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Sonic Fusion
Scientists have reported that by bombarding a liquid with sound they were able to produce nuclear fusion in a tabletop apparatus. But their colleagues doubt it.
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Donald Kennedy, editor of the prestigious journal Science, knew he was in for a row if he published the paper. It’s not that the work was shoddy or came out of left field. On the contrary, the experiments had been performed with great care by well-respected senior scientists at Oak Ridge National Laboratory (ORNL), Rensselaer Polytechnic Institute (RPI) and the Russian Academy of Sciences.
But what the authors were claiming was just so extraordinary: that nuclear fusion reactions, of the sort that power stars and hydrogen bombs, had been created on a lab bench using little more than a vibrating ring, a neutron gun and a beaker of specially prepared acetone. Add to that the fact, reported in the Washington Post, that at least three of the experts to which the article had been sent for peer review urged Science to reject it. And finally there was the follow-up study (not yet subjected to peer review) by another team at Oak Ridge that claimed that the evidence of fusion reactions disappeared when it repeated the experiment with different sensors and analyzed the data in a different way.
"It goes without saying that we cannot publish papers with a guarantee that every result is right," Kennedy hedged in an editorial that accompanied the article in the March 8, 2002, issue of Science. "What we are very sure of is that publication is the right option, even—and perhaps especially—when there is some controversy.
History Repeats?
Controversy is the only thing assured to follow an experiment that so resembles the "cold fusion" fiasco of 1989, when Stanley Pons and Martin Fleishmann of the University of Utah said that they had discovered room-temperature reactions; the announcement became headline news but was soon discredited. There are important differences, however. In this case the scientists who believe they have found a new route to fusion have suggested a plausible mechanism by which it could occur. And they have discovered two genuinely odd anomalies that conventional physics cannot easily explain.
The phenomenon, as described by Rusi Peri Taleyarkhan of ORNL, Richard T. Lahey of RPI and their coinvestigators, happened when they were studying sonoluminescence—light created by sound. German scientists first observed sonoluminescence in the 1930s, when they immersed sonar loudspeakers in water baths. But it wasn’t until the past decade that scientists worked out many of the details.
What we call sound is really a series of moving pressure fronts. The pressure at a fixed point swings from low to high and back as the sound wave sweeps by. If the sound is loud enough and at the right frequency, the pressure at the trough of the wave will be so low that the fluid will boil, producing microscopic bubbles. When the high pressure front at the crest of the sound wave slams into these bubbles, they implode, and shock waves focus the energy of the implosion to a central region of atomic dimensions. The temperature at that central point skyrockets above 10,000 degrees Celsius, the pressure zooms to 10,000 atmospheres and a flash of light emerges for just a few picoseconds. The bigger the bubble, the more energy in the implosion, and the hotter and brighter the sonoluminescent flash.
Star in a Jar
Standard sonoluminescence experiments use water. Taleyarkhan’s group used an organic chemical called acetone, an ingredient in common nail-polish remover, because it is rich in neutron-absorbing carbon and hydrogen atoms. The researchers then loaded up the acetone with extra neutrons in two ways. First, they used acetone made from deuterium, which is hydrogen with an extra neutron. Second, they put the flask of acetone next to a source of neutron radiation, in one case a chunk of plutonium-beryllium and in other cases a neutron pulse gun.
Their hope was that the neutrons shooting into the acetone would collide with the carbon and hydrogen nuclei, and this would create disturbances that would "seed" the bubbles produced by the sound waves. Many more bubbles than normal would be formed at once, and on average the bubbles would grow much larger than usual before they collapsed. Perhaps, the scientists thought, the bubbles would get so big that their collapse would produce temperatures near 10 million degrees—hot enough to cause a few deuterium atoms in the acetone to fuse into helium or tritium (hydrogen with two extra neutrons).
