Distant Threat

Terrorists have used knives, guns and bombs and even turned airliners into missiles, but they haven’t managed to carry out a radiological attack—and University of Maryland scientists are working to make sure they never do.

Researchers in the departments of Electrical and Computer Engineering and Physics are developing a laser-based sensor to spot evidence of hidden radioactive material from 100 or more yards away, alerting officials to a potential attack in the works.

Right now, finding radioactive material in a truck or cargo container typically requires close-up inspection with a Geiger counter.

“What’s needed is standoff detection, because if there’s something you want to inspect, you may not necessarily be able to get close to it,” says Phillip Sprangle, a professor of electrical and computer engineering and physics.

The project is funded by the Pentagon’s Defense Threat Reduction Agency (DTRA), which wants technology that would allow speedy inspection of a large number of distant vehicles or cargo containers, he says, even if physical access to them is blocked.

“One thing DTRA would like to be able to do is determine radioactivity, for example, on barges,” Sprangle says. “That’s a major problem—you could potentially bring a barge into New York Harbor with a nuclear weapon on board and destroy New York.”

Although the worst scenario involves a smuggled conventional nuclear weapon, a far simpler attack could involve a “dirty bomb” that spreads radioactive material like Cobalt-60, common in medical equipment, with a conventional explosive. The damage would be tiny compared to a nuclear weapon’s blast, but such an attack might still kill or injure many people, sow panic and render part of a city uninhabitable.

Instead of measuring radioactivity directly, the theoretical laser device described in a paperthis spring in the journal Physics of Plasmas would spot one effect of radioactivity on surrounding air—oxygen molecules with extra electrons.

A low-power laser directed within a few feet of the radioactive material frees electrons from oxygen molecules, and then a second, more powerful laser energizes them and creates a flashing “avalanche breakdown” of the air, says Josh Isaacs, a physics doctoral student who was lead author on the paper. A faster-than-normal reaction means radioactivity is present.

“A slight difference in the number of electrons means a difference in how long this process takes to complete,” he says. “It looks like a spark on the order of a cubic centimeter. It’s small, but with the right kind of optics you can see it pretty easily.”

Laboratory testing that could lead to a sensing device is under way, Sprangle says, headed up by colleague Howard Milchberg, also a professor of electrical and computer engineering with a joint appointment in physics.

Sprangle envisions a device that mounts on a truck or helicopter and inspects many targets quickly, but he and the team haven’t settled on exactly how the system would be deployed.

“DTRA is funding basic research, and sometimes it’s difficult to predict exactly what the application will be,” he says. “You do the basic research, understand what the limitations and advantages are … and the application follows. Working the other way around doesn’t work.”