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Monday, June 26, 2006

Reading List: Imaginary Weapons

Weinberger, Sharon. Imaginary Weapons. New York: Nation Books, 2006. ISBN 1-56025-849-7.

A nuclear isomer is an atomic nucleus which, due to having a greater spin, different shape, or differing alignment of the spin orientation and axis of symmetry, has more internal energy than the ground state nucleus with the same number of protons and neutrons. Nuclear isomers are usually produced in nuclear fusion reactions when the the addition of protons and/or neutrons to a nucleus in a high-energy collision leaves it in an excited state. Hundreds of nuclear isomers are known, but the overwhelming majority decay with gamma ray emission in about 10-14 seconds. In a few species, however, this almost instantaneous decay is suppressed for various reasons, and metastable isomers exist with half-lives ranging from 10-9 seconds (one nanosecond), to the isomer Tantalum-180m, which has a half-life of at least 1015 years and may be entirely stable; it is the only nuclear isomer found in nature and accounts for about one atom of 8300 in tantalum metal.

Some metastable isomers with intermediate half-lives have a remarkably large energy compared to the ground state and emit correspondingly energetic gamma ray photons when they decay. The Hafnium-178m2 (the “m2” denotes the second lowest energy isomeric state) nucleus has a half-life of 31 years and decays (through the m1 state) with the emission of 2.45 MeV in gamma rays. Now the fact that there's a lot of energy packed into a radioactive nucleus is nothing new—people were calculating the energy of disintegrating radium and uranium nuclei at the end of the 19th century, but all that energy can't be used for much unless you can figure out some way to release it on demand—as long as it just dribbles out at random, you can use it for some physics experiments and medical applications, but not to make loud bangs or turn turbines. It was only the discovery of the fission chain reaction, where the fission of certain nuclei liberates neutrons which trigger the disintegration of others in an exponential process, which made nuclear energy, for better or for worse, accessible.

So, as long as there is no way to trigger the release of the energy stored in a nuclear isomer, it is nothing more than an odd kind of radioactive element, the subject of a reasonably well-understood and somewhat boring topic in nuclear physics. If, however, there were some way to externally trigger the decay of the isomer to the ground state, then the way would be open to releasing the energy in the isomer at will. It is possible to trigger the decay of the Tantalum-180 isomer by 2.8 MeV photons, but the energy required to trigger the decay is vastly greater than the 0.075 MeV it releases, so the process is simply an extremely complicated and expensive way to waste energy.

Researchers in the small community interested in nuclear isomers were stunned when, in the January 25, 1999 issue of Physical Review Letters, a paper by Carl Collins and his colleagues at the University of Texas at Dallas reported they had triggered the release of 2.45 MeV in gamma rays from a sample of Hafnium-178m2 by irradiating it with a second-hand dental X-ray machine with the sample of the isomer sitting on a styrofoam cup. Their report implied, even with the crude apparatus, an energy gain of sixty times break-even, which was more than a million times the rate predicted by nuclear theory, if triggering were possible at all. The result, if real, could have substantial technological consequences: the isomer could be used as a nuclear battery, which could store energy and release it on demand with a density which dwarfed that of any chemical battery and was only a couple of orders of magnitude less than a fission bomb. And, speaking of bombs, if you could manage to trigger a mass of hafnium all at once or arrange for it to self-trigger in a chain reaction, you could make a variety of nifty weapons out of it, including a nuclear hand grenade with a yield of two kilotons. You could also build a fission-free trigger for a thermonuclear bomb which would evade all of the existing nonproliferation safeguards which are aimed at controlling access to fissile material. These are the kind of things that get the attention of folks in that big five-sided building in Arlington, Virginia.

Hafnium isomer hand grenade
Imaginary Hafnium
Hand Grenade
Diameter: 5 inches
Yield: 2 kilotons

And so it came to pass, in a Pentagon bent on “transformational technologies” and concerned with emerging threats from potential adversaries, that in May of 2003 a Hafnium Isomer Production Panel (HIPP) was assembled to draw up plans for bulk production of the substance, with visions of nuclear hand grenades, clean bunker-busting fusion bombs, and even hafnium-powered bombers floating before the eyes of the out of the box thinkers at DARPA, who envisioned a two-year budget of USD30 million for the project—military science marches into the future. What's wrong with this picture? Well, actually rather a lot of things.

