In the skies above Washington, D.C., a low-flying helicopter is searching for evidence of gamma radiation.

If a dirty bomb were to go off in the capital or the authorities were forced to hunt for a nuclear weapon in the city, an established baseline of radiation sources throughout the capital would be important. Radioactive material naturally occurs in soil, water and vegetation. Low levels of uranium, thorium and their decay products are found in cities and the countryside.

An accurate map of naturally occurring harmless radiation sources throughout the city means it would be easier to locate anomalies like nuclear or radiological weapon.

As part of the National Nuclear Security Administration’s security and emergency preparedness procedures, the flights are measuring naturally occurring radiation in the Washington, D.C., area began Dec. 27 and will conclude Jan. 11, 2013.

The helicopter will cover approximately 70 square miles, flying in a grid pattern over the areas. The flyovers are conducted during daylight at 150 feet or higher above the ground at a speed of approximately 80 miles per hour.

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The NNSA says citizens who spy the low-flying helicopter shouldn’t be alarmed.

The goal is to establish baseline levels of natural radiation and provide an assessment for local law enforcement in D.C. The Remote Sensing Laboratory (RSL) at Joint Base Andrews is analyzing the data.

NNSA was established in 2000 as a semi-autonomous agency within the U.S. Department of Energy tasked with improving national security with nuclear science. While NNSA’s profile may be low, the agency has an enormous responsibility: helping to prevent and respond to nuclear or radiological incidents.

The Agency also has a number of missions related to the U.S. nuclear weapons stockpile. It is responsible with ensuring the performance, reliability, safety and security of U.S. nuclear weapons.

For the U.S. Navy, NNSA provides safe and effective nuclear propulsion.

On a global level, the agency works to reduce danger from weapons of mass destruction and responds to nuclear and radiological emergencies on the homeland and abroad.

For example, just over the past few weeks NNSA’s operations combating nuclear smuggling were boosted in South Korea and they completed emergency management preparedness exercises in Taiwan.

Ballet dancer turned defense specialist Allison Barrie has traveled around the world covering the military, terrorism, weapons advancements and life on the front line. You can reach her at wargames@foxnews.com or follow her on Twitter @Allison_Barrie.