Lasers could become an important tool for detecting radioactive materials, such as those in covert nuclear weapons, from a greater distance than ever before.
Conventional radioactivity sensing techniques involve waiting for particles produced during radioactive decay to hit a detector. The method can sense these particles from tens of metres away 鈥� but not much further. at the University of Maryland and his colleagues have now shown that a laser-based method could be effective from as far away as 100 metres from the radioactive object.
The researchers sent pulses of infrared laser light towards a sample of the radioactive element polonium. Its radiation had knocked some electrons out of the air molecules nearby, and the laser light added energy to those particles. Each energised electron then bumped into more molecules and knocked out even more electrons, causing a cascade that culminated in the creation of a small ball of crackling bright plasma around the original free particles.
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鈥淚t鈥檚 hard to detect a single electron in the middle of the air somewhere, but what a laser does is it turns that very undetectable thing into a macroscopically detectable thing,鈥� says Milchberg.
Crucially, incoming laser pulses could now scatter off the plasma. Measuring how intense this scattering process was, and when exactly it started, allowed the researchers to infer the laser had come near radioactive material.
The approach was effective from 10 metres away when the researchers used a very large and powerful laser at Brookhaven National Laboratory in New York. They also have preliminary results from 30 metres away with a more compact laser, and they think it could work at even greater distances, up to 100 metres.
These distances are about ten times greater than past tests of the laser-based method and represent a notable step forward in remote detection techniques, says at the University of Florida. However, the experimental setup is too unwieldy to use outside the lab 鈥� it must be made much smaller and more portable first, he says.
at the University of Michigan says the team鈥檚 approach could be useful in situations where radioactive material is suspected to be at a specific location, because the laser can be aimed in a very controlled way.
But most covert nuclear weapons would be hidden behind some shielding material, which would change the kind of particles that the laser would encounter in the surrounding air, explains Jovanovic. So testing with a more diverse set of radioactive materials is also needed, he says.
Journal reference:
Physical Review Applied, in press
