Hafnium controversy

Last updated May 10, 2024

The hafnium controversy was a debate over the possibility of 'triggering' rapid energy releases, via gamma ray emission, from a nuclear isomer of hafnium, ^178m2Hf. Per event the energy release is 5 orders of magnitude (100,000 times) more energetic 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 of the University of Texas at Dallas reported^[1]^[2] 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. Peter Zimmerman (an American nuclear physicist and arms control expert) described claims of weaponization potential as having been based on "very bad science".^[3]

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 Hf, and it has a long 31-year half life. If much lower-energy radiation from some agent could "trigger" a release of that stored energy before most of this triggering radiation would be dissipated in competing processes, and if the triggering radiation could be regenerated efficiently by the 2.5 MeV gamma, it might be possible 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.^[4]

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.^[5] 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 ^[6] 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, 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 Hf ground state, the absorption of lower-energy triggering x-rays by the bound electrons around the Hf nucleus, and the minute probability to recreate the trigger-capable x-ray starting with the triggered x-ray itself by multiple random scattering. Still, the 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, the equivalent in about 300 kilograms (660 pounds) of the explosive 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 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.
If all the energy in the nucleus could be released within a short time (e.g., one nanosecond), one gram of pure ^178m2Hf would produce an x-ray burst with extremely high power on the order of 1 GJ/ns or 1 exawatt (1 x 10¹⁸ W). However, quantitative estimates indicate that the energy released by the nuclear isomer is much less than the energy needed to initiate the process; the power needed to initiate IGE would have to occur over a shorter time scale than the release of the nuclear energy, and would therefore be even more extremely disproportionate.
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.

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^[7] 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^[8] and at professional levels.^[9]

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 concluded that it could. However, a scientist on that confidential DARPA HIPP review panel "leaked" prejudicial but preliminary concerns to the press.^[10] 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.^[11] 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.^[12]^[13] 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^[14]^[15] 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.^[16]^[17]^[18] The first two described synchrotron experiments in which the X-rays were not monochromatic.
In 2007, Pereira et al.^[19] 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. Any reasonable explosive device, e.g., a hand grenade, may contain from 10 to 100 g of TNT, corresponding to 40 to 400 kJ, at a cost of tens of dollars or at least 10,000 times less than this estimate for isomeric energy content in the nucleus. Such an excessive cost makes any device based on nuclear isomers much too expensive to be practical, and research motivated by potential applications thereof a waste of money (in contrast to research on nuclear isomers purely for scientific purposes that do not claim any practicality).
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.^[20] Quoting from page 33, "Overall, the X-ray ^178m2Hf experiments by Collins et al. are statistically marginal and inconsistent. None of the reported positive triggering results were confirmed by independent groups, including those experiments performed by former collaborators (Carroll). "^[12] In addition, 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.^[21] In addition to 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.

Related Research Articles

Hafnium is a chemical element; it has symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869, though it was not identified until 1922, by Dirk Coster and George de Hevesy, making it one of the last two stable elements to be discovered. Hafnium is named after Hafnia, the Latin name for Copenhagen, where it was discovered.

The Mössbauer effect, or recoilless nuclear resonance fluorescence, is a physical phenomenon discovered by Rudolf Mössbauer in 1958. It involves the resonant and recoil-free emission and absorption of gamma radiation by atomic nuclei bound in a solid. Its main application is in Mössbauer spectroscopy.

Compton scattering is the quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron. Specifically, when the photon hits electrons, it releases loosely bound electrons from the outer valence shells of atoms or molecules.

Electron capture is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. This process thereby changes a nuclear proton to a neutron and simultaneously causes the emission of an electron neutrino.

<span class="mw-page-title-main">Nuclear isomer</span> Metastable excited state of a nuclide

A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state (higher energy) levels. "Metastable" describes nuclei whose excited states have half-lives 100 to 1000 times longer than the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10⁻¹² seconds). The term "metastable" is usually restricted to isomers with half-lives of 10⁻⁹ seconds or longer. Some references recommend 5 × 10⁻⁹ seconds to distinguish the metastable half life from the normal "prompt" gamma-emission half-life. Occasionally the half-lives are far longer than this and can last minutes, hours, or years. For example, the ^180m
₇₃Ta nuclear isomer survives so long (at least 10¹⁵ years) that it has never been observed to decay spontaneously. The half-life of a nuclear isomer can even exceed that of the ground state of the same nuclide, as shown by ^180m
₇₃Ta as well as ^186m
₇₅Re, ^192m2
₇₇Ir, ^210m
₈₃Bi, ^212m
₈₄Po, ^242m
₉₅Am and multiple holmium isomers.

