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THREE years ago, New Scientist warned that the US was developing a new kind of quasi-nuclear weapon, the hafnium bomb (16 August 2003, p 4). It relied not on fission or fusion, but on triggering the release of excess energy stored in the distorted nucleus of the isotope hafnium-178, specifically its isomer hafnium-178m2.

This nuclear isomer is radioactive, emitting its excess energy as a gamma ray, with a half-life of 31 years. The Pentagon was supposedly developing a way to trigger that decay in vast numbers of excited hafnium nuclei at once, producing an explosive burst of gamma rays thousands of times more powerful than conventional explosives, but much less so than a standard nuclear weapon. Because the hafnium bomb would not involve fission or fusion, it might have been able to slip through a loophole in US law banning development of mini-nukes.

In Imaginary Weapons, Sharon Weinberger tells the story behind that story, and explains why the hafnium bomb dropped out of sight despite the Pentagon鈥檚 considerable enthusiasm. It is a solid reporting job, and a disconcerting tale of how fringe science found its way onto the military research agenda.

The idea came from Carl Collins (right), a professor at the University of Texas at Dallas. In the 1980s, he set out to use nuclear isomers to make a gamma-ray laser. That didn鈥檛 work, but in 1998 he announced a breakthrough. He claimed he had triggered gamma-ray emission by zapping the hafnium with X-rays from an old dental X-ray machine. It was no use for a gamma-ray laser, but the rapid release of energy interested the Defense Advanced Research Projects Agency (DARPA), which saw the potential for a 鈥渘uclear hand grenade鈥�, a grapefruit-sized bomb with a two-kiloton yield, the explosive equivalent of two kilotons of TNT.

鈥淭he Pentagon poured millions into a project lacking in credibility鈥�

Nuclear physicists were highly sceptical. Nobody but Collins had reported the crucial triggering effect, and it couldn鈥檛 be reproduced with X-ray sources far more powerful than the dental X-ray machine. DARPA tried to brush off the doubters, claiming that shadowy consultants had verified the triggering results.

The agency charged ahead, starting a study on how to produce enough isomer for bombs, and informed the defence secretary, Donald Rumsfeld, who asked for a prototype in 18 months. The house of cards finally collapsed when a new round of outside tests failed to replicate the X-ray triggering. In November 2004, Congress chopped the hafnium bomb from the DARPA budget and it vanished from public view.

Weinberger calls it a case history of pathological science. She describes the credulity that is the flip side of DARPA鈥檚 willingness to take on high-risk challenges. She chronicles how Collins changed when she put him under pressure, turning from a charming Texas gentleman into a man convinced she was part of a conspiracy out to destroy the hafnium bomb. She makes no secret at her frustration that the Pentagon happily poured millions of dollars into a project so utterly lacking in scientific credibility.

Now the editor-in-chief of Defense Technology International, Weinberger clearly knows her way around the military research establishment. Her book is a good read, sketching sharp images of the military fantasyland of hardware on display when DARPA runs a technology meeting at Disneyland, as well as probing the allure of the hafnium bomb. Yet at times she is sloppy, often failing to clearly distinguish between normal hafnium and the energetic isomer, and failing to check the spelling of John Nuckolls鈥檚 name, which becomes 鈥淣uckles鈥�, a grating error.

I also wonder if she might be missing a deeper layer of deceit and game-playing within the military establishment. Strategists have been bluffing about their capabilities since they invented war. Might the hafnium bomb have been at least in part just another bluff? Could it be that the Pentagon invested a fraction of the cost of a single cutting-edge stealth fighter in creating a smokescreen that would lead potential adversaries to think it possessed a new generation of weapons that no one else knew how to build? In the perverse world of military strategy, it might even have been a reasonable investment.