A team of researchers from Rensselaer Polytechnic Institute, Purdue
University, and the Russian Academy of Sciences has used sound waves to
induce nuclear fusion (sonofusion)!
The results address one of the
most prominent questions raised after publication of the team’s earlier
results in 2004, suggesting that “sonofusion” may be a viable approach
to producing neutrons for a variety of applications.
By bombarding a special mixture of acetone and benzene with
oscillating sound waves, the researchers caused bubbles in the mixture
to expand and then violently collapse. This technique, which has been
dubbed “sonofusion,” produces a shock wave that has the potential to
fuse nuclei together, according to the team.
The telltale sign that fusion has occurred is the production of
neutrons. Earlier experiments were criticized because the researchers
used an external neutron source to produce the bubbles, and some have
suggested that the neutrons detected as evidence of fusion might have
been left over from this external source.
“To address the concern about the use of an external neutron
source, we found a different way to run the experiment,” says Richard
T. Lahey Jr., the Edward E. Hood Professor of Engineering at Rensselaer
and coauthor of the paper. “The main difference here is that we are not
using an external neutron source to kick the whole thing off.”
In the new setup, the researchers dissolved natural uranium in the
solution, which produces bubbles through radioactive decay. “This
completely obviates the need to use an external neutron source,
resolving any lingering confusion associated with the possible
influence of external neutrons,” says Robert Block, professor emeritus
of nuclear engineering at Rensselaer and also an author of the paper.
The experiment was specifically designed to address a fundamental
research question, not to make a device that would be capable of
producing energy, Block says. At this stage the new device uses much
more energy than it releases, but it could prove to be an inexpensive
and portable source of neutrons for sensing and imaging applications.
To verify the presence of fusion, the researchers used three
independent neutron detectors and one gamma ray detector. All four
detectors produced the same results: a statistically significant
increase in the amount of nuclear emissions due to sonofusion when
compared to background levels.
As a cross-check, the experiments were repeated with the detectors
at twice the original distance from the device, where the amount of
neutrons decreased by a factor of about four. These results are in
keeping with what would be predicted by the “inverse square law,” which
provides further evidence that fusion neutrons were in fact produced
inside the device, according to the researchers.
The sonofusion debate began in 2002 when the team published a paper in
Science indicating that they had detected neutron emissions from the
implosion of cavitation bubbles of deuterated-acetone vapor. These data
were questioned because it was suggested that the researchers used
inadequate instrumentation, so the team replicated the experiment with
an upgraded instrumentation system that allowed data acquisition
over a much longer time. This led to a 2004 paper published in Physical
Review E, which was subsequently criticized because the researchers
still used an external neutron source to produce the bubbles, leading
to the current paper in Physical Review Letters.
The latest experiment was conducted at Purdue University. At
Rensselaer and in Russia, Lahey and Robert I. Nigmatulin performed the
theoretical analysis of the bubble dynamics and predicted the
shock-induced pressures, temperatures, and densities in the imploding
bubbles. Block helped to design, set up, and calibrate a
state-of-the-art neutron and gamma ray detection system for the new
experiments.
The research team leaders are all well known authorities in the
field of nuclear engineering. Lahey is a fellow of both the American
Nuclear Society (ANS) and the American Society of Mechanical Engineers
(ASME), and is a member of the National Academy of Engineering (NAE).
Block is the longtime director of the Gaerttner Linear Accelerator
(LINAC) Laboratory at Rensselaer, and he is also a fellow of the ANS
and recipient of their 2005 Seaborg Medal, which recognizes an
individual who has made outstanding scientific or engineering research
contributions to the development of peaceful uses of nuclear energy.
Taleyarkhan, a fellow of the ANS and the program’s director, is
currently the Ardent Bement Jr. Professor of Nuclear Engineering at
Purdue University. Nigmatulin is a visiting scholar at Rensselaer, a
former member of the Russian Duma, and the president of the
Bashkortonstan branch of the Russian Academy of Sciences (RAS).