Technology developed by North Carolina State University researchers allows for the characterization of nuclear materials after they have been removed from an area, leaving no chemical trace. The technique may have applications in nuclear nonproliferation and detection of covert nuclear materials.

How can one measure something that is not there?

The technique exploits the changes imparted by radioactive material on the arrangement of valence electrons – or outer electrons – in insulator materials, such as brick, porcelain and glass. Radiation displaces electrons at defect sites in the crystalline structure of these materials.

By taking samples of multiple materials in a room, applying conventional radiation dosimetry techniques and (Source: John Jones/CC BY-ND 2.0)  (Source: John Jones/CC BY-ND 2.0) evaluating how the electrons at those defect sites are organized, researchers can determine the presence and strength of any nuclear materials that were in that room.

Measuring the radiation dose at various depths in a core sample of the insulating material allows determination of the type of radiation source that was present. This is possible because different radioactive materials have characteristic distributions of gamma rays, X-rays, etc., and each type of energy penetrates materials with different strengths.

“This is not extremely precise, but it does allow us to answer important questions. For example, distinguishing between different kinds of nuclear material such as naturally occurring, medical, industrial and ‘special’ nuclear materials – the latter being used for nuclear weapons,” said Robert Hayes, associate professor of nuclear engineering at North Carolina State University.