New Technology Could Help Thwart Nuclear Terrorism
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.
“U.S. 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 technology.”
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, MD.
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 pulses.
“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 humidity.
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 creating light.
“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 said.
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 radiation readings.
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.”
This material is based upon work supported by the U.S. Department of Homeland
Security under Grant Award Number 2008-DN-077-ARI001-02. The views and
conclusions contained in this document are those of the authors and should not
be interpreted as necessarily representing the official policies, either
expressed or implied, of the U.S. Department of Homeland Security.
Research News & Publications Office
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Writer: Rick Robinson
Georgia Institute of Technology, Research Communications
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