After a long trip across the galaxy, cosmic rays could be employed to find contraband nuclear material hidden in cars, trucks or large ocean-going containers, using a clever new device developed by astrophysicists and national security experts.

The detector can spot dense material, such as a block of radioactive plutonium or uranium possibly destined for a rogue weapons program, by noting how tiny elementary particles called muons pass through it. Muons are castoffs of cosmic rays, created during interactions in the atmosphere.

A typical muon can zip through almost anything, including you, a wall of lead, or about 33 feet (10 meters) of water.

The project to develop the detector, funded in the wake of the Sept. 11 terrorist attacks, was carried out at the Los Alamos National Laboratory (LANL) in New Mexico. A commercial version could be built in about a year, said Christopher Morris, who helped invent the apparatus in research led by Konstantin Borozdin, an LANL astrophysicist.

Cosmic rays are generated by unknown sources throughout the galaxy. They are high-energy, electrically charged particles. Cosmic rays are not part of the electromagnetic spectrum, which includes radio waves, visible light and X-rays.

When cosmic rays enter Earth's atmosphere, travelling at nearly the speed of light, they collide with atoms and break up in a cascading shower of elementary particles, including muons.

A muon is something like an electron but heavier. Unlike larger particles, such as protons, muons are not made out of smaller building blocks. When they travel through matter, muons do lose energy, however. As muons pass through very dense material, they scatter at notably different angles than when passing through steel, plastic or other less dense substances, Morris explained.

"The signature we detect is some number of cosmic-ray muons, all intersecting in a common volume, that scatter to a significantly larger angle than is expected for normal objects," Morris said.

Hiding the dense contraband in a shipment of something else, such as among a herd of sheep, would not sequester it from the muons or the detection device, the researchers will state in the March 20 issue of the journal Nature.

The scientists say monitoring muons could be relatively inexpensive and is safer than using X-rays, and more effective when looking for plutonium or uranium buried among other cargo. There are limitations, however.

"It cannot distinguish between different high atomic weight materials," Morris said. "Gold looks the same as uranium."

Further, the device is not sensitive to common explosives. And while a version could be constructed to monitor moving vehicles, the present approach requires the target to sit still for about half a minute, he said.

The study was supported by funds set aside to work on homeland defense issues in response to the terrorist events of Sept. 11.

William Priedhorsky, who was also involved in the project, is chief scientist in LANL's Nonproliferation and International Security Division. Priedhorsky is also an astrophysicist by training. He said there is a clear connection between national security and basic research in astrophysics, space science, and atmospheric science.

"New ideas for sensors and processing that come from the fundamental science side … are essential for us to carry out our national security mission," Priedhorsky said.

Gary Hogan, Alexander Saunders, Larry Schultz and Margaret Teasdale also worked on the project.

The Los Alamos lab, near Santa Fe, New Mexico, is operated by the University of California for the U.S. Department of Energy’s National Nuclear Security Administration. About 84 percent of the lab's $2.1 billion budget in 2003 is geared toward national security research.