A team of researchers from Oak Ridge National Laboratory, Rensselaer Polytechnic Institute, and the Institute of Mechanics of the Russian Academy of Sciences (RAS) has been using acoustic waves to agitate a flask of deuterated acetone, causing bubbles to expand and contract in phase with the sound. But the researchers really stirred things up in March when they claimed evidence for deuterium-deuterium fusion in their bubbles. 1 Presumably, the shock wave produced by each rapidly imploding bubble was sufficiently intense to force the deuterons together.
So far, however, the evidence proffered by the Oak Ridge-RPI-RAS collaborators, led by Oak Ridge’s Rusi Taleyarkhan, has not convinced many observers. As William Moss of Lawrence Livermore National Laboratory put it, “Extraordinary claims demand unambiguous proof.”
The critics are not saying that fusion couldn’t have occurred, only that its occurrence hasn’t been proven. Unlike claims for “cold fusion,” the idea of “bubble fusion” does not come out of left field. Bradley Barber, Seth Putterman, and their coworkers from UCLA suggested the possibility in 1994, when they were calculating the shock wave that might develop within a collapsing bubble containing deuterium. 2 Moss followed with a detailed numerical simulation of a collapsing bubble containing deuterium, 3 showing that it might be possible to attain high enough temperatures and pressures to overcome the repulsive forces between deuterons. But doing so requires that the bubble remain spherical long enough for the shock wave to converge tightly at the center. Several groups besides the Oak Ridge-RPI-RAS collaboration are now trying to achieve bubble fusion. But it’s not easy to do and it’s even harder to prove that it’s been done, as Taleyarkhan and coworkers are learning.
The new report attracted predictable media attention because of the association with an energy source. “We’re nowhere close to power generation,” says Taleyarkhan. Rather, he says, “the main discovery is the use of simple classical mechanics to induce and control a nuclear phenomenon.”
Interest in bubble fusion has stemmed from work on sonoluminescence (see the article by Lawrence Crum in Physics Today 0031-9228 47
During typical sonoluminescence experiments, spectral emission temperatures range up to tens of thousands of kelvins, and the rapidly imploding bubble walls can generate internal shock waves under certain conditions. The temperatures and pressures inside the bubble are not known. For fusion to occur, the interior would have to reach millions of kelvins, with pressures of hundreds of megabars.
To promote fusion, Taleyarkhan and company tried to achieve more extreme bubble conditions than in previous sonoluminescence experiments. First, they used deuterated acetone (C3D6O) so that fusionable fuel was present. To get a very high compression ratio, they used a beam of energetic (14 MeV) neutrons to generate tiny bubbles in their beakersized container of superheated deuterated acetone, estimating that the resulting bubbles will have a minimum radius of 10–100 nm. That’s five orders of magnitude smaller than the maximum radius the expanded bubble is expected to reach.
To avoid the resistance to collapse that’s frequently produced by residual vapors, the experimenters degassed the acetone. Finally, they drove the liquid with a very intense sound field. Team members performed one-dimensional hydrodynamic shock-code calculations for the conditions of their experiment to determine if fusion was possible.
Signatures of fusion
The fusion of two deuterons (d) can take two equally likely paths:
where t is a tritium nucleus and 3He is a helium-3 nucleus. To prove that fusion has occurred, one should show an increase in tritium and a proportional increase in neutrons having the characteristic energy of 2.5 MeV. To nail the proof, one should demonstrate that the neutrons come out in coincidence with the sonoluminescence generated as the bubble collapses.
After each of three runs of different durations, Taleyarkhan and company took samples from their deuterated acetone to determine how much tritium was present, as measured by its decay rate. For the longest run, the activity was highest: 68.9 ± 2.6 counts/min, compared to the tritium background decay of 53.4 ± 2.3 counts/min. The group estimated that the excess activity corresponds to the emission of 5–7 × 105 tritium atoms per second.
Unfortunately, tritium is a notorious and ubiquitous contaminant, and an impurity in deuterated acetone. To check for spurious readings, the experimenters compared results obtained with C3D6O to those from runs with normal acetone (C3H6O). They also did runs with and without cavitation. One would expect elevated tritium levels only when deuterons are present and when sound waves produce cavitation, and that’s what the Oak Ridge-RPI-RAS group found.