An axion is a hypothetical elementary particle originally postulated by the Peccei–Quinn theory in 1977 to resolve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter.

In physics, induced gamma emission (IGE) refers to the process of fluorescent emission of gamma rays from excited nuclei, usually involving a specific nuclear isomer. It is analogous to conventional fluorescence, which is defined as the emission of a photon by an excited electron in an atom or molecule. In the case of IGE, nuclear isomers can store significant amounts of excitation energy for times long enough for them to serve as nuclear fluorescent materials. There are over 800 known nuclear isomers but almost all are too intrinsically radioactive to be considered for applications. As of 2006 there were two proposed nuclear isomers that appeared to be physically capable of IGE fluorescence in safe arrangements: tantalum-180m and hafnium-178m2.

Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, ²³²Th, is relatively stable, with a half-life of 1.405×10¹⁰ years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of ²³⁰Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Natural hafnium (₇₂Hf) consists of five observationally stable isotopes (¹⁷⁶Hf, ¹⁷⁷Hf, ¹⁷⁸Hf, ¹⁷⁹Hf, and ¹⁸⁰Hf) and one very long-lived radioisotope, ¹⁷⁴Hf, with a half-life of 7.0×10¹⁶ years. In addition, there are 34 known synthetic radioisotopes, the most stable of which is ¹⁸²Hf with a half-life of 8.9×10⁶ years. This extinct radionuclide is used in hafnium–tungsten dating to study the chronology of planetary differentiation.

Dubnium (₁₀₅Db) is a synthetic element, thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was ²⁶¹Db in 1968. Thirteen radioisotopes are known, ranging from ²⁵⁵Db to ²⁷⁰Db, along with one isomer (^257mDb); two more isomers have been reported but are unconfirmed. The longest-lived known isotope is ²⁶⁸Db with a half-life of 16 hours.

Hassium (₁₀₈Hs) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was ²⁶⁵Hs in 1984. There are 13 known isotopes from ²⁶³Hs to ²⁷⁷Hs and 1–4 isomers. The most stable isotope of hassium cannot be determined based on existing data due to uncertainty that arises from the low number of measurements. The half-lives of ²⁶⁹Hs and ²⁷¹Hs are about 12 seconds, whereas that of ²⁷⁰Hs is about 7.6 seconds. It is also possible that ^277mHs is more stable than these, with its half-life likely being 130±100 seconds, but only one event of decay of this isotope has been registered as of 2016.

Darmstadtium (₁₁₀Ds) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was ²⁶⁹Ds in 1994. There are 11 known radioisotopes from ²⁶⁷Ds to ²⁸¹Ds and 2 or 3 known isomers. The longest-lived isotope is ²⁸¹Ds with a half-life of 14 seconds.

Copernicium (₁₁₂Cn) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was ²⁷⁷Cn in 1996. There are 6 known radioisotopes ; the longest-lived isotope is ²⁸⁵Cn with a half-life of 30 seconds.

Yrast is a technical term in nuclear physics that refers to a state of a nucleus with a minimum of energy for a given angular momentum. Yr is a Swedish adjective sharing the same root as the English whirl. Yrast is the superlative of yr and can be translated whirlingest, although it literally means "dizziest" or "most bewildered". The yrast levels are vital to understanding reactions, such as off-center heavy ion collisions, that result in high-spin states.

A nuclear clock or nuclear optical clock is a notional clock that would use the frequency of a nuclear transition as its reference frequency, in the same manner as an atomic clock uses the frequency of an electronic transition in an atom's shell. Such a clock is expected to be more accurate than the best current atomic clocks by a factor of about 10, with an achievable accuracy approaching the 10⁻¹⁹ level. The only nuclear state suitable for the development of a nuclear clock using existing technology is thorium-229m, a nuclear isomer of thorium-229 and the lowest-energy nuclear isomer known. With an energy of 8.35574(3) eV, this corresponds to a wavelength of 148.3821(5) nm in the vacuum ultraviolet region, making it accessible to laser excitation. A comprehensive review can be found in reference.

