The Strange Tale of the Hafnium Bomb: A Personal Narrative

By Peter D. Zimmerman

A slide to promote the Hafnium bomb
A slide used to promote the Hafnium bomb

There is that odd sinking feeling when you realize you’ve let an investigative journalist into your life. You can say what you want to her, but whatever anybody else says about you is out of your control. Sharon Weinberger entered my life almost by accident. While I was working for the Senate Foreign Relations Committee I attended a talk by Steve Younger, then the director of the Defense Threat Reduction Agency, on the future of US nuclear weapons. And I asked Steve what he thought about the DARPA “hafnium bomb” project.

He brushed past the question, but Sharon came up to me afterwards to ask “what’s a hafnium bomb?” As she tells the story in her book Imaginary Weapons, I was fairly mischievous and just said “call me.” The next day we had coffee in one of the Senate’s many coffee shops, and I began the story.

More than eight years on it’s a little hard to remember just exactly how the peculiar properties of the hafnium-178m2 isomeric state, and plans to exploit it, came to my attention. I seem to remember a discussion in the fall of 1998 in my office at the Institute for Defense Analyses with people from Sandia National Laboratories. But perhaps not. In any event, the 25 January, 1999 issue of Physical Review Letters was what really triggered things. (Pun intended.) In that article, University of Texas at Dallas (UTD) physicist Carl Collins reported that he had stimulated decays of the isomer via bombardment with X-rays from a second-hand dental X-ray machine. 

Bohdan Balko (also at IDA) and I considered whether we should write a “comment” in rebuttal, pointing out the obvious failings in the Collins paper’s theory and data analysis. We decided against the idea, and I mostly forgot about hafnium. In any event, I had not been able to discern any actual decay rate enhancement in Collins’s report. There were almost as many lines “suppressed” by the X-ray beam as “enhanced.” The difference spectrum looked mostly like noise.

I was in my last few weeks of waiting for White House security clearance to start a new job as chief scientific advisor of the Arms Control and Disarmament Agency (ACDA) when I received an offer to be briefed by an official of the Defense Intelligence Agency (DIA) on hafnium research. Because I had not yet started my new job, protocol declared that the briefing be on DIA’s turf, and not ACDA’s, and so I crossed the Potomac.

What I heard was extraordinary: DARPA had gone to work on isomers in a big way, despite the fact that there was a six or seven order of magnitude gap between real nuclear theory and the claimed experimental results. The Russians had shown “interest” in isomers, and, most serious of all, they were likely to be far ahead of the US. There was, you see, a mysterious “enigma site” in Russia, the purpose of which we did not know, but intelligence indicated it was both big and expensive. And if the Russians were spending hidden research funds, and were interested in isomers, then the enigma site was probably their isomer R&D facility.

(In retrospect, that part of the briefing was of a piece with some more recently declassified British military intelligence papers. Commenting on the sad state of UK research on carrier pigeons after World War II, the War Office Intelligence Section warned “Pigeon research will not stand still. If we do not experiment, other powers will. ”

Among the more interesting weaponizations of pigeons was a plan to have kamikaze anti-anti-aircraft pigeons carry explosives. These birds would crash into searchlights, destroying them to protect bombers overhead.)

The Pentagon had seemingly estimated that a five pound hand grenade powered by a hafnium explosive could deliver a two kiloton kick. Neither the briefer, Fred Ambrose, nor his colleague, Dr. Eliot Lehman, could explain just how the laws of physics were to be violated quite so grossly.

Other things they didn’t explain included how a soldier was supposed to hold a hafnium grenade, given that it would be fiercely radioactive, at least thousands of curies, or how anybody was supposed to be able to throw a five pound ball far enough to survive a two kiloton blast. Later others were to scale that back to two tons, but I still don’t know how the grenadier was going to come out alive, even if his throwing arm weren’t roasted.

