Researchers Raise New Doubts About 'Bubble Fusion' Reports
By Robert Service
March 17, 2006
Bubble fusion is again generating heat, but not the kind Rusi Taleyarkhan was hoping for. Last week, Purdue University in West Lafayette, Indiana, announced that it was launching a review into allegations that Taleyarkhan, a nuclear engineer at Purdue and the field's chief proponent had obstructed the work of Purdue colleagues by removing shared equipment, declining to share raw data, and trying to stop them from publishing results that countered his own published work.
The allegations, which Purdue University Provost Sally Mason calls "extremely serious," were first made public in last week's print and online issues of Nature. The review also follows a meeting in Taleyarkhan's lab, attended by other researchers trying to replicate his work, at which Taleyarkhan attempted to demonstrate bubble fusion in action. Several participants say the attempt was a dismal failure. And, adding more heat to the debate, a new analysis of data in Taleyarkhan's latest publication casts doubt on the source of a purported signature of fusion.
In an interview with Science, Taleyarkhan says he was blindsided by the charges. "It came as a major shock to me when I first heard about it on Tuesday [7 March]," Taleyarkhan says. The following day, Taleyarkhan met with the university administration and agreed to the review. "We decided we as a university need to provide a point-by-point response." Taleyarkhan says.
Embattled. Fusion researcher Rusi Taleyarkhan says inquiries will support
his lab's evidence of nuclear reactions in hot collapsing bubbles.
Evidence that fusion occurs at the heart of collapsing bubbles has been controversial from the beginning. Fusion, the process that powers the sun, normally takes place under intense pressures and temperatures needed to cause atomic nuclei to smash together with enough force to combine, giving off intense energy in the process. On Earth, fusion researchers have tried to replicate the process with the help of intense lasers and magnetic fields. But 4 years ago, Taleyarkhan, then at Oak Ridge National Laboratory in Tennessee, and colleagues published a paper in Science claiming that the pressure and heat at the center of collapsing bubbles in an organic solvent had also produced the telltale signature of fusion (Science, 8 March 2002. pp. 1808 and 1868). The work held out enormous hope, because if it could be scaled up, it promised near-limitless energy.
In their experiments, Taleyarkhan and his colleagues started with a small cylinder of acetone, a common organic solvent, in which ail the hydrogen atoms had been replaced by deuterium, a sister isotope with an additionaI neutron. The researchers bombarded the cylinder with intense ultrasound and zapped the deuterated acetone with a pulse of neutrons or, in the group's most recent experiment, alpha particles. The combination caused bubbles to form, swell, and then collapse, producing a tiny flash of light, a phenomenon known as sonoluminescence. According to the authors, it also fused pairs of deuterium atoms, creating either tritium and a proton or helium-3 and an extra neutron, which were counted by the group's detectors.
The work drew fire from other researchers who either could not reproduce the results or challenged it on theoretical grounds. Since the original Science paper, Taleyarkhan and colIeagues have published two other papers in Physical Review E and Physical Reviews Letters (PRL) - both prestigious peer-reviewed journals -offering further evidence of bubble fusion. But the effect has yet to be confirmed by researchers who have not been affiliated with Taleyarkhan at one time.
It hasn't been for lack of effort. Last year, the U.S. Defense Advanced Research Projects Agency (DARPA) supported efforts by Seth Putterman, a chemist at the University of California, Los Angeles (UCLA), to replicate Taleyarkhan's results. Taleyarkhan and sonoluminescence expert Kenneth Suslick of the University of Illinois, Urbana-Champaign, also received funding. With independent confirmation still lacking, on 1 March. DARPA convened a contractors' meeting in Taleyarkhan's lab at Purdue in hopes that they could all see tabletop fusion in action. But Putterman and others at the meeting say it didn't go well. "The trip from the point of view of reproducing his experiment was a waste of time," Putterman says.
For starters, the acoustic device that generates the bubbles wasn't working well, says meeting attendee Felipe Gaitan, chief scientist at Impulse Devices, a company in Grass Valley, California, working to commercialize bubble fusion. Instead of creating a largely clear solution with a few bubbles that would concentrate the acoustic energy, the acetone was clouded with bubbles. "We expected he wouldn't see any [results]," Giaitan says. But Taleyarkhan claimed the experiment was producing fusion.
Rather than measuring fusion's excess neutrons with a standard devicee called a scintillation detector, however, Taleyarkhan measured them with plastic neutron traps. The devices are common among nuclear engineers but not among researchers, because they can't measure the precise energy level of recorded neutrons - an important clue to their source. Taleyarkhan says that, unlike scintillation detectors, plastic traps need not he calibrated and they show irrefutable evidence of the presence of neutrons. But Putterman notes that because plastic traps take hours to process, the group had no time for control experiments needed to interpret the results. "It was very frustrating," Gaitan adds.
At the meeting. Putterman also presented calculations made by his graduate student Brian Naranio that questioned the conclusions of Taleyarkhan's most recent paper, published in January. The calculations suggested that the energy levels of the neutrons Taleyarkhan reported are not what the Purdue group should have seen if deuterium atoms were in fact fusing. Instead, Naranjo said, the results are a far better match for what the scintillation detector would have registered in the presence of californium-252, a radioisotope commonly used in nuclear laboratories.
Putterman says Taleyarkhan told him he does have californium-252 in his lab but keeps it enclosed in a shielded vault. Robert Block, a nuclear engineer at Rensselaer Polytechnic Institute in Troy. New York, and a co-author with Taleyarkhan, argues that cosmic rays and other background neutrons in the experiment could have made its readings resemble the expected signature of californium. But Putterman counters that when Naranjo calculated how the detectors would register radioactive cesium and cobalt that are used to calibrate the device, the result was a near-perfect match to the calibration data Taleyarkhan published in his paper. Still, Taleyarkhan says, it's hard for him to assess Naranjo's work until it has been published in a peer-reviewed journal. (Naranjo says he has submitted the work to PRL.) "We are trying to address the issues brought up by UCLA." Taleyarkhan says. And that will be done through publications. "That's how we believe science should be conducted," he adds.
Nevertheless. it now appears that DARPA is preparing to pull the plug on the effort to replicate Taleyarkhan's results. When a DARPA representative at the 1 March meeting suggested that the "successful" experiment be crated up and shipped to UCLA for independent verification, Putterman says, Taleyarkhan balked, saying he was too busy with teaching and research commitments.
Because Putterman's lab has been unable to independently verify the results, the agency says the program won't proceed. "If that had been successful, DARPA would have considered moving into a second phase that would have focused on whether the results can be scaled up," DARPA spokesperson Jan Walker said in a statement on Friday.
Despite the latest round of controversy, Putterman and other sonoluminescence researchers all say the idea of bubble fusion remains worth exploring. Unlike the discredited notion of cold fusion in which deuterium atoms supposedly fuse in a hunk of palladium metal, collapsing bubbles are calculated to produce temperatures in the millions of degrees, possibly high enough in that tiny volume to allow atoms to fuse.
For now, however, the immediate hurdle for Taleyarkhan will be convincing Purdue officials that the effect and his methods are sound. In guidelines issued late last week, Purdue officials said their review would be conducted by three senior Purdue professors and overseen by Peter Dunn, Purdue's associate vice president for research. An initial fact-finding phase will be completed by 1 June. Mason said the results of the review will be made public. Taleyarkhan says he is confident he will be vindicated: "We stand by whatever data we have presented" he says.
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