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Researchers have developed a new way to monitor the spread of nuclear weapons. They have demonstrated a tiny device that detects the heavy water used to produce nuclear explosives. The instrument could also detect biological warfare agents and disease.

U.S. scientist William Priedhorsky, from the Los Alamos National Laboratory, says the government is finding ways to detect the explosive uranium or plutonium used in nuclear weapons. "Those are the two potential components of a bomb: highly enriched uranium or plutonium. We are working on a wide range of technologies to try to detect either the production of plutonium or the enrichment of uranium because these are the two vulnerable paths that could be detected," he said.

Besides looking for these materials, which can be easily concealed, other clues are equally important. One is heavy water. It is used to transform uranium into plutonium, the explosive used in bombs. "There are not many folks who would use heavy water for anything non-nuclear, so if you see heavy water at all, that would be an interesting clue," Priedhorsky said.

Heavy water is heavy because each molecule has two deuterium atoms instead of the two hydrogen atoms found in regular water, and deuterium is twice as heavy as hydrogen. That is because deuterium atoms have a neutron that hydrogen lacks.

Heavy water is hard to detect because it is not common in nature, but California Institute of Technology researchers Kerry Vahala and Andrea Armani have a new approach for finding it.

Their technique measures subtle differences in the composition of compounds such as water. They describe the sensor in the journal "Optics Letters."

Vahala says it is a very small donut-like detector that acts like a tuning fork. By measuring changes in its ring, the device is able to determine what has landed on it. "Each of these little donuts has its characteristic tone that it rings at, and the purity of the tone is so high that it is actually possible to perceptibly detect a change in the tone when even something as small as a molecule lands on the surface of the donut. You imagine one particular donut is ringing away and along comes a molecule that lands on the surface, we can use the change in the ringing frequency to actually know whether something has landed on the surface or not," he said.

With funding from the U.S. military, Vahala's team is working to make the device more sensitive. It is now 30 times more sensitive than others on the market. He says it is sensitive enough to detect very minuscule amounts of heavy water in regular water. "The numbers are around one molecule, so one heavy water molecule in 10,000. That actually represents a number that is right around the level you would find for the natural abundance of heavy water in normal drinking water. So it is already very sensitive as it is, certainly sensitive enough to see abnormal or variations from the normal abundance levels of heavy water in natural water," he said.

Vahala's research partner, Andrea Armani, says the detection of heavy water is only one of the instrument's many applications. "The heavy water detection came about as initially a project we did to characterize the sensor and its behavior in water and heavy water. The true power is that it reacts strongly to the environment that it is in. To a very high precision, it is able to determine what the environment is," she said.

Vahala says these environments include biological compounds. In addition to water, the technique can be modified to sense molecules found in diseases, biological warfare agents, and other threatening situations. "There are large number of applications. The work we are being funded to explore is really targeted on biomolecules," he said.

Vahala believes that with further development, their instrument could be improved dramatically to sense single molecules.