- An exotic kind of nuclear explosive being developed by
the US Department of Defense could blur the critical distinction between
conventional and nuclear weapons. The work has also raised fears that weapons
based on this technology could trigger the next arms race.
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- The explosive works by stimulating the release of energy
from the nuclei of certain elements but does not involve nuclear fission
or fusion. The energy, emitted as gamma radiation, is thousands of times
greater than that from conventional chemical explosives. The technology
has already been included in the Department of Defense's Militarily Critical
Technologies List, which says: "Such extraordinary energy density
has the potential to revolutionise all aspects of warfare."
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- Scientists have known for many years that the nuclei
of some elements, such as hafnium, can exist in a high-energy state, or
nuclear isomer, that slowly decays to a low-energy state by emitting gamma
rays. For example, hafnium178m2, the excited, isomeric form of hafnium-178,
has a half-life of 31 years.
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- The possibility that this process could be explosive
was discovered when Carl Collins and colleagues at the University of Texas
at Dallas demonstrated that they could artificially trigger the decay of
the hafnium isomer by bombarding it with low-energy Xrays (New Scientist,
3 July 1999, p42). The experiment released 60 times as much energy as was
put in, and in theory a much greater energy release could be achieved.
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- Before hafnium can be used as an explosive, energy has
to be "pumped" into its nuclei. Just as the electrons in atoms
can be excited when the atom absorbs a photon, hafnium nuclei can become
excited by absorbing high-energy photons. The nuclei later return to their
lowest energy states by emitting a gamma-ray photon. Nuclear isomers were
originally seen as a means of storing energy, but the possibility that
the decay could be accelerated fired the interest of the Department of
Defense, which is also investigating several other candidate materials
such as thorium and niobium.
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- For the moment, the production method involves bombarding
tantalum with protons, causing it to decay into hafnium-178m2. This requires
a nuclear reactor or a particle accelerator, and only tiny amounts can
be made. Currently, the Air Force Research Laboratory at Kirtland, New
Mexico, which is studying the phenomenon, gets its hafnium-178m2 from SRS
Technologies, a research and development company in Huntsville, Alabama,
which refines the hafnium from nuclear material left over from other experiments.
The company is under contract to produce experimental sources of hafnium178m2,
but only in amounts less than one ten-thousandth of a gram.
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- But in future there may be cheaper ways to create the
hafnium isomer - by bombarding ordinary hafnium with high-energy photons,
for example. Hill Roberts, chief scientist at SRS, believes that technology
to produce gram quantities will exist within five years. The price is likely
to be high- similar to enriched uranium, which costs thousands of dollars
per kilogram- but unlike uranium it can be used in any quantity, as it
does not require a critical mass to maintain the nuclear reaction.
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- The hafnium explosive could be extremely powerful. One
gram of fully charged hafnium isomer could store more energy than 50 kilograms
of TNT. Miniature missiles could be made with warheads that are far more
powerful than existing conventional weapons, giving massively enhanced
firepower to the armed forces using them.
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- The effect of a nuclear-isomer explosion would be to
release high-energy gamma rays capable of killing any living thing in the
immediate area. It would cause little fallout compared to a fission explosion,
but any undetonated isomer would be dispersed as small radioactive particles,
making it a somewhat "dirty" bomb. This material could cause
long-term health problems for anybody who breathed it in.
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- There would also be political fallout. In the 1950s,
the US backed away from developing nuclear mini-weapons such as the "Davy
Crockett" nuclear bazooka that delivered an explosive punch of 18
tonnes of TNT. These weapons blurred the divide between the explosive power
of nuclear and conventional weapons, and the government feared that military
commanders would be more likely to use nuclear weapons that had a similar
effect on the battlefield to conventional weapons. By ensuring that the
explosive power of a nuclear weapon was always far greater, it hoped that
they could only be used in exceptional circumstance when a dramatic escalation
of force was deemed necessary.
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- Then in 1994, the US confirmed this policy with the Spratt-Furse
law, which prevents US military from developing mini-nukes of less than
five kilotons. But the development of a new weapon that spans the gap between
the explosive power of nuclear and conventional weapons would remove this
restraint, giving commanders a way of increasing the amount of force they
can use in a series of small steps.
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- Nuclear-isomer weapons could be a major advantage to
armies possessing them, leading to the possibility of an arms race.
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- André Gsponer, director of the Independent Scientific
Research Institute in Geneva, believes that a nation without such weapons
would not be able to fight one that possesses them. As a result, he says,
"many countries which will not have access to these weapons will produce
nuclear weapons as a deterrent", leading to a new cycle of proliferation.
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- The Department of Defense notes that there are serious
technical issues to be overcome and that useful applications may be decades
away. But its Militarily Critical Technologies List also says: "We
should remember that less than six years intervened between the first scientific
publication characterising the phenomenon of fission and the first use
of a nuclear weapon in 1945."
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- http://www.eurekalert.org/pub_releases/2003-08/ns-gw081303.php
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