A novel technique that can detect weapons-grade nuclear material through layers of heavy metal shielding could help stop smugglers in future.
The technique, developed by a team of American researchers, has so far only been tested in laboratory conditions, but the team believes it could be used to efficiently scan cargo containers to prevent the dangerous materials from getting into the hands of terrorists.
"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 at the Georgia Institute of Technology.
The Georgia Tech team, in collaboration with scientists from the University of Michigan and the Pennsylvania State University, developed a device that relies on the combination of neutrons and photons. The new system not only detects the radioactive material behind layers of shielding – it also provides information about what type of radioactive material it is by measuring its density and atomic number.
"The gamma rays of different energies interact with the material in very different ways, and how the signals are attenuated will be a very good indicator of what the atomic number of the hidden material is, and its potential density," Erickson explained. "We can observe the characteristics of transmission of these particles to understand what we are looking at."
Compared to existing methods, the new technique uses lower radiation doses, which has several advantages. In addition to increasing the detection performance, it also reduces the risk of harming electronics and other cargo inside the containers that might be sensitive to radiation.
The technique, published in the latest issue of the journal Scientific Reports, has been described as the first successful attempt to identify and image uranium using this type of approach.
The device is an ion accelerator producing heavy isotopes of hydrogen, known as deuterons. A beam of deuterons is directed onto a boron surface, where it triggers the creation of neutrons and high energy photons. These particles are then focused into a fan-shaped beam that can be used to scan the cargo container. Both the photons and neutrons penetrate the shielding and excite the nuclear material, which then starts emitting gamma rays and neutrons that can be detected outside the container by transmission detectors.
"If you have something benign, but heavy – like tungsten, for instance – versus something heavy and shielded like uranium, we can tell from the signatures of the neutrons," Erickson said. "We can see the signature of special nuclear materials very clearly in the form of delayed neutrons. This happens only if there are special nuclear materials present."
The team tested the method in collaboration with the Massachusetts Institute of Technology at the Bates Linear Accelerator Center and hopes to start testing in real-world scenarios in near future. The technology could serve in other applications as well, including in material research, medical and industrial imaging and low energy nuclear physics.
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