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 mannernot 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, whears 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.
Kurt Gottfried is an emeritus professor of physics at Cornell
University, and chairman of the UCS Board of Directors.