The National Nuclear Security Administration (NNSA) will spend the four days leading up to the Boston Marathon conducting a helicopter-based survey to measure naturally occurring background radiation in a ten square mile area west of downtown Boston. The measurements are being taken to "establish baseline levels [which] is a normal part of security and emergency preparedness", especially before large public events such as the marathon. A similar survey was conducted in northern New Jersey and New York City before the Superbowl in January.
The NNSA made the announcement to alert residents who may be alarmed by the low-flying aircraft. The twin-engine helicopter out of the military's Joint Base Andrews in Maryland will fly a grid pattern as low as 150 feet above the ground at 80 miles per hour, though only during daylight hours.
A Government Accountability Office (GAO) report in 2006 reported "significant benefits" to such surveys in the event of nuclear terrorism:
There are significant benefits to conducting aerial background radiation surveys of U.S. cities. Specifically, 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.
At the time of the 2006 report, New York City was the only major U.S. city to have conducted a full survey of the entire city, which was done in 2005 at a cost of $800,000.
Washington, D.C. was the site of a 70 square mile, two-week survey in the weeks before President Obama's second inauguration in January 2013.
The NNSA also deployed a team of 33 experts to Japan in 2011 to measure and monitor radiation levels around Fukushima following the tsunami and subsequent nuclear disaster at the power plant in that city.