In a paper appearing in Nature magazine, (March 20 vol. 422, p. 277), a team of Los Alamos National Laboratory astrophysicists and physicists notes that in both laboratory experiments and corresponding computer simulations, dense materials such as uranium can be detected and imaged by tracking the paths of muons as they pass through the target materials.

Unlike X-ray imaging, which depends upon small doses of artificial radiation to create images of dense objects such as bones or smuggled metal objects, muon radiography needs no additional radiation.

Muons, are created naturally when cosmic rays from space interact with the Earth's atmosphere, and pass through the Earth's surface at a rate of approximately 10,000 particles per square meter per minute. Using these common, naturally occurring particles for imagery came naturally to the science team, according to one member.

"There's a lot of work between an idea and any successes.

Most ideas don't pan out, but this one did," said William Priedhorsky, a high-energy astrophysicist on the project. "New ideas for sensors and processing that come from fundamental science are essential for us to carry out our national security mission."

Placing an array of simple sensors above and below a target, the team developed a computer algorithm to detect variations in the muons' paths before and after striking a heavy metal target. The change in direction by the muons was the key to the detection technology, permitting the scientists to build three-dimensional images of the target objects.

"It worked incredibly well, we were really surprised," said Priedhorsky, chief scientist of the Laboratory's Nonproliferation and International Security Division.

"We knew that back in the 1970s physicist Louis Alvarez had created images using muon absorption to map the interior of the Second Pyramid at Giza, but no one had explored the scattering data to see if it was useful," Priedhorsky said.

"We found that we could detect a 1-liter block of uranium in the equivalent of a truckload of sheep, for example, and the greater the efforts at shielding the material, the more obvious it becomes with the muon technology."

A drawback to the infant homeland security project would be the time required for an image, since detection is based on a minimal number of muons impacting the target object, and they fall from space at a rate that cannot be changed.

A 1-minute shower of particles would be required to reasonably scan a suspected smuggler's vehicle, for example, according to lead author Konstantin Borozdin.

But given that the particles are free, available worldwide, and that they provide consistent imaging of any high-atomic-number substance, the concept offers a promising addition to conventional X-ray, gamma and neutron-detection systems.

The team developing this technology comes from both the Laboratory's Physics and Nonproliferation and International Security divisions, and includes Konstantin N. Borozdin , Gary E. Hogan, Christopher Morris, William C. Priedhorsky, Alexander Saunders, Larry J. Schultz and Margaret E. Teasdale (now a student at the University of Hawaii).

Los Alamos National Laboratory is operated by the University of California for the National Nuclear Security Administration (NNSA) of the U.S. Department of Energy and works in partnership with NNSA's Sandia and Lawrence Livermore national laboratories to support NNSA in its mission.

Los Alamos enhances global security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health and national security concerns.

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