Cosmic Particles Could Detect Nuke Materials, Scientists Say

By John Roach
for National Geographic News
March 19, 2003
It takes just a few pounds of plutonium or highly enriched uranium to create a nuclear bomb. That fact frightens security experts around the world. A small chunk of nuclear material that slipped into the wrong hands would be disastrous.

"This threat is a real and difficult problem," said Graham Allison, director of the Belfer Center for Science and International Affairs at Harvard University in Cambridge, Massachusetts, and a leading expert on U.S. national security and defense policy with a special interest in terrorism. "We need to harness all the technology available."

Greater awareness of this threat following the September 11, 2001, terrorist attacks on the United States put a team of scientists at the U.S. Department of Energy's Los Alamos National Laboratory (LANL) in New Mexico on the front lines of defense.

"We are always thinking about some applications of science to the national security task, to homeland defense, especially after September 11," said Konstantin Borozdin, a space scientist at LANL. "It became a high priority for us."

In the March 20 issue of the journal Nature, Borozdin and his colleagues report that they may have found a better way to detect concealed nuclear weapons-grade materials at border crossings around the world. Their method relies on particles generated by cosmic rays as they pass through Earth's atmosphere.

Muon Machine

Cosmic rays are high energy particles that constantly bombard the Earth from outer space. As the rays pass through Earth's atmosphere, nuclear interactions transform many of these particles into heavier particles called muons.

Unlike the particles used in conventional x-ray machines to screen baggage at airports or search for broken bones in human skeletons, muons can penetrate dense objects like uranium and plutonium.

The LANL scientists built an experimental setup that traces the path of these heavy particles as they pass through dense materials and then, using a computer, generates an image of the object.

"You put detectors above and below the thing you're trying to measure and look at the track [of the muons] and see which ones are bent," said William Priedhorsky, a team member with LANL's Nonproliferation and International Security Division.

Muons make a straight line when they pass through organic objects such as human flesh, but bend by several degrees when they pass through dense objects such as uranium or plutonium. Processing where and how the muons bend, the computer generates an image.

Priedhorsky and his colleagues suggest that a machine using muons beamed down on Earth could be an inexpensive and efficient way to screen medium to large objects, such as cars and trucks, for smuggled nuclear materials.

"It sounds plausible enough to me," said Allison, who added that he would hold his final judgment until he got to physically test the technology.

Drive-Through Detection?

The LANL researchers have only completed the initial science for the muon machine and do not yet know what the best application for their technology will be. "The project is in the baby stages," said Priedhorsky. "We have done a little science and none of the engineering."

Funding for the project is currently coming from various parts of the government concerned with national security technology. If fully funded, Priedhorsky believes that the LANL team could produce a full scale system within two years.

Built for national security purposes, Borozdin envisions the system resembling a drive-through garage or a car wash. A car or truck would drive into the structure and stop for a minute or two while muons beamed down from outer space are tracked as they pass through the vehicle.

"During that time, if you have high density material inside the car it could be detected," said Borozdin.

One of the limitations of such a system, said Priedhorsky, is that it does take a few minutes to process an image, rendering the muon machine impractical for use as the primary screening technology at places such as airports where thousands of people and their luggage must be checked each day.

However, unlike a conventional x-ray machine, Priedhorsky says the muon machine introduces no additional radiation and muons are freely available. "Muons are there all the time," he said. "I can take a picture of the inside of something and not give it any radiation."

Thomas Cochran, a senior scientist and nuclear weapons expert at the Natural Resources Defense Council in Washington, D.C., said that it is extremely difficult to detect nuclear materials, especially if the person smuggling them has some knowledge of radiation and shielding techniques. While a muon machine may be an improvement, he suspects sophisticated smugglers could still get around such an apparatus.

"It is useful to have capability at borders to catch unsophisticated smuggling, but I don't think you will catch the sophisticated folk," he said. "The primary focus of the government needs to be on locking the material up at its source and getting out of commercial use of weapons-useable material globally."

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