Muon Detectors Hunt for Fissile Contraband
By Michael Lucibella
Hunting for a concealed nuclear weapon can be harder than looking for a needle in a haystack. Though exposed plutonium’s radiation is easy to detect, uranium’s is less so, and both can be shielded. At the APS April Meeting, researchers presented an improved technique for using the natural radiation of cosmic rays to peer through solid objects and find any hidden fissile material.
When a cosmic ray strikes the nucleus in the atmosphere, it produces a shower of subatomic particles, including pions and kaons which decay into longer-lived muons. These muons travel down at high speeds through the atmosphere and can pass through solid objects. Michael Staib of Florida Institute of Technology and his team are harnessing this natural phenomenon to scan for hidden nuclear materials, and for any shielding hiding it.
“Muon tomography is a passive vehicle interrogation technique designed especially for detecting well-shielded nuclear contraband,” Staib said. “We simply use cosmic ray muons. Those are constantly being produced in the upper atmosphere and passing through us all the time.”
A muon strikes every square centimeter of Earth once a minute on average. Gas electron multiplier (GEM) detectors can detect their location, and when several are stacked on top of each other, they can track the paths of the fast moving particles. The denser the material a muon passes through, the more its path is deflected. Uranium and plutonium are two of the densest elements in the periodic table, so the detectors are used to look for places where the paths of muons are the most disrupted.
“Uranium doesn’t have a very strong signal for radiation detection, but you simply use the fact that uranium is very heavy and very dense and so you can try to find a way to detect it using those characteristics,” Staib said. “No artificial radiation source [is needed] so there’s no exposure of an object to radiation beyond what it would be experiencing anyway.”
To look for nuclear materials, a shipping container is placed between two sets of large GEM detector plates. Two plates on top of the container track the paths of incoming muons, and two plates underneath track them on their way out. If there’s little or no dense material in the container, than the two parts of the muon’s path should line up. Even iron won’t deflect muons a great deal. However, if there’s a lot of dense material, like plutonium, uranium or lead shielding, the paths should veer sharply.
“If I can force them to put five tons of lead around it, I’m good because it’s easier to detect five tons of lead than the radiation,” said Michael Kuliasha from the Defense Threat Reduction Agency. “You have to have a robust radiation detection because it forces them to do something that’s actually easier to detect.”
He added that the difficulty of finding concealed nuclear weapons is not a new problem. “In 1945, Robert Oppenheimer, who was head of the Manhattan project, was actually asked in a congressional hearing by Senator [William] Milliken … how would you detect an atomic bomb hidden somewhere in a city. And [Oppenheimer] says, ‘I’d get a screwdriver and open each and every suitcase and crate,’” Kuliasha said.
GEM detectors were first developed at CERN to detect muons and other particles produced in collisions in accelerators. The idea to use passive scanning to find hidden fissile materials was first developed at Los Alamos in 2003, and has been developed further by the company Decision Sciences. Their method, which uses drift tube detectors, is about to undergo the first test commercial application in the Bahamas. Drift tubes are relatively inexpensive, but take longer to make a measurement than the GEM detectors.
Because both methods rely on the natural rate of muons traveling through the atmosphere, the only way to speed up the detection of illicit materials is by improving the sensitivity of the detectors. Right now Staib’s prototype takes about nine to ten hours to differentiate between different materials, but he says that with more development it should be able to get down to a few minutes.
In addition to scanning incoming cargo, Kuliasha said that the technology is promising for verification of arms reduction treaties like START. He said that a detector could be set up around a missile or submarine to see if nuclear warheads are still inside. At the same time there are limitations to the technology. Using it to scan an entire ship would be logistically impractical, and probably still wouldn’t be as effective as boarding and searching the vessel.
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