astrophysics ,

Semiconductors sniff out nuclear weapons

NORTHWESTERN (US) — New materials that can detect hard radiation could lead to a handheld device for spotting nuclear weapons and materials, like those that could be used in a “nuclear bomb in a suitcase.”

“The terrorist attacks of 9/11 heightened interest in this area of security, but the problem remains a real challenge,” says Mercouri Kanatzidis, professor of chemistry at Northwestern University, who also holds a joint appointment at Argonne National Laboratory.

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“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 materials perform as well as materials that have emerged from five decades of research and development, Kanatzidis says.

To design an effective detector, researchers turned to the heavy element part of the periodic table and 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. Because every element has a particular spectrum, the signal identifies the detected material.

The method, called dimensional reduction, is reported in the journal 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 well.

The problem the researchers faced was that the heavy elements have a lot of mobile electrons, meaning 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 colleagues designed the semiconductor materials to be crystalline in structure, which immobilized their electrons. The materials that 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.”

Bruce W. Wessels, professor of materials science and engineering and Arthur J. Freeman, professor of physics and astronomy, both at Northwestern, contributed to the work, that was supported the Department of Homeland Security and the Defense Threat Reduction Agency.

More news from Northwestern University: www.northwestern.edu/newscenter

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