The isomeric shift is the shift on atomic spectral lines and gamma spectral lines, which occurs as a consequence of replacement of one nuclear isomer by another. It is usually called isomeric shift on atomic spectral lines and Mössbauer isomeric shift respectively. If the spectra also have hyperfine structure the shift refers to the center of gravity of the spectra. The isomeric shift provides important information about the nuclear structure and the physical, chemical or biological environment of atoms. More recently the effect has also been proposed as a tool in the search for the time variation of fundamental constants of nature.

<span class="mw-page-title-main">Photofission</span> Fission of a nucleus via absorption of a gamma ray

Photofission is a process in which a nucleus, after absorbing a gamma ray, undergoes nuclear fission and splits into two or more fragments.

<span class="mw-page-title-main">Nuclear drip line</span> Atomic nuclei decay delimiter

The nuclear drip line is the boundary beyond which atomic nuclei are unbound with respect to the emission of a proton or neutron.

The Breit–Wheeler process or Breit–Wheeler pair production is a proposed physical process in which a positron–electron pair is created from the collision of two photons. It is the simplest mechanism by which pure light can be potentially transformed into matter. The process can take the form γ γ′ → e⁺ e⁻ where γ and γ′ are two light quanta.

George Curriden Baldwin was an American theoretical and experimental physicist. He was a professor of nuclear engineering at Rensselaer Polytechnic Institute and a scientist working at the General Electric Research Laboratory and at the Los Alamos National Laboratory. He wrote a book on Nonlinear Optics and authored or co-authored over 130 technical papers.

References

↑ 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ 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 (5): 054305–054305–7. Bibcode:2000PhRvC..61e4305C. doi:10.1103/PhysRevC.61.054305.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ Peter Zimmerman (June 2007). "The Strange Tale of the Hafnium Bomb: A Personal Narrative". American Physical Society . Retrieved 5 March 2016.
↑ Thomsen, D. E. (1 November 1986). "Pumping Up Hope for a Gamma Ray Laser". Science News. Society for Science & the Public. doi:10.2307/3970900. JSTOR 3970900.
↑ 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". Archived from the original on 2011-07-26. Retrieved 2010-03-31.
↑ 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. Archived from the original on 2011-08-23. Retrieved 2009-05-03.
↑ Bertram Schwarzschild (May 2004). "Conflicting Results on a Long-Lived Nuclear Isomer of Hafnium Have Wider Implications". Physics Today. Vol. 57, no. 5. pp. 21–24. Bibcode:2004PhT....57e..21S. doi:10.1063/1.1768663.
↑ San Jose newspaper article., October, 2003.
↑ C. Rusu (PhD Dissertation, U of Texas at Dallas, 2002)Available from: Proquest (Order Number: 3087127) Archived 2005-10-15 at the Wayback Machine .
1 2 Publications by Center for Quantum Electronics Archived 2007-09-28 at the Wayback Machine , 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): 162–167. Bibcode:2005LaPhL...2..162C. doi:10.1002/lapl.200410154. S2CID 121707178.{{cite journal}}: CS1 maint: multiple names: authors list (link)
↑ 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. S2CID 209833094.
↑ Tkalya, Eugene V. (2003). "Probability of L-shell nuclear excitation by electronic transitions in ¹⁷⁸Hf^m2". Physical Review C. 68 (6): 064611. Bibcode:2003PhRvC..68f4611T. doi:10.1103/PhysRevC.68.064611.
↑ Tkalya, Eugene V. (2005). "Induced decay of ¹⁷⁸Hf^m2: 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): 525–531. Bibcode:2005PhyU...48..525T. doi:10.1070/PU2005v048n05ABEH002190. S2CID 250864125. [Uspekhi Fiz. Nauk 175, 555 (2005)].
↑ Pereira; et al. (2007). "Economics of isomeric energy". Laser Physics. 17 (6): 874–879. Bibcode:2007LaPhy..17..874P. doi:10.1134/S1054660X0706014X. S2CID 122665613.
↑ Hartouni, E.P, et al., "Theoretical Assessment of ^178m2Hf 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" (PDF). Nuclear Instruments and Methods in Physics Research A. 600 (2): 488–497. Bibcode:2009NIMPA.600..488K. doi:10.1016/j.nima.2008.12.001.

