The hafnium controversy is a debate over the possibility of 'triggering' rapid energy releases, via gamma ray emission, from a nuclear isomer of hafnium, 178m2Hf. The energy release is potentially 5 orders of magnitude (100,000 times) more energetic than a chemical reaction, but 3 orders of magnitude less than a nuclear reaction. In 1998, a group led by Carl Collins of the University of Texas at Dallas reported having successfully initiated such a trigger. Signal-to-noise ratios were small in those first experiments, and to date no other group has been able to duplicate these results.
178m2Hf is a particularly attractive candidate for induced gamma emission (IGE) experiments, because of its high density of stored energy, 2.5 MeV per nucleus, and long 31-year half life for storing that energy. If radiation from some agent could "trigger" a release of that stored energy, the resulting cascade of gamma photons would have the best chance of finding a pair of excited states with the inverted lifetimes needed for stimulated emission. While induced emission adds only power to a radiation field, stimulated emission adds coherence. The possibility to manipulate gamma ray coherence, even to a small degree, would be interesting. The lifetime of the hafnium isomer is long enough for tractable amounts of material to be collected into experimental targets. Such samples would hold no hazards for personnel working with the material; 1 microgram of 178m2Hf has an activity of 40 microcuries (1.5 MBq).[jargon]
A proposal to test 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 the results were not published. Nevertheless, 178m2Hf was implied to be of special importance to potential applications of IGE. A controversy quickly erupted.
- 178m2Hf has the highest excitation energy of any comparably long-lived isomer. One gram of pure 178m2Hf would contain approximately 1330 megajoules of energy, the equivalent of exploding about 300 kilograms (660 pounds) of TNT. The half-life of 178m2Hf is 31 years or 1 Gs (gigasecond, 1,000,000,000 seconds) so that a gram's natural radioactivity is 1.6 TBq (terabecquerels) or roughly 40 Ci (curies). The activity is in a cascade of penetrating gamma rays, the most energetic of which is 0.574 MeV. Substantial shielding is needed for human safety.
- All energy released would be in the form of photons; X-rays and gamma rays.
- Discussions also indicate that the energy might be released very quickly, so that 178m2Hf could produce extremely high powers (on the order of exawatts).
- 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 didn't 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 at low concentrations <0.1%.
Chronology of notable events
- Around 1997 the JASONS advisory group took testimony about the triggering of nuclear isomers. The JASON Defense Advisory Group published a relevant public report saying that they concluded that such a thing would be impossible and should not be attempted. Despite intervening publications in peer-reviewed journals of articles written by an international team reporting IGE from 178m2Hf, around 2003 IDA took testimony, again from relevant scientists on matters of the credibility of reported results. Professor Carl Collins, the lead U.S. member of the team publishing the successes, did not testify.
- Around 2003, DARPA initiated exploratory research termed stimulated isomer energy release (SIER) and public interest was aroused, at both popular levels and at professional levels.
- The first focus of SIER was whether significant amounts of 178m2Hf could be produced at acceptable costs for possible applications. A closed panel called HIPP was charged with the task and the conclusion was yes, it could. However, a scientist on that confidential DARPA HIPP review panel "leaked" prejudicial but preliminary concerns to the press. This unsubstantiated assertion set into motion the subsequent cascade of inaccurate reports about the so-called "outrageous costs" of isomer triggering.
- Having satisfied the charge to the HIPP panel to explore the problem of production at acceptable cost, the SIER program turned to the matter of definitive confirmation of the reports of IGE from 178m2Hf. A task of TRiggering Isomer Proof (TRIP) was mandated by DARPA and assigned to a completely independent team from those reporting success previously. The "gold standard" of hafnium-isomer triggering was set as the Rusu dissertation. The TRIP experiment required independent confirmation of the Rusu dissertation. It was successful, but could not be published.
- By 2006, the Collins team had published multiple papers supporting their initial observations of IGE from 178m2Hf. Reprints (available at the link) of articles that were published after 2001 describe work conducted with tunable monochromatic X-ray beams from the synchrotron light sources SPring-8 in Hyogo and SLS in Villigen.
- By 2006 there were 2 articles that claimed to disprove possibilities for IGE from 178m2Hf and three theoretical articles written by the same individual saying why it should not be possible to occur by the particular steps the author envisioned. The first two described synchrotron experiments in which the X-rays were not monochromatic.
- In 2007 Pereira et al. estimated that the cost of the electrical energy to store energy in the nuclear isomer is on the order of $1/J; building and maintaining the particle accelerator needed for the purpose is extra.
- On February 29, 2008 DARPA distributed some of the 150 copies of the final report of the TRIP experiment that had independently confirmed the "gold standard" of hafnium-isomer triggering. Sustained by peer review, the 94-page report is distributed for official use only (FOUO) by the DARPA Technical Information Office, 3701 N. Fairfax Dr., Arlington, VA 22203 USA.