Only a month after I started at ACDA it was folded into the State Department. Ambassador Avis T. Bohlen became assistant secretary of state for arms control, and I became her science adviser. I also had the job of running the research budget for the three State Department bureaus which housed most of the people from ACDA.


Stanford Hooper may be the most important two-star admiral you’ve never heard of. He is the father of electronics in the US Navy. He won the Navy Cross in the First World War as well, and in the late 1930s headed the U.S. Navy’s Technical Division.

But if Hooper’s name rings any bells at all with modern physicists, it is because of a brief and largely fruitless meeting with Enrico Fermi in 1939. Fermi had turned to the Navy to fund research in making explosives using the newly discovered process of uranium fission. Hooper is remembered for dismissing Fermi, allegedly with a racist slur.


I had no ambition to become the Stanford Hooper of hafnium.

I wanted to know that if I opposed funding for hafnium, my instinct that it was a quantitative impossibility was correct. To get the best possible physics analysis I turned to the JASON consulting group, a self-perpetuating “club” of some of the top US scientists who work largely for the Department of Defense on very tough, and very important scientific questions. The State Department research budget had neither the size nor the elasticity of the DOD budget, but we were able to provide a token payment, and JASON agreed to do the work.

Washington bureaucracy works in strange ways. I was immediately asked what stake the arms control groups at State had in a fight over whether or not DARPA was to be allowed to waste $40 million on what my instincts said was very bad science. But we did have a reason to get involved: the proponents of isomer weapons suggested that–although the energy release derived from excited states of nuclei–because the mechanism did not involve either fission or fusion, an isomer bomb would not be a nuclear weapon. That would mean it could be tested even under the terms of the Comprehensive Test Ban Treaty, and could even be tested in the atmosphere, despite the 1963 Limited Test Ban Treaty. 
Our lawyers, of course, said that was nonsense. But it appeared that the only way State could enter the fray was to alert the rest of the government that the178m2Hf isomer could not be used in any kind of weapon because the physics prevented it. I was allowed to contract with JASON, and asked some pointed questions which included:

•  What is the proposed physical mechanism by which Collins claims the decay rate is enhanced?  
•  Is this mechanism in accord with the known principles of nuclear and atomic physics?
•  Have Collins and his co-workers actually demonstrated an enhanced decay rate of 178Hf?
•  Is it likely that 178Hf isomeric nuclei can be produced in useful quantities within the next 20 years? By what mechanism?
•  Is it likely that mechanisms to cause the near-simultaneous de-excitation of large numbers of 178Hf isomers will become practical in the next 20 years?

The answer to all of my questions was “no.” Collins’s experiment was unequivocally dismissed. The question of an explosive was pretty thoroughly undermined by pointing out that the enormous background of photons from the normal decay of the isomer was great enough to ensure a preinitiation fizzle and so the inevitable maximum yield would approach zero, but would scatter thousands of curies of isomer. And there was no known mechanism for producing an exponentiating reaction except the vague hope that one of the photons released in the stimulated decay of the isomer would be exactly the right energy to stimulate another decay. One of the pro-isomer scientists suggested that he could mix the hafnium with another (unknown) element which would provide the necessary photons.

Rather naively for somebody who had worked in Washington for 16 years, I thought the JASON report would end the program. I didn’t even bother to collect any of the “I Believe in Isomers” campaign buttons some of the Sandia National Labs people were handing out, because I didn’t believe.  

In 2001, with a new president and secretary of defense, less interested in taking scientific advice if it conflicted with a desired outcome, DARPA stepped in with two large programs, SIER, or Stimulated Isomer Energy Release, and HIPP, the Hafnium Isomer Production Panel. Heading the effort was an early believer in isomers, C. Martin Stickley, along with Ehsan Khan of Energy, who always seemed to pop up whenever any strange forms of New Energy were reported. Fortunately, they had recruited William Herrmannsfeldt of SLAC to the HIPP. Bill sought to do from the inside of the isomeric world what I had tried to do from the outside: Find out if it made sense, and if it did not, kill it.

