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bubbles over

Bubble fusion

Bubble fusion, also known as sonofusion, is the non-technical name for a nuclear fusion reaction hypothesized to occur during sonoluminescence, an extreme form of acoustic cavitation. Officially, this reaction is termed acoustic inertial confinement fusion (AICF) since the inertia of the collapsing bubble wall confines the energy, causing an extreme rise in temperature. The high temperatures sonoluminescence can produce raises the possibility that it might be a means to achieve thermonuclear fusion.

Original experiments

US patent 4,333,796, filed by Hugh Flynn in 1978, appears to be the earliest documented reference to a sonofusion-type reaction.

In the March 8, 2002 issue of the peer-reviewed journal Science, Rusi P. Taleyarkhan and colleagues at the Oak Ridge National Laboratory (ORNL) reported that acoustic cavitation experiments conducted with deuterated acetone showed measurements of tritium and neutron output that were consistent with the occurrence of fusion. The neutron emission was also reported to be coincident with the sonoluminescence pulse, a key indicator that its source was fusion caused by the sonoluminescence.

Shock wave simulations seem to indicate that the temperatures inside the collapsing bubbles may reach up to 10 megakelvins, i.e. as hot as the center of the Sun. Although the apparatus operates in a room temperature environment, this is not cold fusion (as commonly termed in the popular press) because the nuclear reactions would be occurring at the very high temperatures in the core of the imploding bubbles.

The researchers used a pulse of neutrons in order to nucleate ("seed") the tiny bubbles, whereas most previous experiments started with small air bubbles already in the liquid. Using this new method, the team was able to produce stable bubbles that could expand to nearly a millimeter in radius before collapsing. In this way, the researchers stated, they were able to create the conditions necessary to produce very high pressures and temperatures. The sensitivity of the fusion rate to temperature, which is in turn a function of how small the bubbles get when they collapse, in combination with the likely sensitivity of the latter to fine experimental details, may account for the fact that some research workers have reported to see an effect, while others have not.

Taleyarkhan et al also prepared identical experiments in non-deuterated (normal) acetone and failed to observe neutron emission or tritium production. Taleyarkhan claims his interest in bubble fusion began following a post-dinner chat with a friend, Dr. Mark Embrechts, in 1995.

Oak Ridge failed replication

These experiments were repeated at Oak Ridge National Laboratory by D. Shapira and M. J. Saltmarsh but using more sophisticated neutron detection equipment. They reported that the neutron release was consistent with random coincidence. A rebuttal by Taleyarkhan and the other authors of the original report said that the Shapira and Saltmarsh report failed to account for significant differences in experimental setup, including over an inch of shielding between the neutron detector and the sonoluminescing acetone. Taleyarkhan et al report that when these differences are properly considered, the Shapira and Saltmarsh results are consistent with fusion.

In addition, Galonsky has shown that by Taleyarkhan's own detector calibration the observed neutrons are too high in energy to be from a deuterium-deuterium (d-d) fusion reaction. In a rebuttal comment, Taleyarkhan says the energy is "reasonably close" to that which is expected.

In February 2005, the BBC documentary series Horizon commissioned a collaboration between two leading sonoluminescence researchers, Seth Putterman and Ken Suslick, to reproduce Taleyarkhan's work. Using similar acoustic parameters, deuterated acetone, similar bubble nucleation, and a much more sophisticated neutron detection device, the researchers could find no evidence of a fusion reaction. This work was reviewed by a team of four scientists, including an expert in sonoluminescence and an expert in neutron detection, who also concluded that no evidence of fusion could be observed.

Subsequent reports of replication

In 2004, new reports of bubble fusion were published by the Taleyarkhan group, saying that the results of previous experiments have been replicated under more stringent experimental conditions. These results differed from the original results in that fusion was occurring for a much longer time frame than previously reported. The original report only showed neutron emission from the initial bubble collapse following bubble nucleation, whereas this report showed neutron emission many acoustic cycles later. The data, however, was less than stringent insofar as too large a window of measurement was used to determine a coincidence between neutron emission and sonoluminescent light emission. Furthermore, the energy of the detected neutrons was not consistent with neutrons produced from a fusion reaction.

