A helicopter equipped with radiation-sensing technology will make several low passes over the Boston Marathon route later this week to measure naturally occurring background radiation ahead of the 120th Boston Marathon next week.

Between Tuesday and Friday, the National Nuclear Security Administration will use the chopper to measure background radiation along the 26.2-mile marathon route and slightly beyond, flying a twin-engine Bell 412 helicopter i

n a grid pattern at about 150 feet above the ground at speeds of about 80 miles per hour, the agency said.

"We are flying the survey at the request of the marathon coordinators and the state and local authorities," Shelley Laver, deputy director of public affairs for the NNSA, told the News Service. "That data that we collect is provided to said parties and gives them a baseline of what the situation looks like today."

The NNSA has conducted a radiation survey in the Boston area before the Boston Marathon each of the last two years. Three years ago, terrorists detonated two pressure cooker bombs near the marathon finish line, killing three and injuring more than 260 spectators.

According to the United States Government Accountability Office, "the surveys can be used to compare changes in radiation levels to (1) help detect radiological threats in U.S. cities more quickly and (2) measure contamination levels after a radiological attack to assist in and reduce the costs of cleanup efforts."

"If something were to occur, they would have that, and if there was suspicion of some sort of a use of nuclear or radiological source in any kind of event, federal, state and local teams would come in and assess and be able to do a comparison to the baseline," Laver said.

The NNSA said it wants the public to be aware of the scheduled flights so as to not be alarmed when a 56-foot helicopter buzzes overhead. Laver said the agency routinely fields calls about the flights from residents, often mothers upset that the helicopter disturbed their child's nap time.