Improved X-ray vision to stop nuke smugglers

• 20 October 2005
• NewScientist.com news service
• Celeste Biever

IT'S been a long day at the Port of New York and New Jersey. Officials have wasted precious time and money opening up or X-raying at least 150 incoming freight containers. They turn out to be full of cat litter, ceramic tiles or bananas - all of which happen to be naturally radioactive.

What they don't realise is that they have also nodded through a container in which is stowed a 50-kilogram canister of stolen highly-enriched uranium. Unlike the bananas, the low-energy gamma rays it emits are easily absorbed by the 2-centimetre-thick sheet of lead around it, so it passes through the radiation monitors unnoticed. Some time later, home-grown terrorists build two nuclear bombs, take one across the country in a car and set off simultaneous explosions in New York and San Francisco.

Nightmare scenarios like this have already prompted the US Department of Homeland Security (DHS) to spend $300 million installing radiation-detection equipment at the nation's ports. But despite this, US ports remain vulnerable, according to the many scientists, government and port officials who testified on the technology for detecting smuggled nuclear materials before a special hearing of the House Committee on Homeland Security in June.

"We'd be crazy to assume that the bad guys aren't thinking of this," says Bruce Schneier, a security expert based in Mountain View, California.

Around 80 per cent of freight containers coming into the US are already screened by a radiation portal monitor, which detects any gamma rays and neutrons that escape from the container, and that figure is expected to rise to 100 per cent by the end of the year. But these scanners have significant limitations.

Firstly, they wrongly flag around 2 per cent of all containers as suspect, mainly because they cannot distinguish between a plutonium bomb and the radioactive potassium-40 found in bananas. More importantly, they fail to detect the most dangerous nuclear material of all: highly enriched uranium (HEU).

Unlike plutonium, which emits neutrons and high-energy gamma rays that are almost impossible to shield, HEU emits only low-energy gamma rays. The monitor detects gamma rays using a piece of plastic called a scintillator, which emits photons when it is hit by gamma rays. These photons are then converted into an electrical signal. But if the uranium is shielded by just a thin layer of lead, or even wood, the detectors miss it. And HEU is much easier to turn into a bomb than plutonium.

The number of false positives may be reduced by a new set of detectors known as the Advanced Spectroscopic Portal, to be introduced in 2006. These will distinguish between radioactive isotopes such as plutonium and the potassium-40 in bananas by replacing the plastic scintillator with an inorganic crystal such as sodium iodide. These crystals also emit photons when hit by gamma rays, but the photons have different, clearly defined energies depending on which isotope the gamma rays come from, allowing the isotope's identification.

But even this detector will not be able to detect uranium. So the DHS is devoting further millions to fund the development and testing of a new generation of active screening technologies that bombard containers with high-energy particles or rays and then scan for the resulting signature emissions. Instead of relying on the low-energy gamma rays that uranium naturally emits, the bombardment will spark the emission of higher-energy neutrons or gamma rays that can penetrate lead shields and be picked up by the detectors.

One idea being tested by Dennis Slaughter at Lawrence Livermore National Laboratory in California is to bombard the container with a beam of neutrons (New Scientist, 16 July, p 8). If the neutrons hit uranium or plutonium, they induce fission reactions that split the nuclei into smaller fragments, but do not start a chain reaction as the density of neutrons is too low. These fragments emit high-energy gamma rays as they decay.

Meanwhile James Jones of Idaho National Laboratory in Idaho Falls is using a similar idea in reverse. He bombards the containers with high-energy X-rays to induce fission reactions in uranium and plutonium and then looks for the neutrons that are emitted in the process (New Scientist, 11 January 2003, p 17).

Both these technologies have been used to detect HEU behind lead shields and have been developed to prototype stage. But both have their drawbacks. Neutrons are scattered by materials rich in hydrogen, such as food, fuel, clothes and wood. As a result, the illuminating beam in Slaughter's system was unable to get to the sample in tests on cargoes containing any of these common materials, and the signature emissions of Jones's technology could not pass out of the container.

Both have come up with methods to overcome this problem (see "Fission or food?"), but others think that since much of the freight passing through ports every day contains hydrogen-rich material, the answer lies not in neutrons but in photons. "Photons are much more penetrating of hydrogenous materials," says physicist William Bertozzi at the Massachusetts Institute of Technology.

Bertozzi has set up a company called Passport Systems, based in Acton, Massachusetts, which is working on a rival technology known as nuclear resonance fluorescence imaging. In NRFI, an X-ray beam excites all the nuclei of all the atoms in the container. A gamma-ray detector then identifies the atoms according to the unique energies and intensities of the photons emitted as the nuclei return to their original state without undergoing fission.

Bertozzi decided to apply NRFI to cargo containers when he realised that an X-ray beam of high enough energy to penetrate a lead shield would also span the range of energies needed to excite each of the atoms in the periodic table - between 2 and 9 million electronvolts.

Unlike its rivals, Bertozzi's technique can identify both radioactive and non-radioactive isotopes in a container. All nuclei are made up of clouds of protons and neutrons that rotate and vibrate with respect to one another when they are hit by photons at their resonant energy. The exact energies emitted when the nuclei fall from these different energy states depends on the number of protons and neutrons in the nucleus, and provides a unique signature for every element and isotope.

By carefully positioning the detectors and the beam, Bertozzi can illuminate each 20-centimetre cube of the space within the container individually, so putting the resulting images together will give a 3D image of the location of all the radioactive and non-radioactive isotopes in the cargo (see Diagram).

This means the system can be used to detect traditional explosives or contraband as well as nuclear materials, and could be useful to customs officials who simply want to ensure that a container holds what its owners claim it does.

Being able to pinpoint the location of the various contents within the container might also allow a more intelligent analysis of its contents. Bob Ledoux, CEO of Passport Systems, intends to program the system to distinguish between a nuclear bomb, which would be attached to a traditional explosive and require a higher alert, and the components for a bomb, which might simply be surrounded by a lead shield.

But Jones regards this as a waste of time, preferring to simply pinpoint the nuclear material. "I don't need to know what the element is at every single point," he says. "If there is no nuclear material declared at all in this cargo I don't care what else there is." His detector is able to find the approximate location of the nuclear signals. The three technologies will be tested against each other in late 2006, at a site being built in Nevada by the DHS.

Slaughter also helped write a report on technologies for detecting smuggled nuclear materials, commissioned by the DHS but not yet released. It concludes that the final system is likely to be a combination of all the technologies being developed. "There is no silver bullet that works for all cargo mixes," Slaughter says.

Preventing a terrorist nuclear attack is obviously about more than scanning containers. Increasing security at reactors so that the material cannot be stolen in the first place will be a major part of the equation. But should terrorists get their hands on any uranium, Schneier is confident the technology to prevent it being smuggled is within reach. "It's the kind of thing that technology can actually help with, unlike so many security problems."