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Chopper Maps Out DC Radiation

It's a bird, it's a plane, it's the Nuclear Safety Administration.
A helicopter passing the Washington Monument
Image Courtesy: Frank Gruber

If you're a DC resident, don't be surprised by the low-flying chopper you'll likely hear crisscrossing the city for the rest of the week. The helicopter, outfitted with a gamma-ray detector, is mapping out DC's natural radiation levels. This map of existing radiation will act as a baseline for future radiation detection.

Gamma rays are the most energetic kind of light in the universe. They can travel enormous distances across space and are emitted by huge supernova explosions and microscopic atomic decay. Unlike visible light, the wavelength of gamma rays is so small that gamma rays can pass right through the space between atoms in most materials. Instead of lenses and mirrors, scientists in nuclear reactor laboratories or astronomers studying gamma rays from neutron stars and pulsars use special gamma ray detectors that rely on the fact that gamma rays strongly interact with electrons in atoms.

Image Source: NASA
The key process in most gamma ray detectors is the transfer of some of the energy of the incoming gamma ray to the electron of an atom in the detector (ionization). The gamma ray and the electron scatter, colliding with many other atoms and freeing more electrons. The resulting voltage of the freed electrons is proportional to the energy of the incoming gamma ray.

For the next two weeks, one such gamma ray detector is getting a special helicopter ride across 70 square miles over DC, flying as low as 150 feet above ground.

"If sometime in the future you have a reason to be looking for something radiological, it's very necessary to have the original background so you don't chase a high radiation area that's part of the background," said Joseph Krol of the National Nuclear Safety Administration to CNN. The National Nuclear Safety Administration is leading the mapping efforts.

Many common materials give off low levels of radiation. The soil, the concrete pavement, and the iconic granite in DC's architecture all contain a small percentage of elements like uranium, thorium, and potassium. The small quantities of radioactive isotopes emit low levels of radiation that can be measured.

In the event of a radiation leak, a "dirty bomb," or nuclear weapon, the map will provide a baseline that allows scientists and administrators to distinguish existing radiation levels (like a hotspot due to a granite statue -- a common appearance in DC!) from unknown or insidious sources of radiation.

With a detailed current radiation map, any new radiation source "would stick out like a sore thumb," Randy Larsen with the Institute for Homeland Security told CNN, "Without (the map), it would be like looking for a needle in a haystack."

Since the 1960's, the Department of Energy has made radiation maps for environmental regulation and remediation and background radiation records; the National Nuclear Safety Administration has scanned DC before, along with New York City, Baltimore, and San Francisco.


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