Magnetic resonance and luminescence techniques can be used to determine what radioactive, or nuclear, materials have been present at a site even after those materials have been removed. In an age of growing concerns about "rogue states" and terrorists with so-called "dirty bombs" there is a pressing need for such detection technology that could allow the authorities and international peacekeeping forces to identify problems with respect to nuclear proliferation and security.

"Basically, we can see nuclear material that is no longer there, where there are no chemical residues of any kind" explains Robert Hayes of North Carolina State University. "For example, we could identify and characterize a dirty bomb based on samples taken from a room the bomb was in a year ago." The technique can reveal exactly where radioactive material has been situated, how much was present and what kind of radiation source it was.

The technique exploits the phenomenon whereby a radioactive material will damage materials with which it has been exposed to, rearranging valence electrons in insulators, such as brick, porcelain, glass - even hard candy, by displacing the electrons at defect sites in the crystalline structure of such materials. The researchers or future inspectors only then need to take samples of various materials at a site and evaluate how the electrons at defect sites have been disturbed by any radioactive materials that were at that location. The physics are similar to that used for radiation worker dosimetry using thermoluminescent dosimeters but designed to enable nuclear forensics applications at any inhabited location at any time, past, present or future.

"If the samples were taken at regular intervals in a grid pattern, the relative radiation dose profile can be used to triangulate where in a room the source was located, in three dimensions," Hayes suggests. "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. In other words, it turns basically any wall, floor or array of insulators (such as coffee cups or electronic components) into a gamma camera." [Hayes et al., Health Phys (2017); DOI: 10.1097/HP.0000000000000680].

Additionally, by taking a core sample of any 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 given the known penetration power of X-rays, gamma rays, beta particles and alpha particles.

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," Hayes adds. The team is now exploring the detection limits of and spatial and energy resolution of which their approach might be capable with further development.

"The ultimate goal of this work was to support nuclear non-proliferation efforts for the US government," Hayes told Materials Today. "The technique has even been postulated to be capable of measuring historical uranium enrichment levels. The research could also help with radiological emergency response by providing dosimetry to members of the public in the event that their exposure was unknown and triage was needed.

"This is a big deal for nuclear non-proliferation efforts, because it means you can't handle nuclear material in secret anymore," Hayes adds. "It means the world is now densely blanketed by low-resolution integrating gamma-ray spectrometers, so we can always go back and measure what was present. There's no hiding."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.