busiest shipping ports in the United States. Together, ports in Long Beach, Los Angeles, and the nearby Port of Oakland carry approximately 50 percent of the nation's total container cargo volume. But with high traffic comes risk. In 2003, a team assembled by ABC News managed to smuggle depleted uranium, a byproduct of uranium enrichment, from Indonesia into the Los Angeles port. While depleted uranium itself is not dangerous, in most other ways it effectively mimics enriched uranium—the material that would be used to make a bomb. So how did this material get by the layers of radiation detection currently in place?

The business of importing cargo into the United States is one of high volume and brutal efficiency. In the Port of Oakland alone, the port's operators process dozens of containers every minute. Paperwork is checked, spot inspections are administered, and containers are scanned for anything out of the ordinary. However, by the very design of the system, the total time available for radiation detection amounts to less than a minute per container. The situation is further complicated by the fact that hazardous nuclear material is not the only thing that undergoes radioactive decay.

Radioactivity is simply the transformation of an unstable atomic nucleus (the "parent") into a more stable one (the "daughter"), resulting in the emission of radiation in the form of particles or waves. Many consumer products, from cocoa powder to pottery, are naturally radioactive at low levels, but present no danger to consumers. At the Port of Oakland, this translates into dozens of false positives on a daily basis when containers are scanned. To combat this issue, the Department of Homeland Security's Domestic Nuclear Detection Office (DNDO) has made improving radiation detection systems one of its primary goals, and has provided grant money to a number of institutions willing to take on the task.

That's where UC Berkeley's Department of Nuclear Engineering comes in. In the latter half of 2007, the department was awarded two major grants from the DNDO to study ways to improve radiation detection. The first, totaling $7.1 million over five years, sponsors a collaboration called DONUTS, or DOmestic NUclear Threat Security. The team is led by Ed Morse, Eric Norman, Jim Siegrist, and Brian Wirth, professors in the physics and nuclear engineering departments. Together, they hope to develop new techniques to increase detection accuracy, limiting false positives and improving spatial resolution. An additional DNDO grant for two million dollars over five years was awarded to nuclear engineering professor Kai Vetter to investigate gamma ray imaging.

Detecting radiation is fairly easy to do. Determining precisely what materials set off a radiation detector is not. Take uranium, for instance. "Uranium can be found everywhere. On average, there are about three grams of it in a cubic meter of soil," says Morse. Uranium is a naturally radioactive element that exists in the environment in two isotopic forms, U-238 and U-235; however, only U-235 can be used to sustain fission in a reactor or weapon. Both isotopes, as well as their daughters, decay by emitting the same types of radiation, including alpha particles, beta particles, and gamma rays. While alpha and beta particles cannot travel very far, gamma rays can easily penetrate container walls and be detected. However, this radiation does not clearly signal whether the source is harmless U-238 or dangerous U-235. Other naturally radioactive elements, such as thorium and potassium-40 (found in kitty litter, bananas, and many other common non-hazardous materials), further compound the problem, as does background radiation from cosmic rays and material in the earth itself.

"The logistical problem is enormous," says Wirth, who is studying materials for improved radiation detection. A major detection method at the Port of Oakland is gamma spectroscopy, the measurement of the energy and intensity of gamma rays. The traditional material used for most gamma spectroscopy is germanium, a semiconductor that must be cooled to very low temperatures—almost -170°C (-340°F)—to be an effective sensor. Such low temperatures are very impractical at facilities like the Port of Oakland. "What would be nice is if one could make small tabletop room-temperature detectors that operate at a resolution like germanium," explains Wirth. If there is success in developing a new material that can replace germanium, portable gamma detectors will both decrease the cost and increase the efficiency of radiation detection.

Researchers are also working to increase the sensitivity of radiation detectors. However, says Morse, "due to increased detection sensitivity in the last few years, what we've now done is increase the rate of false positives." Eric Norman is addressing detection accuracy by looking at detailed emission signatures of potentially dangerous isotopes using "active interrogation." Unlike passive detection, where the goal is to detect the radiation passing out of the container, active interrogation is the process of sending in radiation to extract more precise information. One active interrogation technique, called neutron activation analysis, involves sending in neutrons that are absorbed by isotopes in all kinds of materials. These isotopes, in turn, emit characteristic patterns of radiation. In many cases, this technique is a precise way to determine elemental composition, including differentiating between benign and potentially dangerous radioactive elements. However, the type of neutron must be carefully chosen—if a nuclear bomb happened to be sitting in a container, bombarding it with neutrons of the wrong energy might actually cause an explosion.

Luckily, the team will also be taking the work in less dangerous directions. A top priority for the DONUTS project is improving the spatial resolution of radiation detectors—that is, the ability to locate the exact source of radiation in a large space, such as a shipping container. Jim Siegrist, a physics professor at UC Berkeley as well as the Physics Division Director at LBNL, is working to reduce spatial uncertainty by measuring Compton scattering. Compton scattering occurs when gamma rays emitted by nuclear material interact with electrons in matter, in this case the detection medium. Once inside the detector, the rays interact with the medium and the original source of the radiation can be identified. Siegrist is hoping to increase the resolution of these detectors to help better triangulate the source of the gamma rays.

In addition to the approaches laid out by the DONUTS team, Kai Vetter will tackle the problem of detecting weak signals from radioactive materials amidst high levels of background radiation. Vetter is studying a technique called electron tracking-based Compton imaging, which measures Compton scattering by monitoring the scattered electrons instead of the gamma radiation itself. These instruments have the potential to significantly increase the sensitivity in the detection of nuclear materials, particularly in scenarios characterized by weak signals from dangerous material in the midst of stronger natural background sources, such as a small amount of enriched uranium hiding among goods that contain naturally occurring uranium, like granite. Vetter's research with gamma ray imaging could also increase spatial resolution while reducing background noise, leading to improved detection of small amounts of nuclear material in large volumes.

Improving radiation detection at our nation's ports is a monumental task that will take many years. As with any problem of this magnitude, a multidisciplinary approach is required, and Berkeley researchers in a variety of fields are just getting started. But with some luck and ingenuity, the Port of Oakland may yet be able to distinguish bombs from bananas.

Michael Calvert is a graduate student in chemistry.

Want to know more? Check out:
donuts.berkeley.edu

Comments on this article? Drop us a line at with 'letter to the editor' in the subject!