for National Geographic News
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.
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.
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