Radiation survey will be performed on Marathon route

Low-flying helicopters may be visible along the Boston Marathon route this weekend, according to the National Nuclear Security Administration.

The federal agency is seeking to measure naturally occurring background radiation to establish baseline levels, as part of a security and emergency preparedness plan for Monday’s Boston Marathon, according to a statement from the agency.

The helicopter will fly over the Athlete’s Village at the starting line area in Hopkinton and survey the entire route, plus an additional 1,000 feet on either side of the path, according to the agency.

One twin-engine Bell 412 helicopter equipped with the radiation sensing technology will fly over areas at 150 feet, or higher, at about 80 miles per hour. Flyovers will occur during the day, said the statement.

The helicopter began surveying Thursday and will continue sweeping the route until Sunday, said the statement.

The security agency seeks to make the public aware of these flights so they are not alarmed, said the statement.

The background radiation survey is done so that if radiation is detected during the Marathon, authorities can distinguish between normal levels and possible terrorist activity.

The agency performs such surveys for many high-profile events annually, said the agency.