Sharon tells the story of Bill’s efforts, and the help I tried to give him, in her book. At about this time in the story, she walked into my life. Because I wanted to stop the waste of money, and also the assault on arms control treaties, I agreed to talk. Why not? I wasn’t talking about anything that was classified. A rather misguided State Department had been snookered into putting isomer weapons on the arms export control list (arguing that while they were presently impossible, it had taken less than seven years to go from the discovery of nuclear fission to Hiroshima, so just as with pigeon research, we must stay ahead of Other Powers), but I knew that wouldn’t pass the giggle test.

When you tell a reporter a story, you become a “source.” But reporters need several sources, and so I knew that Sharon would find others. She would certainly talk to people like Stickley and the Sandia group who probably weren’t happy with what I had done. They would be “sources” too, and only Sharon Weinberger would get to sort out what she thought was the truth, writing only what she chose. Hence the sinking feeling.

Hafnium became the subject of the cover article of a Washington Post Sunday Magazine few months later, complete with a cover photo of Collins, and a screen shot of the isomer hand grenade. That got the House and Senate appropriators in the game, and within months the law forbade DOD to spend its money on isomers. I thought we had killed the hafnium bomb with laughter. Victory for the good guys? Well, not exactly.  DOE supported the work with Stickley apparently getting funds that way. A TRiggered Isomer Proof experiment, TRIP, was scheduled and apparently was conducted at the synchrotron light source at Brookhaven although all reviewers recommended against the experiment. Many of the critics were invited to critique the experimental plans on the condition that if Pat McDaniel’s TRIP showed triggering and our suggestions were followed, we would agree that triggering was real; my contribution was to suggest some target-out/target-in procedures and taking data from ordinary hafnium, all under the usual computer controls so that the experimenters were effectively blindfolded and couldn’t tune their instruments to maximize any signal from the isomer. All my suggestions were rejected.

Did “TRIP” become “Stumble-Fall?” Did McDaniel ever show that isomer triggering occurred? I have no idea, because the results have never been published. Perhaps they have been trapped in the government’s highest level of classification: TS/E, Top Secret/Embarrassing. Surely they showed no prospects for a weapon, because even Carl Collins conceded that he needed 11 keV photons to trigger the isomer, and that only one photon in 600 would interact leaving a net energy deficit of several MeV per trigger. 

But this is Washington. Last summer Ehsan Khan circulated a strange letter to the remains of the HIPP warning them not to talk to Sharon and to inform him if she contacted them. Khan wrote “[T]his is really important.” And he added that TRIP had been so successful that an Independent Evaluation Board had recommended further “exploratory research,” which he defined as “high risk/high payoff” with only the “most seasoned and outstanding individuals” selected by DOE/DOD allowed to be engaged. No such category as “exploratory research” appears in DOD’s budget documents. We’ll never know about TRIP if they don’t publish, and if the research has been classified TS/E there will never be a paper.

So it almost worked out. I let an investigative reporter have a crack at me, and wound up being featured in a news magazine and a book. However much money is wasted on “hafnium bombs,” former presidential science adviser Jack Gibbons, whom I admire greatly, called me a hero in his review of Imaginary Weapons in Physics Today. That’s good enough.

Peter D. Zimmerman is Chair of Science and Security and Director of the Centre for Science & Security Studies at King’s College London. He was the last chief scientist of the US Arms Control and Disarmament Agency, science adviser for arms control in the State Department, and chief scientist of the Senate Foreign Relations Committee.

APS encourages the redistribution of the materials included in this newspaper provided that attribution to the source is noted and the materials are not truncated or changed.

Editor: Alan Chodos
Contributing Editor: Jennifer Ouellette
Staff Writer: Ernie Tretkoff

June 2007 (Volume 16, Number 6)

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