Georgia Tech Research Institute researchers Brent Wagner (l) and Bernd Kahn are using novel materials and nanotechnology techniques to develop improved radiation detection. Photo: Gary Meek
Among terrorism scenarios that raise the most concern are attacks involving
nuclear devices or materials. For that reason, technology that can effectively
detect smuggled radioactive materials is considered vital to U.S. security.
To support the nation’s nuclear-surveillance capabilities, researchers at
the Georgia Tech Research Institute (GTRI) are developing ways to enhance the
radiation-detection devices used at ports, border crossings, airports and
elsewhere. The aim is to create technologies that will increase the
effectiveness and reliability of detectors in the field, while also reducing
cost. The work is co-sponsored by the Domestic Nuclear Defense Office of the
Department of Homeland Security and by the National Science Foundation.
security personnel have to be on guard against two types of nuclear attack—true
nuclear bombs, and devices that seek to harm people by dispersing radioactive
material,” said Bernd Kahn, a researcher who is principal investigator on the
project. “Both of these threats can be successfully detected by the right
The GTRI team, led by co-principal investigator Brent Wagner, is utilizing
novel materials and nanotechnology techniques to produce improved radiation
detection. The researchers have developed the Nano-photonic Composite
Scintillation Detector, a prototype that combines rare-earth elements and other
materials at the nanoscale for improved sensitivity, accuracy and robustness.
Details of the research were presented April 23, 2012 at the SPIE Defense,
Security, and Sensing Conference held in Baltimore,
Scintillation detectors and solid-state detectors are two common types of
radiation detectors, Wagner explained. A scintillation detector commonly
employs a single crystal of sodium iodide or a similar material, while a
solid-state detector is based on semiconducting materials such as germanium.
Both technologies are able to detect gamma rays and subatomic particles
emitted by nuclear material. When gamma rays or particles strike a
scintillation detector, they create light flashes that are converted to
electrical pulses to help identify the radiation at hand. In a solid-state
detector, incoming gamma rays or particles register directly as electrical
“Each reaction to a gamma ray takes a very short time—a fraction of a
microsecond,” Wagner said. “By looking at the number and the intensity of the
pulses, along with other factors, we can make informed judgments about the type
of radioactive material we’re dealing with.”
But both approaches have drawbacks. A scintillation detector requires a
large crystal grown from sodium iodide or other materials. Such crystals are
typically fragile, cumbersome, difficult to produce and extremely vulnerable to
A germanium-based solid-state detector offers better identification of
different kinds of nuclear materials. But high-purity single-crystal germanium
is difficult to make in a large volume; the result is less-sensitive devices
with reduced ability to detect radiation at a distance. Moreover, germanium
must be kept extremely cold—200 degrees below zero Celsius—to function
properly, which poses problems for use in the field.
The nanoscale advantage
To address these problems, the GTRI team has been investigating a wide variety
of alternative materials and methodologies. After selecting the scintillation
approach over solid-state, the researchers developed a composite material—composed
of nanoparticles of rare-earth elements, halides, and oxides—capable of
“A nanopowder can be much easier to make, because you don’t have to worry
about producing a single large crystal that has zero imperfections,” Wagner
A scintillator crystal must be transparent to light, he explained, a quality
that’s key to its ability to detect radiation. A perfect crystal uniformly
converts incoming energy from gamma rays to flashes of light. A
photo-multiplier then amplifies these flashes of light so they can be
accurately measured to provide information about radioactivity.
However, when a transparent material—such as crystal or glass—is ground into
smaller pieces, its transparency disappears. As a result, a mixture of
particles in a transparent glass would scatter the luminescence created by
incoming gamma rays. That scattered light can’t reach the photo-multiplier in a
uniform manner, and the resulting readings are badly skewed.
To overcome this issue, the GTRI team reduced the particles to the
nanoscale. When a nanopowder reaches particle sizes of 20 nanometers or less,
scattering effects fade because the particles are now significantly smaller
than the wavelength of incoming gamma rays.
“Think of it as a big ocean wave coming in,” Wagner said. “That wave would
definitely interact with a large boat, but something the size of a beach ball
doesn’t affect it.”
Rare Earths and Silica
At first the team worked on dispersing radiation-sensitive crystalline nanoparticles
in a plastic matrix. But they encountered problems with distributing the
nanopowder uniformly enough in the matrix to achieve sufficiently accurate
More recently, the researchers have investigated a parallel path using glass
rather than plastic as a matrix material, combining gadolinium and cerium
bromide with silica and alumina.
Kahn explained that gadolinium or a similar material is essential to
scintillation-type particle detection because of its role as an absorber. But
in this case, when an incoming gamma ray is absorbed in gadolinium, the energy
is not efficiently emitted in the form of luminescence.
Instead, the light emission role here falls to a second component—cerium.
The gadolinium absorbs energy from an incoming gamma ray and transfers that
energy to the cerium atom, which then acts as an efficient light emitter.
The researchers found that by heating gadolinium, cerium, silica and alumina
and then cooling them from a molten mix to a solid monolith, they could successfully
distribute the gadolinium and cerium in silica-based glasses. As the material
cools, gadolinium and cerium precipitate out of the aluminosilicate solution
and are distributed throughout the glass in a uniform manner. The resulting
composite gives dependable readings when exposed to incoming gamma rays.
“We’re optimistic that we’ve identified a productive methodology for
creating a material that could be effective in the field,” Wagner said. “We’re
continuing to work on issues involving purity, uniformity and scaling, with the
aim of producing a material that can be successfully tested and deployed.”