Electron-like particles better than X-rays

By CHARLES Q. CHOI   |   Feb. 22, 2005 at 9:00 AM
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WASHINGTON, Feb. 22 (UPI) -- Electron-like particles called muons that rain down naturally from the sky are helping scientists to peer inside Mexican pyramids for royal chambers, Japanese volcanoes for hints of erupting magma, and cargo inbound to the United States for illicit nuclear materials.

"We're all familiar with the use of X-rays in medical applications to take pictures inside bodies," researcher Rick Chartrand of Los Alamos National Laboratory in New Mexico told United Press International. "(But X-rays) are not so well-suited for going through large amounts of rock and metal. Now we can use a natural source of radiation that's completely harmless to do so."

Several teams of scientists are investigating muons, particles that in every way resemble electrons except they are 207 times more massive. They are formed when cosmic rays from deep-space sources collide with Earth's upper atmosphere. Roughly 10,000 muons shower every square meter of the planet's surface every minute, and though they can pass through large volumes of rock and metal with ease, their electrical charge makes them easy to track.

Advances in electronics now make muon detectors practical, Los Alamos researcher Christopher Morris told UPI at the American Association for the Advancement of Science annual meeting. Detecting how muons are deflected or absorbed can reveal the presence of chambers or the makeup of materials.

Arturo Menchaca-Rocha, director of the physics institute at the National Autonomous University of Mexico, is deploying muon detectors in a tunnel 26 feet below the base of the Pyramid of the Sun in Teotihuacan, some 30 miles northeast of Mexico City. The 2,000-year-old pyramid was built by an unknown people thought to be contemporaries of the Mayans.

"Archaeologists have speculated an important person is buried somewhere inside," Menchaca-Rocha told UPI. "Our aim is to try and locate empty space -- a royal cavity."

He said his team expects to finish building the muon detectors by the end of summer, and begin collecting data on the pyramid's interior, which is expected to take a year. The tunnel predates the pyramid, Menchaca-Rocha noted.

The pyramid group's detector measures only about three feet square, it still can image nearly all of the interior of the 215-foot-tall pyramid, which is 740 feet long on each side.

"You can use a very small eye to see a large spot," Menchaca-Rocha said.

Meanwhile, Kanetada Nagamine of the KEK Muon Science Laboratory in Tsukuba, Japan, and his group are using muons to image the innards of volcanoes to look for hints of eruptions. Instead of placing detectors under volcanoes and waiting for muons to rain down, Nagamine and colleagues space detectors around volcanoes, taking advantage of the fact that some muons travel almost horizontally when they reach the Earth's surface.

"We have successfully observed two active volcanoes in Japan," Nagamine told UPI.

At Los Alamos, scientists are conducting perhaps the most urgent work with muon detectors. There, they are developing large-scale devices to scan shipping containers at border points. Existing X-ray machines cannot readily penetrate a well-shielded cache of nuclear material. Two inches of shielding can make 5 kilograms of uranium practically invisible to X-rays from three feet away.

On the other hand, more-dense metals -- such as weapons-grade uranium and plutonium -- deflect muons more than less-dense substances, and efforts to shield nuclear-weapon components with metals such as lead only make such objects easier to image with muons.

Most muons can penetrate about six feet of lead, the scientists explained, and any attempt to deflect them electrically would be fruitless, because cosmic-ray muons each pack billions of electron volts of energy.

The Los Alamos team envisions cavernous detectors into which shipping containers could proceed on conveyor belts with minimal delay.

"We estimate it should take 20 seconds to two minutes to detect kilogram-quantities of uranium and plutonium," Morris said.

Chartrand estimated the cost of a portside muon detector at about $2 million.


Charles Choi covers research and technology for UPI Science News. E-mail: sciencemail@upi.com

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