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    Helicopter measuring city radiation before Marathon

    A low-flying helicopter will be criss-crossing downtown Boston throughout the week as part of a federal operation to measure naturally occurring radiation as part of the security preparation for the Boston Marathon.

    The effort by the US Department of Energy’s National Nuclear Security Administration runs from Tuesday through Friday.

    The program is intended to give emergency officials a picture of the normal levels in the area for comparison in case there is an emergency involving radiation at the event. In a news release, the agency called the flights “a normal part of security and emergency preparedness.”

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    “NNSA is making the public aware of the upcoming flights so that citizens who see the low-flying aircraft are not alarmed,” officials said.

    The helicopter will fly at about 80 miles per hour over downtown in a grid pattern at an altitutde as low as 150 feet above the ground. The flyovers will happen only during the day.

    Andy Rosen can be reached at andrew.rosen@globe.com. Follow him on Twitter at @andyrosen.