Vigilance about the smuggling of weapons-grade uranium and plutonium is an international priority, but effective surveillance is impossible at busy ports that require cargo scans of 40-foot shipping containers to finish within two minutes. Now, a team of researchers at U-M and Georgia Tech think they’ve developed the right method.
The problem with conventional techniques, such as X-ray scans, is that they require high doses of radiation to probe through the steel walls of the shipping containers and other obstacles inside—including the casing of shielding that smugglers would place around the nuclear contraband.
“Once heavy shielding is placed around weapons-grade uranium or plutonium, detecting them passively using radiation detectors surrounding a 40-foot cargo container is very difficult,” said Anna Erickson, an assistant professor of mechanical engineering at Georgia Tech. “One way to deal with this challenge is to induce the emission of an intense, penetrating radiation signal in the material, which requires an external source of radiation.”
The team used a combination of gamma rays—the same type of radiation as visible light but much more energetic and penetrating—and neutrons, the neutral particles found in atomic nuclei. The gamma rays give a reading of the densities of materials and the chemical elements present inside the container, allowing the researcher to pick out uranium and plutonium—or a suspicious lead box that might be hiding these materials.
By using gamma rays of a single energy, rather than a broad spectrum of radiation, the team reduced the radiation exposure needed to scan cargo.
“It turns out that when the energies of the interrogating radiation are not well defined, as in the case of conventional approaches, most of the radiation does not contribute to improving the quality of image,” said Igor Jovanovic, a professor of nuclear engineering and radiological sciences at U-M.
Their single-energy scan lowers the dose significantly. This is important because some shipments, such as electronic devices, do not hold up well under radiation exposure. It could also save lives as human traffickers have hidden people in shipping containers.
The researchers used neutrons and high-energy gamma rays to look for particular radioactive forms of uranium and plutonium that make effective nuclear weapons. When the neutrons hit these so-called “special nuclear materials,” the materials reply by sending out more radiation, which the researchers could then detect. In particular, materials that can undergo a nuclear reaction called fission will send out radiation after a delay, showing that something inside the material continued generating neutrons after the probing beam stopped.
“This happens only if there are special nuclear materials present,” said Erickson.
Erickson’s group developed the gamma ray detectors while Jovanovic’s group, then at Pennsylvania State University, developed the neutron detectors. The team tested their method using a particle accelerator at the Massachusetts Institute of Technology, identifying various materials through the gamma-ray probing while also uniquely detecting uranium based on its characteristic emission of delayed radiation. The two teams contributed equally to the detection concept as well as the analysis of the data, much of which happened after Jovanovic’s arrival at Michigan.
“This provided proof that the physics works, and that we can use these particles to actually distinguish among various materials, including special nuclear materials,” said Jovanovic.
The study is described in a paper titled “Uncovering special nuclear materials by low-energy nuclear reaction imaging,” in the Nature journal Scientific Reports.The work was supported the National Science Foundation and the U.S. Department of Homeland Security.
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