have developed new materials that can detect hard radiation, a very difficult
thing to do. The method could lead to a handheld device for detecting nuclear
weapons and materials, such as a “nuclear bomb in a suitcase” scenario.
“The terrorist attacks of 9/11 heightened interest in
this area of security, but the problem remains a real challenge,” says
Mercouri G. Kanatzidis, who led the research. “We have designed promising semiconductor
materials that, once optimized, could be a fast, effective, and inexpensive
method for detecting dangerous materials such as plutonium and uranium.”
The Northwestern materials perform as well as materials that
have emerged from five decades of research and development, Kanatzidis says.
To design an effective detector, Kanatzidis and his team
turned to the heavy element part of the periodic table. The researchers
developed a design concept to make new semiconductor materials of heavy
elements in which most of the compound’s electrons are bound up and not mobile.
When gamma rays enter the compound, they excite the electrons, making them
mobile and thus detectable. And, because every element has a particular
spectrum, the signal identifies the detected material.
The method, called dimensional reduction, will be published
in Advanced Materials.
In most materials, gamma rays emitted by nuclear materials
would just pass right through, making them undetectable. But dense and heavy
materials, such as mercury, thallium, selenium, and cesium, absorb the gamma
rays very well.
The problem the researchers faced was that the heavy
elements have a lot of mobile electrons. This means when the gamma rays hit the
material and excite electrons the change is not detectable.
“It’s like having a bucket of water and adding one drop—the
change is negligible,” Kanatzidis explains. “We needed a heavy
element material without a lot of electrons. This doesn’t exist naturally so we
had to design a new material.”
Kanatzidis and his colleagues designed their semiconductor
materials to be crystalline in structure, which immobilized their electrons.
The materials they developed and successfully demonstrated
as effective gamma ray detectors are cesium-mercury-sulfide and
cesium-mercury-selenide. Both semiconductors operate at room temperature, and
the process is scaleable.
“Our materials are very promising and
competitive,” Kanatzidis says. “With further development, they should
outperform existing hard radiation detector materials. They also might be
useful in biomedicine, such as diagnostic imaging.”