Also note:

Weinberger, Sharon (2006). Imaginary Weapons: A Journey Through the Pentagon's Scientific Underworld . Nation Books. ISBN 1560258497.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.

[1] 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.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[2] 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 (5): 054305–054305–7. Bibcode:2000PhRvC..61e4305C. doi:10.1103/PhysRevC.61.054305.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[3] Peter Zimmerman (June 2007). "The Strange Tale of the Hafnium Bomb: A Personal Narrative". American Physical Society . Retrieved 5 March 2016.

[Thomsen1986-4] Thomsen, D. E. (1 November 1986). "Pumping Up Hope for a Gamma Ray Laser". Science News. Society for Science & the Public. doi:10.2307/3970900. JSTOR 3970900.

[5] Proceedings of the NATO-ARW are collected in Hyperfine Interactions, 107, pp 3–492 (1997).

[6] "Link to review of "Isomer Triggering history from one participant". Archived from the original on 2011-07-26. Retrieved 2010-03-31.

[7] 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.

[8] S. Weinberger (28 March 2004). "Scary things come in small packages". Sunday Supplement Magazine. Washington Post. Archived from the original on 2011-08-23. Retrieved 2009-05-03.

[9] Bertram Schwarzschild (May 2004). "Conflicting Results on a Long-Lived Nuclear Isomer of Hafnium Have Wider Implications". Physics Today. Vol. 57, no. 5. pp. 21–24. Bibcode:2004PhT....57e..21S. doi:10.1063/1.1768663.

[10] San Jose newspaper article., October, 2003.

[11] C. Rusu (PhD Dissertation, U of Texas at Dallas, 2002)Available from: Proquest (Order Number: 3087127) Archived 2005-10-15 at the Wayback Machine .

[p-12] 1 2 Publications by Center for Quantum Electronics Archived 2007-09-28 at the Wayback Machine , University of Texas at Dallas Retrieved on 2010-12-12.

[13] 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): 162–167. Bibcode:2005LaPhL...2..162C. doi:10.1002/lapl.200410154. S2CID 121707178.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[14] 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.

[15] 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. S2CID 209833094.

[16] Tkalya, Eugene V. (2003). "Probability of L-shell nuclear excitation by electronic transitions in ¹⁷⁸Hf^m2". Physical Review C. 68 (6): 064611. Bibcode:2003PhRvC..68f4611T. doi:10.1103/PhysRevC.68.064611.

[17] Tkalya, Eugene V. (2005). "Induced decay of ¹⁷⁸Hf^m2: Theoretical analysis of experimental results". Physical Review C. 71 (2): 024606. Bibcode:2005PhRvC..71b4606T. doi:10.1103/PhysRevC.71.024606.

[18] Tkalya, Evgenii V (2005). "Induced decay of the nuclear isomer ^178m2Hf and the 'isomeric bomb'". Physics-Uspekhi. 48 (5): 525–531. Bibcode:2005PhyU...48..525T. doi:10.1070/PU2005v048n05ABEH002190. S2CID 250864125. [Uspekhi Fiz. Nauk 175, 555 (2005)].

[19] Pereira; et al. (2007). "Economics of isomeric energy". Laser Physics. 17 (6): 874–879. Bibcode:2007LaPhy..17..874P. doi:10.1134/S1054660X0706014X. S2CID 122665613.

[20] Hartouni, E.P, et al., "Theoretical Assessment of ^178m2Hf De-excitation, LLNL Report TR-407631, October 9, 2008, p.33. https://e-reports-ext.llnl.gov/pdf/366265.pdf

[21] Karamian, S. E.; et al. (2009). "Spallation and fission products in the (p+¹⁷⁹Hf) and (p+^natHf) reactions" (PDF). Nuclear Instruments and Methods in Physics Research A. 600 (2): 488–497. Bibcode:2009NIMPA.600..488K. doi:10.1016/j.nima.2008.12.001.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]