In July 2005, two of Taleyarkhan's students at Purdue University published evidence confirming the previous result. They used the same acoustic chamber, the same deuterated acetone fluid and a similar bubble nucleation system. In this report, no neutron-sonoluminescence coincidence was attempted. Once again, the neutron energies measured were not consistent with those of neutrons produced by a d-d fusion reaction.

A paper published in the journal Physical Review Letters by researchers from Rensselaer Polytechnic Institute reports statistically significant evidence of fusion: The initial news report, however, shows that the reaction does not always work correctly and it is not known what parameters change to cause the reaction to function properly or not function at all.

In November 2006, in the midst of charges leveled at Taleyarkhan as regards his research standards, Dr. Edward R. Forringer and undergraduates David Robbins and Jonathan Martin of LeTourneau University presented two papers at the American Nuclear Society Winter Meeting that reported replication of neutron emission during a visit to the meta-stable fluids research lab at Purdue University. Their experimental setup was similar to the preceding experiments in that it used a mixture of deuterated acetone, deuterated benzene, tetrachloroethylene and uranyl nitrate. Notably, however, it operated without an external neutron source and used two types of neutron detectors. They claimed a liquid scintillation detector measured neutron levels at 8 standard deviations above the background level, while plastic detectors measured levels at 3.8 standard deviations above the background. These measurements were within one standard deviation for the same experiment with a non-deuterated control liquid, indicating that the neutron production had only occurred during cavitation of the deuterated liquid.

Doubts prompt investigation

Reports as spectacular as the above arouse a lot of doubt. In March 2006, Nature published a "special report" "silencing the hype" that called into question the validity of the results of the Purdue experiments. The report quotes Brian Naranjo of the University of California, Los Angeles to the effect that spectrum measured in these sonofusion experiments is consistent with radioactive decay of the lab equipment and hence does not reliably demonstrate the presence of nuclear reactions.

The response of Taleyarkhan et al, published in Physical Review Letters, attempts to refute Naranjo's hypothesis as to the cause of the neutrons detected.

Doubts at Purdue University's Nuclear Engineering faculty as to whether the positive results reported from sonofusion experiments conducted there were truthful prompted the university to initiate a review of the research, conducted by Purdue's Office of the Vice President for Research. In a March 9, 2006 article entitled "Evidence for bubble fusion called into question", Nature interviewed several of Taleyarkhan's colleagues who suspected something was amiss.

In his piece "On Science, Journalism, and Nature", New Energy Times writer Steven Krivit poses some questions about Nature's coverage of the affair.

On February 7, 2007, the Purdue University administration determined that "the evidence does not support the allegations of research misconduct and that no further investigation of the allegations is warranted". Their report also stated that "vigorous, open debate of the scientific merits of this new technology is the most appropriate focus going forward." In order to verify that the investigation was properly conducted, House Representative Brad Miller requested full copies of its documents and reports by March 30, 2007.

On March 7, 2008, Taleyarkhan filed a civil complaint against his boss Tsoukalas and colleague Jevremovic with the state of Indiana regarding events that occurred in the Purdue School of Nuclear Engineering.

In June 2008, a multi-institutional team including Taleyarkhan publishes a paper in Nuclear Engineering and Design to "clear up misconceptions generated by a webposting of UCLA which served as the basis for the Nature article of March 2006", according to a press release.

On July 18, 2008, Purdue University announced that a committee with members from five institutions has investigated 12 allegations of research misconduct by Rusi Taleyarkhan. It concluded that two allegations were founded—that Taleyarkhan had claimed independent confirmation of his work when in reality the apparent confirmations were done by Taleyarkhan's former students and was not as "independent" as Taleyarkhan implied, and that Taleyarkhan had included an additional colleague's name on one of his papers who had not actually been involved in the research ("the sole apparent motivation for the addition of Mr. Butt was a desire to overcome a reviewer's criticism," the report concluded). However, in a previous press release in July 2005, Purdue University had said that Butt's replication was independent from Taleyarkhan

Taleyarkhan appealed the conclusions in the report, but this was rejected. He said the two allegations of misconduct were trivial administrative issues and had nothing to do with the discovery of bubble nuclear fusion or the underlying science, and that "all allegations of fraud and fabrication have been dismissed as invalid and without merit — thereby supporting the underlying science and experimental data as being on solid ground".

On August 27, 2008 he was stripped of his named Arden Bement Jr. Professorship, and forbidden to be a thesis advisor for graduate students for at least the next 3 years.

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