The central premise of nuclear forensics is that differences in how and where radioactive materials are mined, processed, enriched and stored will leave tell-tale markers in the materials’ chemical and physical properties. Behind that relatively straightforward premise, however, the science can be rather complicated.
For example, as a strong alpha emitter, plutonium-238 is a significant radiation hazard, and the ratio of Pu-238 to its “daughter” U-234 in a sample of nuclear fuel tells you something about the type of reactor that made it. But Pu-238 looks the same in a mass spectrometer as the comparatively benign U-238, while its alpha emission spectrum is identical to that of Pu-239 and Pu-240. Researchers are working on a modified mass-spectrometry technique that will, they hope, make it easier for nuclear forensics experts to pick Pu-238 out of the “lineup” of potential culprits.
In 2009 isotopic signatures helped Australian law-enforcement officials trace a glass jar labelled “gamma source” back to a disused uranium mine in northwest Queensland, after the jar was seized in a drugs raid. In the Portuguese and Cornish ores, however, Dunne found that the U235/U238 ratios differed not only between mines, but also between different veins of ore within the same mine, and even between samples taken from different places within the same vein – making the ratio pretty useless for detective work. Radiogeochemistry is, he noted ruefully, a complex science.