Published online 8 March 2006 | Nature | doi:10.1038/news060306-3


Bubble bursts for table-top fusion

Data analysis calls bubble fusion into question.

The detection spectrum of neutrons from a fusion reaction has a hump, and then dies down to zero.The detection spectrum of neutrons from a fusion reaction has a hump, and then dies down to zero.

Data claimed in January to be evidence for bubble fusion are actually a much better match for the radioactive decay of a standard lab source - by a factor of more than 100 million.

That is the key claim in an analysis by physicist Brian Naranjo, who is in the group of Seth Putterman at the University of California, Los Angeles. If correct, it casts serious doubt on the claims made by Rusi Taleyarkhan of Purdue University in West Lafayette, Indiana, that he has produced energy by nuclear fusion in collapsing bubbles in a liquid.

Putterman has been a key critic of Taleyarkhan's work since 2002, when Taleyarkhan first published his claim to have achieved bubble fusion1. Putterman and others argue that Taleyarkhan has not been able to rule out several potential sources of error in his experiment. In particular, they were concerned that the source of neutrons Taleyarkhan used to seed bubble formation in the liquid could have been responsible for the neutrons detected during the experiment and cited as evidence for fusion.

In January, Taleyarkhan and his colleagues published further positive results, in which they again cited the detection of neutrons as evidence for bubble fusion2. This time, Taleyarkhan sought to allay doubts over the neutron detection by seeding bubble formation with alpha particles rather than neutrons.

Sound footing?

But Naranjo and Putterman say that the spectrum that Taleyarkhan claims proves neutrons were generated by fusion looks nothing like it should given the equipment used. The fusion of deuterium nuclei produces neutrons that have a particular energy of 2.45 mega-electronvolts (MeV). "The published spectrum is totally inconsistent with that of 2.45 MeV neutrons, raising doubt over the fusion claim," says Naranjo.

The spectrum for such neutrons should have a hump in the middle and a sharp cut-off at higher energies. Yet both features are strikingly absent from Taleyarkhan's data, Naranjo says. The probability of getting a spectrum that is such a poor match for neutrons produced by fusion is one in more than 100 million - virtually impossible, Naranjo calculates.

But the data are a very good match for the spectrum that would be expected if neutrons were produced by the normal fission of a standard lab radioactive source: the decay of californium-252.

That raises a serious question over Taleyarkhan's methodology, because it suggests that an external neutron source was present, although his paper claims that this was not the case.

Another possibility, which Naranjo considered but believes is less likely based on the published data, is that the reported spectrum was produced by cosmic neutrons that had accumulated in the detector over time. But if this was the case, it would not explain why Taleyarkhan consistently saw many more neutrons during the positive experimental runs than during the negative control runs, as he claims. One possibility is that the experimental runs were longer than the control runs, but this would conflict with the method described in the paper, Naranjo says.

Naranjo has submitted his analysis to the arXiv preprint server and to Physical Review Letters as a comment on Taleyarkhan's paper, he says. The work is also posted on his own prlpreprint.pdf">website.

Naranjo is familiar with neutron spectra produced by fusion as in April 2005 he co-authored a paper on a table-top fusion device that produces neutrons3 - although it is not a potential energy source.

Taleyarkhan's most recent paper2 came with much more data than his previously published works, allowing for more analysis. Naranjo took advantage of the fact that all of the graphs were published in editable form within PDF files, which allowed him to work out the numerical values behind them. Crucially, Naranjo says that he was able to validate his calculations by showing that they produce a very good match to Taleyarkhan's own published calibration data.

Taleyarkhan has so far declined comment on the new claims. Putterman says that Taleyarkhan was made aware of the concerns during a meeting at Purdue on 1 March. Ken Suslick of the University of Illinois at Urbana-Champaign, who also attended the meeting, says that the analysis looks definitive to him. "At the very least it's not fusion, and it looks remarkably like californium," he says.


Problems over Taleyarkhan's work do not change the fact that bubble fusion is a promising idea, nor do they undermine the intrinsic scientific interest in studying the behaviour of collapsing bubbles, Putterman says. But if Taleyarkhan's work is wrong, it would open the field up to a range of different approaches. Taleyarkhan's initial choice of deuterated acetone as a fluid may have been a poor one because it has a high vapour pressure, making it hard to collapse the bubbles. Putterman suggests that using a fluid with a low vapour pressure, such as sulphuric acid, might prove more productive.

But for the time being, Putterman is still being funded by the US Defense Advanced Research Projects Agency (DARPA) to replicate Taleyarkhan's set-up because that is what the prior claim was, says William Coblenz of the agency. "That's the scientific method," says Coblenz, who also attended the recent Purdue meeting. Coblenz declined comment on Naranjo's analysis.

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  • References

    1. Taleyarkhan R. P., et al. Science, 295 . 1868 - 1873 (2002). | Article | PubMed | ISI | ChemPort |
    2. Taleyarkahn R. P., et al. Phys. Rev. Lett., 96 . 034301 (2006).
    3. Naranjo B., Gimzewski J.K., Putterman S., et al. Nature, 434 . 1115 - 1117 (2005). | Article | PubMed | ISI | ChemPort |