Case Study

Using Nanotechnology to Detect Gamma Radiation

Published: November 2, 2009

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One of the many challenges in homeland security is detecting materials associated with potential nuclear threats while effectively filtering out the many legitimate radioactive objects commonly found in commerce and the environment.

One type of gamma radiation detector contains an inorganic single-crystal scintillator, such as sodium iodide, which absorbs the high energy radiation and coverts it to light pulses. The light-pulse intensities are then measured and compared to the energies of known nuclides. While crystals typically have a high stopping power, meaning not much radioactive energy is lost during interaction with the crystal, they are plagued by low energy resolution and long decay times, and typically need to be protected from the environment.

Researchers at the Georgia Tech Research Institute (GTRI) are investigating replacing the crystals with composite materials made of nanoparticles or quantum dots embedded in a polymer matrix.

"Scintillators made of quantum dots or nanoparticles may have many advantages over a single crystal, including better resolution, meaning they can better distinguish differences in energy intensity, and have better stopping power," said Bernd Kahn, associate director of GTRI's Environmental Radiation Center (ERC).

Fabricating the composites is also less expensive than growing a single crystal, and the size and shape of the fluorescent material is not constrained by crystal growth. Also, the particles are automatically sealed off from the environment because they are suspended in plastic.

Because nanoparticles and quantum dots are so small, the properties they exhibit differ substantially from the properties of the same material in bulk. Quantum dots become more efficient as they become smaller and their decay times become faster, meaning more gamma rays can be detected in a given amount of time.

"Quantum dots are promising because they can be tuned to certain wavelengths by making them a specific size, and suspending several different types of quantum dots in the plastic allows for a broader wavelength range to be utilized," explained principal research scientist Brent Wagner. "But to get the best performance and higher stopping power, we need to use quantum dots made of heavier elements."

In the proper format, nanoparticles of standard scintillator materials, like sodium iodide, could also have a major advantage over the crystalline form - a reduction in light scattering.

The researchers are currently developing and testing polymer composites containing several different types of quantum dots or nanoparticles, including barium and lanthanum halides, lead sulfide, and cerium-doped yttrium aluminum garnet (YAG).

ERC director and principal research scientist Robert Rosson, research engineer Jason Nadler, research scientist Zhitao Kang, and senior research scientists David Roberts and Hisham Menkara are also working on this project, which is supported by the U.S. Department of Homeland Security.