In searching for 2.5-MeV neutrons that might have been produced by fusion, the Oak Ridge-RPI-RAS team had to cull any fusion neutrons from the substantial background of 14-MeV neutrons used to seed the bubbles. At a rate of 200 pulses per second, the experimenters were sending about one million neutrons per second (n/s) into their chamber.
To determine whether excess neutrons were present, the experimenters used a scintillation counter to detect neutrons as a function of energy, using a pulse-shape discriminator to block out gamma rays. They took the difference in neutron counts with and without cavitation, first for energies at or below 2.5 MeV and then for energies above that. They saw a 4% excess of neutrons below 2.5 MeV, which corresponded to the generation of about 4–8 × 104 n/s, about 10 times smaller than the rate of 7 × 105 n/s that would be consistent with the tritium levels they found. Taleyarkhan now estimates that their neutron number would have been higher if his group had corrected for neutron losses within the chamber.
Moss would like to see the full energy spectrum for the neutrons with a maximum energy at the expected 2.5 MeV. Others have expressed concern about the neutron background, because 14-MeV neutrons from the incoming pulse are being scattered around the room and degraded in energy. Taleyarkhan counters that this effect should be the same in runs with or without cavitation.
Putterman points out that the amount of tritium reported by Taleyarkhan and his team amounts to about 1000 tritium atoms for every incoming neutron that passes through the active region of the cavitation cell. “That’s an enormous effect,” he says. He’s surprised that the best data show a neutron excess of only 4%. Overall, he comments, “the experiment is an excellent example of high-risk, high-gain research. While I can’t rule out the existence of a major discovery waiting to be uncovered, the paper does not provide evidence for it.”
The final signature of bubble fusion is the coincidence between the sonoluminescent light flashes from collapsing bubbles and the neutron emission. Both should be generated during the final stages of bubble collapse, within a time span much less than 10 ns.
For the coincidence measurements, Taleyarkhan and his collaborators triggered on the sonoluminescent light flash and looked for all signals from a scintillation counter that occurred within a specified time interval after a flash. (The scintillation counter for this measurement recorded both neutrons and gammas.) As seen in figure 1, the coincidence counts peaked in the 2-µs windows surrounding the light flash, but only when the cavitation was on and deuterated acetone filled the chamber.
Of course, a 2-µs window is very wide when one is looking for coincidences between events that occur less than 10 ns apart. Taleyarkhan says that “our goal was to see if we had a net effect of increased nuclear activity around a sonoluminescence peak.”
When Lee Riedinger, a nuclear physicist and deputy director of science and technology at Oak Ridge, learned late last spring about the research by Taleyarkhan and his colleagues, he suggested that the coincidence measurements be repeated by Daniel Shapira and Michael Saltmarsh of the lab’s physics division. Shapira and Saltmarsh took data with a bigger neutron detector and more sophisticated electronics, while Taleyarkhan conducted the rest of the experiment. 4
Shapira and Saltmarsh recorded the time sequence of coincidences between neutrons (or gammas) and light flashes (see figure 2), as well as the time sequence of single light flashes and single neutron (or gamma) events. To best reproduce the results of the Oak Ridge-RPI-RAS collaboration, they set the coincidence window open at 20 µs and looked for coincidences between neutrons or gamma rays, without discriminating between them (although they recorded the pulse height of each count so they could go back later and separate the two).
Shapira and Saltmarsh found that the observed coincidences were consistent with the expected rate of random coincidences. Looking at neutrons only, they reported an increase in background neutron counts for the run with cavitation over that without cavitation, but these counts were not correlated in time with the bubble collapses.
Taleyarkhan and company do not concur with Shapira and Saltmarsh’s conclusions. They point out, among other things, that because the pair’s detector did not fit within the experimental enclosure, it was placed outside, beyond some shielding. They also feel that Shapira and Saltmarsh’s detector threshold was set too high to capture many of the lower-energy neutrons. 5
In July, Shapira and Saltmarsh wrote up their results for the lab management only. Just before appearance of the Oak Ridge-RPI-RAS paper in Science, 1 however, the existence of this second measurement came to light, raising complaints from several Science referees that they had not seen all the relevant information. Now the formerly internal report is on the Oak Ridge Web site 4 and is being prepared for publication.
Richard Lahey Jr, an RPI participant in the experiment claiming fusion, said he’s very sure they’ve achieved d–d fusion. Instead of offering opinions, he says, critics need to try to reproduce their results.