  • No other researcher had been able to reproduce the results from the original experiment. This included a team of senior experimentalists who used the Advanced Photon Source at Argonne National Laboratory and state of the art instrumentation and found no evidence whatsoever for triggering of the hafnium isomer with X-rays—in two separate experiments.
  • As noted above, well-understood nuclear theory predicted the yield from triggering, if it occurred, to be six orders of magnitude less than reported in Collins's paper.
  • An evaluation of the original experiment by the independent JASON group of senior experts in 1999 determined the result to be “a priori implausible” and “inconclusive, at best”.
  • A separate evaluation by the Institute for Defense Analyses concluded the original paper reporting the triggering results “was flawed and should not have passed peer review”.
  • Collins had never run, and refused to run, a null experiment with ordinary hafnium to confirm that the very small effect he reported went away when the isomer was removed.
  • James Carroll, one of the co-authors of the original paper, had obtained nothing but null results in his own subsequent experiments on hafnium triggering.
  • Calculations showed that even if triggering were to be possible at the reported rate, the process would not come close to breaking even: more than six times as much X-ray energy would go in as gamma rays came out.
  • Even if triggering worked, and some way were found to turn it into an energy source or explosive device, the hafnium isomer does not occur in nature and would have to be made by a hideously inefficient process in a nuclear reactor or particle accelerator, at a cost estimated at around a billion dollars per gram. The explosive in the nuclear hand grenade would cost tens of billions of dollars, compared to which highly enriched uranium and plutonium are cheap as dirt.
  • If the material could be produced and triggering made to work, the resulting device would pose an extreme radiation hazard. Radiation is inverse to half-life, and the hafnium isomer, with a 31 year half-life, is vastly more radioactive than U-235 (700 million years) or Pu-239 (24,000 years). Further, hafnium isomer decays emit gamma rays, which are the most penetrating form of ionising nuclear radiation and the most difficult against which to shield. The shielding required to protect humans in the vicinity of a tangible quantity of hafnium isomer would more than negate its small mass and compact size.
  • A hafnium explosive device would disperse large quantities of the unreacted isomer (since a relatively small percentage of the total explosive can react before the device is disassembled in the explosion). As it turns out, the half-life of the isomer is just about the same as that of Cesium-137, which is often named as the prime candidate for a “dirty” radiological bomb. One physicist on the HIPP (p. 176) described a hafnium weapon as “the mother of all dirty bombs”.
  • And consider that hand grenade, which would weigh about five pounds. How far can you throw a five pound rock? What do you think about being that far away from a detonation with the energy of two thousand tons of TNT, all released in prompt gamma rays?

But bad science, absurd economics, a nonexistent phenomenon, damning evaluations by panels of authorities, lack of applications, and ridiculous radiation risk in the extremely improbable event of success pose no insurmountable barriers to a government project once it gets up to speed, especially one in which the relationships between those providing the funding and its recipients are complicated and unseemingly cozy. It took an exposé in the Washington Post Magazine by the author and subsequent examination in Congress to finally drive a stake through this madness—maybe. As of the end of 2005, although DARPA was out of the hafnium business (at least publicly), there were rumours of continued funding thanks to a Congressional earmark in the Department of Energy budget.

This book is a well-researched and fascinating look inside the defence underworld where fringe science feeds on federal funds, and starkly demonstrates how weird and wasteful things can get when Pentagon bureaucrats disregard their own science advisors and substitute instinct and wishful thinking for the tedious, but ultimately reliable, scientific method. Many aspects of the story are also quite funny, although U.S. taxpayers who footed the bill for this madness may be less amused. The author has set up a Web site for the book, and Carl Collins, who conducted the original experiment with the dental X-ray and styrofoam cup which incited the mania has responded with his own, almost identical in appearance, riposte. If you're interested in more technical detail on the controversy than appears in Weinberg's book, the Physics Today article from May 2004 is an excellent place to start. The book contains a number of typographical and factual errors, none of which are significant to the story, but when the first line of the Author's Note uses “sited” when “cited” is intended, and in the next paragraph “wondered” instead of “wandered”, you have to—wonder.

It is sobering to realise that this folly took place entirely in the public view: in the open scientific literature, university labs, unclassified defence funding subject to Congressional oversight, and ultimately in the press, and yet over a period of years millions in taxpayer funds were squandered on nonsense. Just imagine what is going on in highly-classified “black” programs.

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