The previous post examined the dirtiest of the so-called dirty bombs, which, if ever built, would release high-energy radioactivity by converting cobalt-59 into cobalt-60. This post examines an element potentially even more frightening—partly because of its nukelike fury, partly because it might be perfectly legal to stockpile this weapon.

Cobalt-59 and cobalt-60 are called isotopes of cobalt—they're the same element, but with different numbers of neutrons. Elements also can form isomers, a subtle variation. If a regular atom is called A, then an isomer of it (call it A*) is the same element and has the same number of neutrons. But the A* isomer has an overabundance of energy in its nucleus—it's in an excited state. That energy must be released somehow (atoms cannot stay in an excited state forever). And when the nucleus finally relaxes, it releases the extra energy as a gamma ray, the same type of malignant radiation that dirty bombs often produce.

Many different elements can form isomers, but only a few elements on the periodic table, like hafnium, can form isomers that last more than fractions of a second—and might therefore be turned into weapons. Some scientists claim—although these claims are contentious—that they can form deadly isomers with simple X-rays and that hafnium can multiply the power of these X-rays to an astounding degree, converting them into gamma rays up to 250 times more potent than the X-rays. (These claims were published in peer-reviewed journals, but other scientists think the experiments were flawed.)

The unresolved status of the field didn't stop the U.S. government from pursuing a hafnium bomb, however. The Defense Advanced Research Projects Agency, known as DARPA, reportedly spent $30 million in 2004 and 2005 on the feasibility of hafnium bombs. (It also considered unmanned aerial drones powered by hafnium.) The experiments were pretty hush-hush, and many scientists remain skeptical they produced anything worthwhile. (Indeed, other government scientists published a report to debunk the science behind hafnium bombs. Some went so far as to lump hafnium triggering with cold fusion.)

What really has some observers worried is that hafnium weapons, if possible, might not technically violate current nuclear nonproliferation treaties. Those treaties cover the movement and use of elements like uranium or plutonium, not elements like hafnium, and they focus on fission and fusion, not novel processes like exciting isomers. Any hafnium bombs would probably take decades to create, but at least one other country, Russia, has an active isomer-weapon research program. And unless DARPA and the scientists supporting hafnium isomer research back off their claims, Element 72 will continue to provoke controversy. As one physicist put it, hafnium studies "have aroused a public debate whose acrimony [goes] far beyond the interesting but normally staid physics of nuclear isomers."

The specter of cobalt bombs in the 1960s—the first "doomsday devices"—scared many people, and works like the movie Dr. Strangelove and the novel Canticle for Leibowitz, made cobalt a villain. But if anyone ever remakes Strangelove or adapts Canticle for the big screen, Hollywood may very well substitute hafnium for cobalt as the periodic table bad guy. From the rain of the hafnium, O Lord, deliver us ...

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