Now, nuclear material can be traced even after they are moved

Scientists have developed a new technique to detect traces of nuclear material even after it has been removed from a location, an advance that would make it difficult to handle radioactive elements in secret anymore.
The technique takes advantage of the fact that radioactive material changes the arrangement of valence electrons - or outer electrons - in insulator materials, such as brick, porcelain, glass - even hard candy.
The radiation displaces electrons at defect sites in the crystalline structure of these materials, researchers said.
"Basically, we can see nuclear material that is no longer there," said Robert Hayes, associate professor at North Carolina State University in the US.

By taking samples of multiple materials in a room, applying conventional radiation dosimetry techniques, and evaluating how the electrons at those defect sites are organised, researchers were able to determine the presence and strength of any nuclear materials that were in that room.
If the samples were taken at regular intervals in a grid pattern, the relative radiation dose profile can be used to triangulate where in the room the source was located, in three dimensions, researchers said.

"It can also provide a very rough idea of the physical size of the source, but that depends on various factors, such as how close the source was to the materials being sampled." Hayes said.

By taking a core sample of the insulating material, and measuring the radiation dose at various depths in the material, researches can also ascertain what type of radiation source 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 strength.
It is not extremely precise, but allows us to answer important questions, like distinguishing between different kinds of nuclear material such as naturally occurring, medical, industrial, and 'special' nuclear materials - the latter being used for nuclear weapons, researchers said.
"This is a valuable tool for emergency responders, nuclear nonproliferation authorities and forensics, because it allows us to get a rough snapshot of the size of a radiation source, where it was located, how radioactive it is, and what type of radioactive material it is," Hayes said.

"This is a big deal for nuclear nonproliferation efforts, because it means you can't handle nuclear material in secret anymore," he added.
The study was published in the journal Health Physics.