A novel nuclear technology is making a comeback - but why are the researchers keeping quiet about it?

There was uproar in the scientific community in 2003 when it emerged that the US military was funding research into miniature warheads based on nuclear isomers. The science behind the project was attacked and even ridiculed.

Nuclear isomers are atoms with a "metastable" nucleus. Perhaps the best analogy is to consider the nucleons as snooker balls. In a normal atom, the balls are all flat on the table. In a metastable isomer, they are piled in a pyramid, ready to collapse and release energy, given the right sort of prod.

Everyone agrees that nuclear isomers can store vast amounts of energy - less than is released in other nuclear reactions, but thousands of times more than chemical fuels. The question is whether the isomeric decay is random, or whether it can be triggered by interaction with an x-ray or other high-energy photon. Triggering could release an intense burst of gamma rays, lethal to both humans and electronic devices. The planned weapon was dubbed "the death ray bomb" and "the atomic hand grenade" (US military pioneers death ray bomb, August 2003).

Trigger happy

Triggering of the 178m2 isomer of Hafnium was reported by Carl Collins of the University of Texas in 1999. But others, notably the Argonne National Laboratory, were not able to repeat his results.

Academic opposition was intense, and a panel of scientists urged a review of the evidence "before proceeding to study applications that may not make physical sense". Unsurprisingly, the US Congress cut the project's funding.

Many still treat isomer triggering as a classic example of pseudoscience. But the phenomenon is becoming more accepted, and several military isomer projects are under way. The focus is now on a slow energy release for batteries - but not everyone has completely given up on bombs.

Youngstown State University, Ohio, recently announced funding from the Pentagon for fundamental research into the physics of nuclear isomers. This academic effort is led by James (Jeff) Carroll. He regards applications such as exotic batteries as speculative, since much more solid data is needed to prove they are feasible, but believes that any form of weapon is unlikely. "One should differentiate between an 'isomer programme' and what might be called the 'Hafnium programme', " says Carroll. "The former has added significantly to the body of science, as documented by many peer-reviewed and published results that have achieved acceptance in the nuclear physics community.

"The Hafnium program was focused, to my knowledge, on proving a specific claimed effect that has even now not been successfully observed by any independent and sceptical group. It would be a shame to throw the baby out with the bath water by equating these two overlapping, yet distinct programmes with quite different aims and outcomes."

The US army has been quietly running an isomer project for years. The researchers have published several papers about using isomers for micro-batteries and portable power generation. Rather than Hafnium, they are working with isomers of silver, lutetium and holmium.

This team also distances itself from earlier work, noting: "This approach differs dramatically from suggestions for use of isomeric materials in explosives applications".

However, the army estimates that isomer power plants will be too expensive for anything except small-scale use. Nuclear isomers have to be charged up with energy, and there is no cheap way of doing this.

In Britain, the Atomic Weapons Establishment is looking at whether an ultra-short pulse laser can trigger uranium isomers by stimulating the electron shell around the nucleus into an energetic state. The technique is called Nuclear Excitation by Electron Transition, and involves zapping a quantity of material so it becomes a plasma. "This process requires a resonance between the nuclear and atomic electron transition energies," says Peter Thomson, a scientist at the Atomic Weapons Establishment. "In principle these might be adjusted within a high-temperature plasma to bring an appropriate atomic transition into resonance with the nucleus."

Voodoo science

The AWE's emphasis is on basic research and the potential for energy storage. But Thomson does not believe that an isomer bomb can be ruled out, saying that the controversy over Collins' claims has not been fully resolved. He adds: "A secondary role of AWE is to provide independent advice to the UK government on potential threats to the security of the UK, so an understanding of the fundamentals of isomer physics is essential."

The US navy also has an isomer programme. A recent presentation lists nuclear isomers as a possible replacement for explosives in warheads. This continued belief may stem from the Triggering Isomer Proof Experiment, which was sponsored by the US Department of Defense to determine conclusively whether high-energy Hafnium isomer triggering was possible. In spite of failures to reproduce results elsewhere, there are strong hints that the results of the experiment were positive.

Meanwhile, isomer triggering is edging towards the mainstream. Lawrence Livermore National Laboratory was previously a stronghold of isomer scepticism. Last year, researchers announced they were working on triggering energy release from a thorium isomer. They say this would be a world first, showing continued disbelief in the earlier triggering claims.

The earlier results have been disowned by many, who refer to their work as "quantum nucleonics" rather than the sensitive "isomer triggering". Nobody wants to be accused of voodoo science, and military funding is at the whim of politicians who can be swayed by the scientific community. No wonder researchers are treading very carefully.