Nuclear Needles in Cargo Haystacks

By Shannon Palus

APS April Meeting 2015, Baltimore — Every day, some 60,000 ship-borne cargo containers pull into United States ports. Each metal box is about 2.4 m by 2.4 m by 12 m, which is plenty of space to hide a nuclear bomb, says Areg Danagoulian, a nuclear scientist at MIT. In fact, there are bombs that could blow up several city blocks, and could fit in a backpack. Danagoulian asks: “How do we detect something so small?”

At the April Meeting, Danagoulian presented data from a proof-of-concept demonstration that peers into cargo with beams of gamma rays. Within the decade, he hopes, the method will be used at ports to accurately scan cargo at a rate of about two minutes per container. His is just one of the technologies that physicists are developing to prevent the proliferation of nuclear materials, whether through terrorism or war.

It’s been decades since school children were advised to “duck and cover” to shield themselves from Cold War atomic bombs. According to the National Academy of Engineering (NAE), preventing nuclear terror is still an important goal, and the NAE put it on the list of 14 Grand Challenges of Engineering to be solved in this century.

“Peaceful energy programs could mutate into weapons programs,” says Danagoulian. “You could use a reactor for synthesizing plutonium, and [make] a weapon out of that.” Also, weapons can be stolen from existing stockpiles; there are 17,000 warheads in the arsenals of Russia and the U.S.

Today, port inspectors use passive methods to detect nuclear contraband, like looking for radiation coming from a container. That is easy to block with lead if the smuggler has any competence, says Danagoulian. Inspectors also use broadband x-ray beams, ranging in photon energy from 1-6 MeV, to gauge the density of the material inside the container. But when used to discern the atomic number, the method is inefficient, and requires a high dose to work.

In Danagoulian’s cargo interrogation method — 10 times as efficient as the broadband method — two monochromatic gamma-ray beams, one at 4.4 MeV and one at 15.1 MeV, pass through a container to a detector on the other side. The flux of the 4.4 MeV beam through the container reveals the density of the material; combined with the 15.1 MeV flux, the method also yields information on the atomic number Z.

Danagoulian has completed a proof-of-concept test of the technique with several materials, from aluminum to iron to copper to lead, but not plutonium, which is hard to get, even for a scientist studying how to stop it from spreading. The data clearly show that as Z increases, the number of photons that go through the container decreases.

But his system alone isn’t enough to conclusively determine what’s inside the container. “If you start mixing materials, you are going to measure the effective Z,” says Danagoulian. Adding low-Z materials to a container with plutonium could throw off the system.

One method that might complement gamma-ray inspection is neutron radiography. “Neutron radiography is good at analyzing materials with low Z,” says Danagoulian. That includes plastics and organic material. “I see it as a way of augmenting the range of Z reconstruction,” he explains.

When it comes to capturing a warhead, or the material for one, “There is no silver bullet,” says Danagoulian. Ultimately, he envisions a combination of methods employed at ports, the data from each utilized in a decision-making algorithm. And the methods work for catching more banal contraband, too. Coffee importers smuggling beans to avoid import taxes may not be as threatening of a scenario, but it’s one customs agents are more likely to see on a regular basis.