Has Nuclear Fusion Been Observed in a Bubble Tank?

A team of scientists has claimed evidence for deuterium-deuterium fusion in a tabletop apparatus at Oak Ridge National Lab (Taleyarkhan et al., Science, 8 March 2002), but other scientists (including a separate group at Oak Ridge) are raising serious concerns about the validity of the result.

In their experiment, Taleyarkhan et al. (a collaboration of scientists from Oak Ridge, Rensselaer Polytechnic Institute and the Russian Academy of Sciences) utilize sonoluminescence (SL), itself a well-studied and highly regarded area of research (see, for example, Updates 34, 299, 307, 327, and 355), in which powerful sound waves sent into a liquid tank trigger the creation of single or multiple bubbles which then collapse and release short flashes of light.

Sonoluminescence, literally the conversion of sound into light, is a remarkable process in that sound itself is not a densely packed form of energy. Even the sound in the most powerful car stereo has a much lower energy density than the light in a penlight laser beam.

In an SL experiment, however, the energy from the sound wave gets focused into a very small region, namely a collapsing bubble. This highly concentrated energy heats the gas inside the bubble to incandescent temperatures resulting in the release of light. The conversion of sound energy into light energy represents an energy concentration of over a trillion.

Researchers have long speculated whether the conditions inside the collapsing bubbles could be made to approach the high temperatures and densities necessary to trigger energy-producing nuclear fusion reactions such as those that occur inside the sun. This is a great matter of debate, as some details of the bubble collapse and light emission are still incompletely understood.

With this incomplete knowledge, researchers cannot discount the possibility that the conditions can be tweaked to generate nuclear fusion, modest as these fusion reactions are likely to be.

However, according to leading sonoluminescence theorist William Moss of Lawrence Livermore National Laboratory, "We are all pretty sure that normal SL conditions are nowhere near fusion temperatures--typical SL temperatures don't exceed 11,000 degrees Kelvin or so, at least from theoretical estimates"---as opposed to the millions of degrees that nuclear fusion would typically require.

In the newly reported experiment, many details are similar to a traditional SL setup: researchers aimed 19.3-kHz sound waves at a glass flask containing deuterated acetone. But here's the novel part of the experiment: a pulsed neutron generator injected 14.3 MeV neutrons into the flask, in sync with the sound waves.

The researchers claim that the neutrons trigger the creation of extremely small bubbles which then grow to relatively large sizes and then collapse to generate pulses of light. In conjunction with the light pulses, the researchers report the detection of significant amounts of tritium and evidence for neutrons with an energy of 2.5 MeV. Such neutrons would be produced in the fusion of deuterium atoms in the glass flask. They repeated the experiment with normal acetone (lacking deuterium) and did not detect the tritium or neutrons.

However, another group at Oak Ridge, consisting of D. Shapira and M.J. Saltmarsh, attempted to reproduce the experiment, except for the fact that they used a larger neutron/gamma-ray detector and what they report to be a more sophisticated data acquisition system. They found a 1% increase in the neutron/gamma ray signal when the experiment was set up to trigger cavitation (formation of bubbles), as opposed to when the sound wave was turned off.

However, they did not find the 10-fold increase that they expected if the reported tritium levels occurred as a result of deuterium-deuterium fusion. And they found nothing when they looked for neutrons or gamma rays being emitted in coincidence with the light pulses.

Outside researchers who have studied the Science paper have expressed very significant concerns about its validity. According to Moss, the key measurement is the 2.5 MeV neutron peak.

"If measured neutrons are thermonuclear in origin, then there must be a peak at 2.5 MeV, and measuring and reporting that peak constitutes a minimum requirement to support the claim of thermonuclear origin," he says. "Tritium production (claimed in the paper) is not sufficient evidence, since it is difficult to determine the source."

Moss rejects the conclusions of the paper based on the "lack of a properly resolved neutron peak." He says, "Extraordinary claims require unambiguous data, which they did not provide. This doesn't mean that thermonuclear neutrons from a sonoluminescence source are impossible, only that they didn't show data to support the claim."

Seth Putterman, a leading sonoluminescence experimentalist at UCLA, points out that the researchers claim a 1000-to-1 production of output neutrons to input neutrons that hit the acoustically sensitive region of the resonator.

It should be possible, he says, to turn this data into a huge signal and a clearly detectable neutron spectrum, but this is not presented in the paper. Putterman also points out that no other sonoluminescence paper to his knowledge has ever reported detecting a single neutron as a result of the SL process.

The authors of the Science paper have invited other researchers to attempt to reproduce the experiments. They say that they have reanalyzed the Shapira and Saltmarsh data and find that these data are actually compatible with sonofusion and provide an independent confirmation of their controversial claim.

However, according to Putterman and Moss, the experiment by Taleyarkhan et al. does nothing to resolve the question of whether acoustic cavitation can generate nuclear fusion reactions. "The actual scientific experiment appears to be flawed," Putterman says. "If confirmed, however," adds sonoluminescence pioneer Lawrence Crum of the University of Washington, "it would be a remarkable result, demonstrating that mechanical systems could induce nuclear reactions." However, Crum also adds, "I am very skeptical that their results will ever be duplicated."

"This is an interesting, high-risk direction of research that should go on," Putterman says. "These results may be so premature and so flawed, however, that it may taint future attempts in the field."