Lasers can detect weapons-grade uranium from miles away
It's hard enough to identify nuclear materials when you can directly scan a suspicious suitcase or shipping container. But if you can't get close?
Now, the researchers have shown that a technique often used to identify chemicals at a distance can also distinguish between ordinary uranium-238 and the fission-prone uranium-235. Just three fewer neutrons make a big difference in the element's potential for destruction.
"It's a much harder problem to try to measure different isotopes of the same element," said Igor Jovanovic, professor of nuclear engineering and radiological sciences at the University of Michigan, who conducted parts of the research while at Pennsylvania State University.
"Different isotopes are very important in the case of uranium because some of them can be used for the production of nuclear weapons."
With ordinary chemical detection—the approach used by the Mars Curiosity rover, for instance—a laser strikes a surface and causes electrons to jump off the atoms and molecules, forming a plasma.
When the electrons jump back into the atoms and molecules, and then come down from higher-energy states within them, they emit light in a particular set of colors that serve as a fingerprint for that atom or molecule.
Jovanovic and his team—Kyle Hartig, assistant professor of nuclear engineering at the University of Florida, and Isaac Ghebregziabher, a postdoctoral scholar at Penn State—showed that this technique can tell the difference between uranium-235 and uranium-238 when the uranium is bonded with oxygen.
"Not only is it possible to make measurements in air, but some constituents of air in fact make this detection more readily achievable," Jovanovic said.
The technique takes advantage of a phenomenon known as laser filamentation.
When very short—and very intense—laser pulses run through the air, they create a plasma channel along the beam line.
The channel serves as a sort of an optical fiber, keeping the laser pulses focused so that they strike their targets in a small spot, even at distances of a kilometer or more.
The intense laser pulses create a plasma from the uranium as well as the air, which gives uranium plenty of opportunity to bond with oxygen. When it does, the energy stored in the bond between the oxygen and the uranium-235 or -238 is just different enough to be detectable.
"These molecules radiate just slightly different colors, depending on whether we are looking at uranium-235 or uranium-238," Jovanovic said.
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