New technique may detect concealed nuclear materials

Scientists have designed a novel low-energy nuclear reaction imaging technique to detect the presence of nuclear materials such as weapons-grade uranium and plutonium in cargo containers.
The technique could significantly improve the ability to prevent the smuggling of dangerous nuclear materials and their potential diversion to terrorist groups, researchers said.
The method relies on a combination of neutrons and high-energy photons to detect shielded radioactive materials inside the containers.
The technique can measure the suspected material's density and atomic number using mono-energetic gamma ray imaging, while confirming the presence of special nuclear materials by observing their unique delayed neutron emission signature.

The research from the Georgia Institute of Technology and the Pennsylvania State University is the first successful effort to identify and image uranium using this approach.
"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, from Georgia Tech.

The technique begins with an ion accelerator producing deuterons, heavy isotopes of hydrogen. The deuterons impinge on a target composed of boron, which produces both neutrons and high-energy photons.
The resulting particles are focused into a fan shaped beam that could be used to scan the cargo container.

The transmission of high-energy photons can be used to image materials inside the cargo container, while both the photons and neutrons excite the special nuclear material - which then emits gamma rays and neutrons that can be detected outside the container.
Transmission imaging detectors located in the line of sight of the interrogating fan beam of photons 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 said.

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.
Researchers demonstrated that the technique works in a laboratory setting by detecting uranium plates and rods.
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, researchers said.

The study was published in the journal Scientific Reports.