- On October 9, 2008 LLNL released the 110-page evaluation of the DARPA TRIP experiment. Quoting from page 33, "The only experiment that shows statistical significance is the coincidence experiment described in the thesis by Rusu ." However, the report summary states, page 65: "Our conclusion is that the utilization of nuclear isomers for energy storage is impractical from the points of view of nuclear structure, nuclear reactions, and of prospects for controlled energy release. We note that the cost of producing the nuclear isomer is likely to be extraordinarily high, and that the technologies that would be required to perform the task are beyond anything done before and are difficult to cost at this time."
- In 2009 S.A. Karamian et al. published the results of a four nation team's experimental measurements at Dubna for the production of quantities of 178m2Hf by spallation at energies as low as 80 MeV. Besides significantly lowering the projected cost of production, this experimental result proved the accessibility to sources of 178m2Hf to be within the capabilities of the several idle cyclotron devices scattered around the world.
- Collins, C.B., Davanloo, F., Iosif, M. et al. (1999). "Accelerated Emission of Gamma Rays from the 31-yr Isomer of 178Hf Induced by X-Ray Irradiation". Physical Review Letters 82 (4): 695–698. Bibcode:1999PhRvL..82..695C. doi:10.1103/PhysRevLett.82.695.
- Collins, C.B., Davanloo, F., Rusu, A.C. et al. (2000). "Gamma emission from the 31-yr isomer of 178Hf induced by x-ray irradiation". Physical Review C 61: 054305–054305–7. Bibcode:2000PhRvC..61e4305C. doi:10.1103/PhysRevC.61.054305.
- Dietrick E. Thomsen Pumping up hope for gamma ray laser, Science News, Nov 1, 1986
- Proceedings of the NATO-ARW are collected in Hyperfine Interactions, 107, pp 3–492 (1997).
- Link to review of "Isomer Triggering history from one participant.
- N. Lewis, R. Garwin, D. Hammer, W. Happer, R. Jeanloz. J. Katz, S. Koonin, P. Weinberger, E. Williams (October 1997). High Energy Density Explosives (PDF). JSR-97-110. Sect. 4, p. 13.
- S. Weinberger (28 March 2004). "Scary things come in small packages". Sunday Supplement Magazine. Washington Post. Retrieved 2009-05-03.
- Bertram Schwarzschild (May 2004). "Conflicting Results on a Long-Lived Nuclear Isomer of Hafnium Have Wider Implications". Physics Today: 21.
- San Jose newspaper article., October, 2003.
- C. Rusu (PhD Dissertation, U of Texas at Dallas, 2002)Available from: Proquest (Order Number: 3087127).
- Publications by Center for Quantum Electronics, University of Texas at Dallas Retrieved on 2010-12-12.
- C.B. Collins, N.C. Zoita, F. Davanloo, Y. Yoda, T. Uruga, J.M.Pouvesle, and I.I. Popescu (2005). "Nuclear resonance spectroscopy of the 31-yr isomer of Hf-178". Laser Physics Letters 2 (3): 162. Bibcode:2005LaPhL...2..162C. doi:10.1002/lapl.200410154.
- Ahmad, I. et al. (2001). "Search for X-Ray Induced Acceleration of the Decay of the 31-Yr Isomer of 178Hf Using Synchrotron Radiation". Physical Review Letters 87 (7): 072503. Bibcode:2001PhRvL..87g2503A. doi:10.1103/PhysRevLett.87.072503. PMID 11497887.
- Ahmad, I. et al. (2003). "Search for x-ray induced decay of the 31-yr isomer of 178Hf at low x-ray energies". Physical Review C 67 (4): 041305R. Bibcode:2003PhRvC..67d1305A. doi:10.1103/PhysRevC.67.041305.
- Tkalya, Eugene V. (2003). "Probability of L-shell nuclear excitation by electronic transitions in 178Hfm2". Physical Review C 68 (6): 064611. Bibcode:2003PhRvC..68f4611T. doi:10.1103/PhysRevC.68.064611.
- Tkalya, Eugene V. (2005). "Induced decay of 178Hfm2: Theoretical analysis of experimental results". Physical Review C 71 (2): 024606. Bibcode:2005PhRvC..71b4606T. doi:10.1103/PhysRevC.71.024606.
- Tkalya, Evgenii V (2005). "Induced decay of the nuclear isomer 178m2Hf and the 'isomeric bomb'". Physics-Uspekhi 48 (5): 525. Bibcode:2005PhyU...48..525T. doi:10.1070/PU2005v048n05ABEH002190. [Uspekhi Fiz. Nauk 175, 555 (2005)].
- Pereira et al. (2007). Laser Physics 17: 874. Bibcode:2007LaPhy..17..874P. doi:10.1134/S1054660X0706014X. Missing or empty
- Hartouni, E.P, et al., "Theoretical Assessment of 178Hfm2 De-excitation, LLNL Report TR-407631, October 9, 2008, p.33. https://e-reports-ext.llnl.gov/pdf/366265.pdf
- Karamian, S. E. et al. (2009). "Spallation and fission products in the (p+179Hf) and (p+natHf) reactions". Nuclear Instruments and Methods in Physics Research A 600: 488–497. Bibcode:2009NIMPA.600..488K. doi:10.1016/j.nima.2008.12.001.