 Image: courtesy of Rusi Taleyarkhan SOUND OF NEUTRONS. Click here to download a Quicktime Movie showing the nucleation of tiny--smaller than a molecule--vapor pockets when neutrons from a source strike the nucleus of atoms of acetone. These vapor bubbles then grow in the "stretched" liquid (in which the pressure is about minus 250 psi) to a cloud of hundreds of bubbles about six millimeters in size. The bubbles then collapse when the pressure turns positive. Collapse speeds reach near 10 kilometers per second or so and the final pressures reach to more than 50 million atmospheres upon which sufficient heat and compression is built up; neutrons and tritium are emitted. The intense collapse results in shock waves that travel outwards of the chamber through the glass walls and make an audible sound. |
Creating even small numbers of fusing atoms would be a big deal. Fusion reactions release lots of energy, hence their usefulness for lighting stars and making mushroom clouds. The energy comes out in the form of neutrons humming along at 2.5 million electron volts (MeV), fast-moving protons and hot tritium and helium atoms. When the Taleyarkhan group checked the samples for tritium, the researchers found that it had indeed increased—but only in the deuterium-laced acetone that had been zapped with both sound and neutrons. Tritium levels didn’t change significantly in normal acetone put through the process, nor in deuterated acetone shot just with neutrons or subjected only to a good ringing.
They also looked for neutrons emerging from the flask after the neutron gunshot had dissipated and the bubbles had burst. Sure enough, their scintillation detector started scintillating about twice as fast within a few microseconds of the strongest sonoluminescent flashes. Working through a complicated set of calculations, the researchers reckoned that they observed a four-percent increase in 2.5 MeV neutrons just after the onset of bubble formation. That is certainly not enough to start a chain reaction (thank goodness), or even enough to produce as much energy as the apparatus consumes. But if it were confirmed, it would be an entirely new approach to generating fusion energy.
The Race to Test the Results
Unsurprisingly, many research groups around the world are scrambling to try this out for themselves. But the only one to make a report so far has disputed on several technical grounds the evidence that any atoms were fusing, though the group did allow that something strange was going on. Dan Shapira and Mike Saltmarsh, the group's leaders, had been asked last May by science managers at ORNL to check the Taleyarkhan group’s findings.
Shapira and Saltmarsh brought in a different kind of neutron detector that is 30 times the size of the scintillator that the first team used. (A bad idea, Taleyarkhan complained in a rebuttal, because it is more likely to pick up background radiation and to overload the electronics.) The new detector system was triggered by a neutron or gamma-ray strike, and then matched that to any sonoluminescent flash that happened within 10 microseconds before or after the strike. (But that dilutes the signal, because neutron/gamma hits are much more common than flashes, complains the Taleyarkhan group, whose detectors worked the other way around.)
Saltmarsh and Shapira did not check the tritium observations. "Those look like they were handled correctly," Shapira says. He can offer no explanation for the apparent increase in tritium levels. So that is one mystery.
A second mystery, Shapira reports, is that "right after the neutrons hit the acetone, there are light flashes as the bubbles collapse, then there is a quiet period, and then thousands of flashes—90 percent of the light—comes out after about a millisecond. Why that happens I don’t know."
Shapira does know what he would do differently to answer the question more clearly. "For starters I would not use neutrons to create the bubbles—I would use a laser or even a charged particle beam, something you can really control. You cannot guide neutrons." And it would be better, he suggests, not to set the acetone flask on a steel table, which can reflect neutrons back toward the detector. Finally, he advises, use a more advanced detector that uses boron or an ionization chamber. That will filter out gamma rays, which confounded both his and Taleyarkhan’s measurements.
With stakes so high and so many reputations on the line, the debate over this discovery is certain to produce lots of sound and heat—but perhaps also a flash of illumination.
RELATED LINKS:
Ask the Experts: Can Ultrasound Bubbles in Water Initiate Nuclear Fusion?
Tabletop Nuclear Fusion Claims Meet with Skepticism
"Sonoluminescence: Sound into Light," by Seth J. Putterman (Scientific American, February 1995), is available for purchase from the Scientific American Archive.
Follow up by Shapira and Saltmarsh