Every few days for the past four years, Smith, a Berkeley Lab physicist, stops by the City of Berkeley's Public Works Department and picks up an armful of police car air filters. He takes the filters to Berkeley Lab's Low Background Facility , where a high-resolution gamma spectrometer analyzes them for the telltale signs of radioactive substances, which could indicate that terrorists have released harmful radiation somewhere in the Bay Area.

Physicist Al Smith, holding a very sensitive radionuclide collector — also known as a car air filter. (Photo Roy Kaltschmidt)

"We haven't detected anything that shouldn't be there," says Smith, who has analyzed about a thousand filters so far.

Now, Smith and several colleagues are working to take this cheap but highly sensitive early warning system nationwide. They're patching together a network of high-resolution gamma spectrometers located near major population centers throughout the U.S. The project, which was competitively selected from over 3,000 submissions in 2003, is funded by the Department of Homeland Security and managed by the Technical Support Working Group, a government group that coordinates R&D projects aimed at combating terrorism.

So far Smith, with the help of retired Berkeley Lab physicist Richard McDonald and project leader Kevin Lesko of the Nuclear Science Division, has secured informal agreements from DOE, other federal agency, and university labs located near New York City, Boston, Washington, D.C., Seattle, and several other cities.

"We have chosen metropolitan areas with nearby labs that have detectors that allow scientists to quickly analyze air filters," says Smith. "We envision a wide-ranging network of detectors that can be deployed quickly and inexpensively."

If a dirty bomb is detonated, air filters analyzed by these detectors could provide an early and effective indication of the epicenter and scope of the attack. They could also help officials pinpoint contaminated areas that must be evacuated, people at particular risk, and safe havens where people could seek refuge.

Smith's idea to use air filters to monitor for airborne radiation dates back to 1986, when the Chernobyl disaster spewed a radioactive cloud into the atmosphere. He and colleagues set up a sampling station at the Lab's firehouse to test for the cloud. The first radionuclides appeared about two weeks after the accident, representing a round-the-world journey that the radioactive cloud would make four more times.

"When the radionuclides arrived, it occurred to me to also examine a car filter. And sure enough, there they were," says Smith. "So I made a mental note that we could see radionuclides using car air filters."

Fifteen years later — and a few weeks after September 11, 2001 — he again thought of car air filters. Perhaps they could monitor the air for the signs of a dirty bomb or terrorist-related radiation release. In order for this to work, however, he needed a steady supply. He chose Berkeley's police cars for several reasons: they have regular routes, are in constant use, and have a routine maintenance schedule — meaning that in the event of an attack, it's possible to quickly determine which areas have been affected by radiation and which have not. Smith and Berkeley Lab employee Donna Hurley also routinely analyze air filters from sheriff's cars that patrol Oroville, CA, where another Berkeley Lab-operated gamma spectrometer is located.

Smith and colleagues are also working to determine the background levels of radionuclides normally captured by air filters throughout the U.S. He has collected and analyzed filters from 12 police departments that serve major cities. This work is critical because air filters are highly sensitive, able to capture very low concentrations of beryllium 7, a naturally occurring isotope formed high in the atmosphere by cosmic rays. They also capture cesium 137, a very rare isotope left over from the nuclear weapons tests of the 1950s and 1960s.

Ultimately, Smith envisions a national network of highly sensitive detectors like Berkeley Lab's gamma spectrometer, which can determine the type of isotopes present in filters. These detectors would be fed by a much larger number of less sensitive but cheaper detectors used to screen filters for the presence of radioactive substances. More funding is needed to make this system a reality, but Smith believes Berkeley Lab is uniquely qualified to shepherd the project along.

"In times like this, it is important for national labs like Berkeley Lab to pitch in," says Smith. "The involvement of this Lab in homeland security projects is beneficial both to the nation we serve and to the Lab itself."

Additional information

• More about the Technical Support Working Group of the U.S. Department of Homeland Security