A prototype of a device that could someday detect nukes through layers of steel just passed its first test. The detector, which uses technology that was developed for particle physics experiments at the Large Hadron Collider, can tell the difference among iron, lead and other heavy metals.

By detecting the signature of heavy elements that could be used to build nuclear weapons, the new machine could someday find nuclear contraband hidden in shielded vehicles.

"This is the first time that we actually built and operated successfully the equipment to actually do this in real life, rather than in a computer," said high-energy physicist Marcus Hohlmann of the Florida Institute of Technology, a co-author of the study.

The device takes advantage of charged particles called muons, which are created in the atmosphere and zip through every square centimeter of material on Earth – human bodies and armored trucks alike – at a rate of one per minute.

"They sort of rain upon us like a light drizzle all the time," Hohlmann said.

Despite their high energies, muons don't interact very strongly with matter. "They can go through 6 to 8 feet of steel without being stopped," Hohlmann said. "That's nice for our application, because what we're trying to do is look into things that are shielded."

But though matter typically doesn't stop muons in their tracks, heavy elements like uranium and metals like lead can deflect the charged particles. By tracking the muons' paths, scientists can construct a 3-D image of whatever material got in their way.

The new prototype uses detectors called GEMs, or Gas Electron Multipliers, to trace muons' trajectories before and after they hit a bit of heavy material. The detectors are thin plates filled with gas that were originally developed for particle physics experiments at places like CERN and Fermilab. When a muon plows through the detector, it rips electrons from the gas, leaving a distinctive trail readable by electronics on the detector's surface.

"This is a very common technique," Hohlmann said. "When you look at the fancy pictures of experiments from the LHC, and they say here's this particle and here's that particle, that's how they get those tracks. In some sense, this whole thing is a spin-off from experiments from particle physics."

Working in a lab at CERN, Hohlmann and his colleagues positioned two detectors above a 250-cubic-centimeter volume, and two below. Because their target area was so small, the researchers could collect only about 1,000 muons per day, so each trial took at least two days. The team tested the device on a block of iron, a block of lead and a cylinder of the dense rare metal tantalum. Each object was left in the detector until it had been hit by 3,000 to 5,000 muons.

Using computer imaging techniques, the researchers successfully resolved the raw data from the detectors into plots of each muon strike, which revealed the composition and the shape of each target. Heavier elements deflect muons more strongly, so the average angle of the muons' post-impact path tells physicists the material's identity.

"I was surprised it worked as well as it did, especially that we could tell the difference in shape between cylinder and cube," Hohlmann said. The results are reported in a paper submitted to Nuclear Instruments and Methods A.

The prototype is not practical as it stands now, Hohlmann said. For one thing, it's far too small to drive a truck through. It also takes days to collect enough muons to make an image. Using bigger detectors will let physicists collect more muons, just as putting a bigger bucket out in a storm collects more raindrops. The researchers are working on a larger version that would surround the target on four sides, not just two.

"We're hoping we can get sort of an alarm – yes, there is something in there, or no, there is nothing – within a few minutes," Hohlmann said. The team hopes to ultimately build a box similar to an airport security scanner, or a tunnel to drive trucks through, that could probe incoming packages at borders and ports in a manner of minutes. He expects a version big enough to test luggage in the next year, and big enough for cars and trucks within three or four years.

Hohlmann's team is not the first to try using muons to detect nuclear contraband. That distinction goes to a group at Los Alamos National Laboratory, which built a prototype using drift-tube detectors in 2005. But the GEM detectors used in Hohlmann's device can resolve features one-quarter the size detectable by earlier devices.

"This looks to be a solid piece of detector technology development based on the well-established GEM technique," said physicist Roy Schwitters of the University of Texas at Austin, who has used the muon technique to peer inside Mayan ruins. "Whether the GEM approach will supersede the drift-tube detectors used by LANL is more of a detailed engineering question."

Images: 1) A lump of lead waits in the detector for a muon strike.

2) Computer-resolved images of a cube of iron (left) and a cylinder of tantalum (right). The colors depict how much the muon was deflected.

Credit: Marcus Hohlmann.

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