New materials could help find hidden nuclear weapons

Posted by Kate Taylor

Northwestern University scientists say they've taken a big step towards a handheld device for detecting nuclear weapons and materials such as a 'bomb in a suitcase'.

The technology is based on a new material that can be used to detect  hard radiation - previously a very difficult thing to do.

"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."

Kanatzidis and his team developed a method called dimensional reduction 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.

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 effectively.

Unfortunately, the heavy elements have a lot of mobile electrons, meaning that when the gamma rays hit the material and excite electrons the change is impossible to detect.

"It's like having a bucket of water and adding one drop - the change is negligible," says Kanatzidis. "We needed a heavy element material without a lot of electrons. This doesn't exist naturally, so we had to design a new material."

The team's semiconductor materials were designed to be crystalline in structure, immobilizing their electrons. They comprise cesium-mercury-sulfide and cesium-mercury-selenide, both of which operate at room temperature, and the process is scaleable.

"Our materials are very promising and competitive," says Kanatzidis. "With further development, they should outperform existing hard radiation detector materials. They also might be useful in biomedicine, such as diagnostic imaging."