That helicopter you may have seen swooping low over the nation’s capital, over the heads of the knowing and the goggle-eyed, the confident and the wary: Is it for a movie? Preparing for the inauguration? Carrying the rich, the famous or the powerful?

Not according to a little-known arm of the U.S. government called the National Nuclear Security Administration. The agency said it is trying to measure radiation of a sort that is usually produced in nature by nuclear decay. Specifically, it is trying to measure gamma rays.

The idea, according to the NNSA, is to get an idea of Washington’s natural, normal output of gamma rays, a potentially harmful form of nuclear radiation. Such knowledge would aid in the response to a situation in which levels of radiation spiked, possibly as the result of a “dirty bomb.”

A helicopter fitted out with radiation-sensing technology has become part of the scene in Washington in recent days. The flights began last month and are expected to continue through the middle of January.

The helicopter could fly as low as 150 feet, the NNSA said. It has already been spotted over parts of Capitol Hill and the Columbia Heights and U Street areas.