If there’s something strange in your neighbourhood, then Hollywood tradition suggests that you’re gonna call the Ghostbusters. But if that “something strange” looks less like an ectoplasmic manifestation, and more like an unidentified radiation source, you might want to pick up the phone to Mansie Iyer instead.
As an expert in nuclear forensics, Iyer is part of a multidisciplinary group of scientists who swing into action whenever a radioactive object turns up in an unexpected location. During these so-called “interdictions”, nuclear forensic scientists work to identify what the object is, where it came from, who it belongs to and whether there might be more of it. This, Iyer acknowledges, is a tough job. “It’s a little rough and tumble out there,” she told attendees in her keynote address at the NuFor conference in Bristol, UK, last week. “We’ll know we’ve made it when we have a TV show called CSI: Nuclear Forensics. We’re not there yet.”
The challenging nature of the field was evident throughout the conference, which took place on 10-11 July and was co-organized by the Atomic Weapons Establishment (AWE), the Institute of Physics (which publishes Physics World) and the University of Bristol’s South West Nuclear Hub. 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. In CSI terms, these markers play the role of fingerprints or DNA, providing clues to the material’s origins and helping law enforcement officials find out who’s been spreading it around.
We want to be able to go back and say, ‘Where did this come from?’
Jacquelyn Dorhout, Los Alamos National Laboratory
Behind that relatively straightforward premise, however, the science can be rather complicated. The photo above comes from a talk by Jacquelyn Dorhout, a post-doctoral researcher at Los Alamos National Laboratory in the US. In the photo are 22 vials of a powdery substance, ranging in colour from pale yellow to bright orange. Despite their visual differences, every one of those vials contains the same chemical: ammonium diuranate (ADU), popularly known as yellowcake. ADU is present at many stages of the nuclear fuel cycle, and Dorhout and her colleagues found that its colour and texture variations are due to differing amounts of ammonium nitrate. In principle, Dorhout says, these differences could reveal forensically useful information about a sample’s history. “We want to be able to go back and say, ‘Where did this come from?’,” she explains.
Unfortunately, not every potential marker turns out to be useful. In another talk, AWE scientist James Dunne described a series of experiments on uranium ore from mines in Portugal and in Cornwall, UK. The “isotopic signature” of ore samples – including the ratio of uranium-235 to the more common uranium-238 – can be an important tool in nuclear forensics. In 2009, for example, 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.
In some cases, merely identifying a radioactive substance is difficult, never mind working out where it came from. 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. (Specifically, it tells you the level of enrichment used and the amount of time the fuel spent in the reactor.) But as Jeremy Inglis pointed out in his talk, 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. Inglis and his colleagues at Los Alamos 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.
The importance of such advances was brought home in a talk by John Simm, an officer in London’s Metropolitan Police who specializes in counter-terrorism operations. After a presentation that centred around the Met’s current tools for handling a “nuclear security event”, I asked Simm what tools he’d like to have in the future. “A list of everything something could be and how long it would take to identify it,” he replied, gesturing at the Periodic Table of the Elements hanging on the wall behind him. His second wish, he added half-jokingly, was for an internationally recognized series of terms for how scientists report probabilities to law enforcement, running from “possibly” all the way up to “definitely”.
During the conference’s poster session, I spoke briefly with Erin Holland, a Bristol PhD student who is developing forensic markers for thorium. Her background is in earth sciences, and she pointed out that because nuclear forensics draws on research from chemistry, physics and geology, conferences in the field are great for “informed non-experts” because they are generally free of discipline-specific jargon. As an informed non-expert myself, I have to agree, and I’m already looking forward to next year’s follow-up gathering. And who knows? Maybe by then, the organizers will be able to screen episodes of CSI: Nuclear Forensics in the coffee breaks.
