BETWEEN 1992 and 2007, according to Ian Hutcheon of the Lawrence Livermore National Laboratory, in California, 17kg of highly enriched uranium was seized from smugglers around the world, along with 400 grams of plutonium. In neither case is that enough for a proper atom bomb, but it is still worrying. Presumably, more is out there. Even if it is not, the material that has been found could have been used to make a “radiological” weapon, by blowing it up and scattering it around a city using conventional explosives. Dr Hutcheon is one of those charged with analysing this captured material, to discover how dangerous it really is and where it came from—and thus whether it has been stolen from legitimate nuclear projects or made on the sly. At the AAAS meeting in San Diego, he showed off some of the tricks of his trade.

His main tool is a device called a secondary-ion mass spectrometer. This measures the flight path of ions (electrically charged atoms) through a magnetic field. The lighter an ion is, the more the field bends its trajectory. The spectrometer can thus distinguish between, say, 235U (the fissile sort, from which bombs are made) and 238U (which has three extra neutrons in its nucleus and is much less fissile). Natural uranium has only seven atoms per thousand of the former. Weapons-grade uranium is 95% 235U. The “depleted” uranium used in armour-penetrating shells, by contrast, is almost pure 238U.

Uranium that has been in a reactor, though, has other isotopes in it, 233U and 236U, for example. The quantities of these, plus isotopes of elements such as plutonium that are also created in reactors, vary from one reactor to another. The isotopic signature is changed, too, by the centrifuges used to separate 235U from 238U during the process of enrichment, and radioactive decay after processing creates yet further elements that can be detected this way. These give some idea of a sample's age.

The result is a profile that is often characteristic of a particular type of reactor or centrifuge, and sometimes of an individual machine—and can also indicate how long ago the processing took place. That enables the good guys to improve security in the case where something has been pinched, the bad guys to be admonished if they have been up to something they should not have been doing, and everyone else to sleep more easily in their beds.