Improving detection of concealed nuclear materials

create the image of the cargo.

“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.”

When the neutrons interact with fissile materials, they initiate a fission reaction, generating both prompt and delayed neutrons that can be detected despite the shielding. The neutrons do not prompt a time-delayed reaction with non-fissionable materials such as lead, providing an indicator that materials of potential use for development of nuclear weapons are inside the shielding.

“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.”

Earlier efforts at active detection of radioactive materials used X-rays to image the cargo containers, but that technique had difficulty with the heavy shielding and could harm the cargo if the radiation dose was high, Erickson said. Because it uses discrete energies of the photons and neutrons, the new technique minimizes the amount of energy entering the container.

Researchers at Georgia Tech — led by Erickson — and at University of Michigan and Penn State University — led by Igor Jovanovic, professor of nuclear engineering and radiological sciences — demonstrated that the technique works in a laboratory setting by detecting uranium plates and rods.

Georgia Tech notes that in testing conducted in collaboration with the Massachusetts Institute of Technology at the Bates Linear Accelerator Center, the researchers used a fan-like pattern of particles created by an ion accelerator and emitted at 4.4 and 15.1 MeV. The particles passed through a shielded radioactive material, and were measured on the other side with Cherenkov quartz detectors connected to photomultiplier tubes.

“This provided proof that the physics works, and that we can use these particles to actually distinguish among various materials, including special nuclear materials,” Jovanovic said. The technique has not yet been tested under the real-world conditions of a steel cargo container, but such demonstration may take place in the near future.

Beyond the potential homeland security uses, the technology could also find application in materials science, medical imaging, low-energy nuclear physics and industrial imaging.

— Read more in Paul Rose et al., “Uncovering Special Nuclear Materials by Low-energy Nuclear Reaction Imaging,” Scientific Reports 6, Article number: 24388 (18 April 2016) (doi:10.1038/srep24388)