But could a hidden dirty bomb be detected from a helicopter?
"Absolutely," said Larsen, but only after a detailed scan has been done. "This would tend to make something stick out like a sore thumb. Without it, it would be looking for a needle in a haystack."
Two pilots, a scientist, a technician and equipment for detecting gamma radiation are aboard the flights.
Everyday radiation can come from several common urban sources, including industrial sites, construction sites, hospitals that use radiological materials, and even the stone used in buildings and monuments.
The agency has already scanned the capital once before, as well as New York, Baltimore, and San Francisco.
Larsen said the New York scan turned up unexpected sources of radiation -- industrial accidents that were never been reported. "They were surprised how many hot spots they found," he said.
The helicopter in Washington is making an exception for the National Zoo, giving it a wider berth to avoid scaring the animals. But Krol said the public should not be worried about the survey's potential impact on people.
"These scanners are passive," he said. "All they're doing is measuring the emanations coming from the ground."