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New Method Could Optimize Detection of Nuclear Materials Hidden Within Containers

A new low-energy nuclear reaction imaging method was put to test by a team of researchers recently. This method has the capability of detecting whether there was any weapons-grade plutonium and uranium present, and can be applied to check cargo containers entering into the US ports. The method relies on a mixture of high-energy photons and neutrons to detect radioactive materials shielded within the containers.

University of Michigan Professor Igor Jovanovic and Graduate Student Jason Nattress are shown in front of an ion accelerator at the Michigan Ion Beam Laboratory. Beams produced by this type of source have been used to interrogate shielded special nuclear materials. (Courtesy Igor Jovanovic)

The method has two key benefits. It can concurrently measure the density and atomic number of the suspected material with the aid of mono-energetic gamma ray imaging, as well as validate the presence of weapons-grade plutonium and uranium just by analyzing their distinctive delayed neutron emission signature.

The distinctive radiation source’s mono-energetic nature may result in a lower radiation dose, in comparison to traditionally used techniques. The method could optimize the detection performance while preventing harm to electronics and other cargo, which may otherwise be affected by radiation.

If this method could be scaled up and tested in real world conditions, it could considerably optimize the ability to prevent dangerous nuclear materials from being smuggled and the probable diversion to terrorist groups.

This research was published in the Scientific Reports journal by Nature Publishing and was supported by the US Department of Homeland Security and the National Science Foundation. The team consisted of researchers from the Georgia Institute of Technology, the University of Michigan, and the Pennsylvania State University. This research is considered the first successful attempt to detect and image uranium using this novel method.

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. 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.

Anna Erickson, Assistant Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, University of Michigan

The method uses an ion accelerator to produce heavy isotopes of hydrogen and deuterons. The deuterons impinge on a target made up of boron, which generates both high-energy photons and neutrons. The particles that are formed are focused into a fan-shaped beam, which can then be used to examine the cargo container.

The transmission of highly active photons can be applied to image materials concealed within a cargo container, while both the neutrons and photons excite the weapons-grade nuclear material, which subsequently emits neutrons and gamma rays that can be detected from outer side of the container. Transmission imaging detectors placed in the line of sight of the fan beam of photons produce 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. We can observe the characteristics of transmission of these particles to understand what we are looking at.

Anna Erickson, Assistant Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, University of Michigan

When the neutrons come into contact with fissile materials, they set off a fission reaction, producing both timely and delayed neutrons which can be identified regardless of concealment.

The neutrons are crucial to this method as they do not prompt a time-delayed reaction with materials that are non-fissionable, such as lead, providing an indication that there are potential nuclear weapon creation materials present 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. 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.

Anna Erickson, Assistant Professor, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, University of Michigan

X-rays were used to actively detect the presence of radioactive materials in the past. Erickson stated that that method was not successful when there was heavy shielding and because of the potential damage to the cargo caused by higher doses of X-rays. The new method, however, uses discrete energies of neutrons and photons, reducing the quantity of energy flowing into the container.

Georgia Tech’s team was headed by Erickson, and the University of Michigan and Penn State University teams were led by Igor Jovanovic, professor of nuclear engineering and radiological sciences. Together they illustrated that the method functions in a laboratory setting by detecting uranium rods and plates.

The testing was performed in partnership with the Massachusetts Institute of Technology at the Bates Linear Accelerator Center, and the researchers used a fan-like outline of particles developed by an ion accelerator and emitted at 4.4 and 15.1 MeV. The particles were passed via a shielded radioactive material, and were measured on the other side using Cherenkov quartz detectors, which were 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.

Igor Jovanovic, Professor of Nuclear Engineering and Radiological Sciences, Penn State University

The method is yet to be tested on steel cargo containers in real life. But the team believes that these types of investigations will soon take place.

In addition to the probable homeland security applications, the technology can also be applied in materials science, low-energy nuclear physics, medical imaging, and industrial imaging. The research also included graduate students Paul Rose, Jr. (Georgia Tech) and Jason Nattress (University of Michigan) and postdoctoral research associate Michael Mayer (Penn State University).


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