Gamma Ray Weapons? A Premature Speculation

By Kurt Gottfried, Union of Concerned Scientists, 7 October 2003

Recently there has been speculation that a gamma ray arms race may be in the offing.1 The U.S. Department of Defense is considering the development of a novel and exotic explosive that would release a flood of high-energy gamma rays from the nuclei of certain atoms.2 If a weapon that exploits this energy source could be devised, it would have a power approaching that of a nuclear weapon.

Gamma rays are quanta of light of very short wavelength and very high energyon the order of a million times more energetic than visible light photons, and at least 10 times more energetic than X-ray photons. When a nucleus undergoes a spontaneous transition from an excited state to a lower-energy state, it emits one or more gamma rays, just as an excited atom emits visible photons when it de-excites.

There are some nuclear species that have very long-lived excited states, called isomers. (Here “long-lived” means relative to the usual lifetimes of excited nuclear states, which are miniscule fractions of a microsecond.) When a nucleus is in such a state, it “stores” this excitation energy, which is subsequently released spontaneously in the form of gamma rays.

A particularly striking example is an isomer of the nucleus hafnium-178, which we here designate by Hf*. (The element hafnium is a heavy metal with the atomic number 72.) This isomer has a half-life of 31 years, and an excitation energy of 2.5 million electron volts (MeV). To put this into everyday terms, the energy stored in one ounce of pure Hf* could, in principle, heat 120 tons of water at room temperature to the boiling point. The energy content of Hf* is therefore enormous compared with chemical explosives, and about 100 times smaller than that of the fissile materials in nuclear weapons.

To put this energy to use, either in a weapon or for benign purposes, such as a gamma ray laser, a mechanism is required to release the energy quickly, on demand, and in a controllable manner—not at the useless pace of several decades.

Interest in an “isomer bomb” has been stimulated by a collaboration led by C.B. Collins of the University of Texas at Dallas, who reported that irradiating samples of Hf* with X-rays produces a several percent enhancement of gamma ray emission by the isomer.3 This experiment therefore suggested that the isomer could be triggered to release its energy by irradiating it with a much lower-energy beam.

Such speculations are premature, however, because another collaboration,4 using the very intense and sophisticated X-ray source at Argonne National Laboratory, has announced that it does not reproduce the phenomenon reported by the Texas group; furthermore, the experiment at Argonne sets limits on the effect more than a thousand times below the magnitudes reported in the Texas papers.5 This does not quite settle the matter, however. There are certain differences between the experiments that might have prevented the experiment at Argonne from detecting the effect reported by the Texas group. On the other hand, the results reported by the Argonne experiment are consistent with well-established knowledge about nuclear structure and processes, wheras those from the Texas group are in flagrant disagreement with such knowledge.

Until this disagreement is resolved, there is neither cause for alarm or celebration, nor for diverting substantial sums from the U.S. Treasury to programs built around what may well not be a real effect. Even if the Texas result were to be confirmed, putting it to use would require overcoming a series of enormous hurdles, the first being the astronomical cost of fabricating significant amounts of the isomer. In fact, the Institute for Defense Analysis has taken “a hard, in-depth technical look at the [Texas] results,” reaches a very skeptical verdict regarding their validity, and paints a deeply pessimistic picture of the prospects for putting the effect to use were it to be real.6

It is puzzling, therefore, that the Pentagon's Defense Advanced Research Projects Agency (DARPA) appears to be seriously interested in devoting resources to so implausible a prospect.7 What this situation clearly calls for is an independent evaluation of the competing experiments, and perhaps another independent experiment. These are the only activities related to this matter to which the U.S. government should now commit any resources.

Technical Details

A more detailed explanation of what is afoot may be of interest to some readers. The hafnium nucleus has an ellipsoidal shape and, as a consequence, its excitation spectrum is similar to that of molecules. (See the figure.) Below the isomer at 2.5 MeV, which has a rather prodigious angular momentum of 16, there are two rotational bands. The undisturbed isomer decays spontaneously by first de-exciting to the higher rotational band with the half-life of 31 years, then cascading rapidly to the bottom of this band, from where it de-excites to the ground state band with a half-life of four seconds. The Texas claim is that irradiation by X-rays excites the isomer to one or more somewhat higher states whose decay is not suppressed by the stringent selection rules associated with very high angular momentum states. (These selection rules account for the enormously long lifetime of the isomer.) Moreover, and of importance for applications, the Texas group reports that the transitions from the states they excite above the isomer do not all pass through the state with the four-second half-life.

