Researchers at Oak Ridge National Laboratory, Rensselaer Polytechnic Institute and the Russian Academy of Sciences have reported results, soon to be published in Science, that suggest the possibility of nuclear reactions during the explosive collapse of bubbles in liquid, a process known as cavitation.
The bubbles, which grow in the presence of sound waves, collapse to produce locally high pressures and temperatures. These pressures and temperatures can be sufficiently high to result in light emissions, called sonoluminescence, from the collapsing bubbles.
The collaboration was led by Rusi Taleyarkhan, a senior scientist in ORNL’s Engineering Science and Technology Division, and Richard Lahey Jr., the Edward Hood professor of engineering at Rensselaer. The team used 14 million electron volt (MeV) neutrons shot into the liquid by a pulsed neutron generator to nucleate the bubbles.
These conditions are believed to result in a significant increase in the final pressure of the collapsing bubbles. This suggests the possibility of producing densities and temperatures necessary for nuclear reactions. In particular, a long-sought goal of sonoluminescence research has been the possibility of achieving nuclear reaction conditions.
Experiments suggest the presence of small but statistically significant amounts of tritium above background resulting from cavitation experiments using chilled deuterated acetone. This tritium could result from the nuclear fusion of two deuterium nuclei. Tritium was not observed during cavitation of normal acetone, which does not contain deuterium.
Attempts to confirm these results by looking for the telltale neutron signature of the deuterium fusion reaction have yielded mixed results. While there are indications of neutron emission in the newly published results, subsequent experiments with a different detector system show no neutron production.
Theoretical estimates of the conditions in the collapsing bubbles are consistent with the possibility of nuclear fusion, under certain assumptions concerning the relevant hydrodynamics.
These results suggest the need for additional experiments, said ORNL’s Lee Riedinger, deputy director for Science and Technology. In particular, the difference in the two sets of neutron measurements must be clarified. Additional tritium experiments would also allow a better understanding of the tritium observations.
Until confirmatory experiments are completed, a cautionary view is appropriate, according to Riedinger, who said, ‘The manuscript has been through external peer review, but the scientific record shows that tritium and neutron measurements at these levels are difficult, and one must do further tests before firm conclusions can be drawn.’