The hafnium controversy was a debate over the possibility of "triggering" rapid energy releases, via gamma-ray emission, from 178m2Hf, a nuclear isomer of hafnium. The energy release per event is 5 orders of magnitude (100,000 times) higher than in a typical chemical reaction, but 2 orders of magnitude less than a nuclear fission reaction. In 1998, a group led by Carl Collins in the University of Texas at Dallas reported what they interpreted as evidence of such a trigger, but these results were not independently reproduced and remain controversial. Signal-to-noise ratios were small in those first experiments, and to date no other group has reproduced these results. Peter Zimmerman (an American nuclear physicist and arms-control expert) described claims of weaponization potential as having been based on "very bad science".
Background
178m2Hf is a particularly interesting candidate for induced gamma emission (IGE) experiments, because 178m2Hf's energy is 2.5 MeV per nucleus higher than that of ground-state 178Hf, and it has a long (31-year) half life. If lower-energy radiation could induce gamma emission in the isomer before competing processes dissipated that energy, it might, in principle, start a cascade of gamma photons. The long half-life of 178m2Hf might make it possible to engineer a substance with enough of these energetic nuclei needed for stimulated emission, i.e. a gamma-ray laser. While induced emission of a high-energy photon by a lower-energy photon adds power to a radiation field, stimulated emission adds coherence. With all the caveats about dissipation of the triggering photon, and its efficient recreation by the energetic photon that is being triggered, the process could, in principle, lead to nuclear reaction engines, along with more precise radiometric devices. A proposal to show the efficacy for "triggering" 178m2Hf was approved by a NATO Advanced Research Workshop (NATO-ARW) held in Predeal in 1995. Although the proposal was to use incident protons to bombard the target, α-particles were available when the first experiment was scheduled. It was done by a French, Russian, Romanian and American team. Results were said to be extraordinary but were not published. Nevertheless, 178m2Hf was implied to be of special importance for potential applications of IGE. A controversy quickly erupted, mostly between the original proponents of 178m2Hf as having potential military applications as a gamma-ray laser weapon or a non-neutronic but still nuclear-like explosive, and critics who discounted such possibilities due to practical obstacles along the way: 178m2Hf is difficult to make and virtually impossible to separate from the ground-state 178Hf, the absorption of lower-energy triggering X-rays by the bound electrons around the Hf nucleus, and the minute probability of recreating the trigger-capable X-ray starting with the triggered X-ray itself by multiple random scattering. Still, the potential military application was enticing enough to try to make 178m2Hf into something useful (rather than an intriguing nucleus suitable for academic study only).
Importance
178m2Hf has the highest excitation energy of any comparably long-lived isomer. One gram of pure 178m2Hf contains approximately 1330 megajoules of energy, about 300 kilograms (660 pounds) of TNT equivalent. The half-life of 178m2Hf is 31 years, or 1 Gs (gigasecond, 1,000,000,000 seconds), so that natural radioactivity of one gram is 2.40 TBq (65 Ci). The activity is in a cascade of penetrating gamma rays, the most energetic of which is 0.574 MeV. Substantial shielding would be needed for human safety if the sample were to be one gram of the pure isomer. However, so far the nuclear isomer exists only at low concentrations (<0.1%), within multi-isotopic hafnium. All energy released would be in the form of photons: X-rays and gamma rays. In theoretical estimates, if all stored energy were released rapidly, a gram of pure 178m2Hf might emit an intense photon burst; however, these conditions have not been achieved experimentally. The characteristic scales of times for processes involved in applications would be favorable for consuming all of the initial radioactivity. The process for triggering a sample by IGE would use photons to trigger and produce photons as a product. The propagation of photons occurs at the speed of light, while mechanical disassembly of the target would proceed with a velocity comparable to that of sound. Untriggered 178m2Hf material might not be able to get away from a triggered event if the photons did not interact first with the electrons. Both the proposal to the NATO-ARW and the fragmentary results from the subsequent experiment indicated that the energy of the photon needed to initiate IGE from 178m2Hf would be less than 300 keV. Many economical sources of such low-energy X-rays were available for delivering quite large fluxes to target samples of modest dimensions. Samples of 178m2Hf were and remain available only at low concentrations (<0.1%), without any clear way to increase this concentration.
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