Figure: The spectrum of Hf-178 showing the 31-year isomer, and the state at the bottom of the second rotational band with the four-second half-life (also called an isomer). The heavy arrows are the transitions that Collins et al. report as enhanced; the numbers on the transitions are gamma ray energies in kilo-electron volts. The Texas collaboration claims that the states above the 31-year isomer, which it populates by X-ray absorption, decay rapidly to the ground state band. (From Ahmad et al., Physical Review Letters 87, 072503-1; 2001. Copyright 2001 by the American Physical Society.)

The Texas trick sounds simple enough, so one may ask why it was not done long ago. If the phenomenon proves to be real, the reason would be that the reaction rate for X-ray photons to resonantly jiggle the isomer into higher states is far larger than what would be expected on the basis of well-established knowledge about nuclei. In contrast, the low bounds reported by the Argonne experiment are consistent with such expectations.

Now to some differences between the experiments. The original Texas collaboration experiments used a dental X-ray source, but their more recent experiments have also used far more intense X-ray sources. The experiments at Argonne have all been done with the Advanced Photon Source, a state-of-the-art electron accelerator designed to produce very intense X-ray beams. The Texas group argues that these intense beams damage the target and produce backgrounds that mask the effect, but the authors of the experiment at Argonne have stated that their measurements produce results that are valid despite these problems.8

Finally, given the difficulty of triggering the simultaneous decay of most nuclei in a large sample of Hf* (assuming that stimulated decay exists), some have speculated that it may be possible to achieve release of the energy stored in the whole sample by means of a chain reaction initiated by triggering a small fraction of such a sample. This is a really far-fetched notion. In contrast to a neutron chain reaction in fissile materials, where the neutrons are not lost as they travel from their place of birth to where they induce a fission reaction, the gamma ray photons emitted by any nucleus have a very high probability of quickly disappearing by knocking electrons out of atoms. In addition, a chain reaction would require each isomer decay cascade to promptly emit, on average, more than one photon in the energy band required by the triggering mechanism. If all the cascades pass through the four-second isomer, a chain reaction would be impossible.

Notes

1. New Scientist, “Gamma-ray weapons could trigger new arms race,” August 16, p.4 (2003).

2. See www.dtic.mil/mctl/DCT/DCTSec02.pdf. This Pentagon site states that isomers have “the potential to revolutionize all aspects of warfare,” a reckless statement which has, of course, been picked up in the media.

3. C.B. Collins et al., Physical Review Letters 82, 695 (1999); C.B. Collins et al., Hyperfine Interactions 135, 51 (2001); C.B. Collins et al., Europhysics Letters 57, 677 (2002). See also www.utdallas.edu/research/quantum/isomer.

4. This collaboration included researchers from Argonne National Laboratory, Los Alamos National Laboratory, and Lawrence-Livermore National Laboratory.

5. I. Ahmad et al., Physical Review Letters 87, 072503-1 (2001); I. Ahmad et al., Physical Review C 67, 041305 (R) (2003).

6. B. Balko, J. Silk and D. Sparrow, “An Examination of the Possibility of Controlled Extraction of Energy from Nuclear Isomers,” Institute for Defense Analysis, September 12, 2002, briefing for the Deputy Under-Secretary of Defense (Science & Technology).

7. K. Davidson, “Superbomb ignites science dispute,” San Francisco Chronicle, September 28, 2003, p. A1.

8. J.A. Becker, D.S. Gemmell, J.P. Schiffer, and J.B. Wilhelmy, “The Hf-178m2 Controversy,” Argonne, Livermore, and Los Alamos National Laboratories, UCRL-ID-155489, July 2003.