In a recent Radsafe post, Mr. Estabrooks asked to start a radsafe discussion on a report of Acoustic Fusion by Taleyarkhan, West, Cho, Lahey, Nigmatulin, and Block (Evidence for Nuclear Emissions During Acoustic Cavitation, Science, March 8, 2002), an unpublished rebuttal by Shapira and Saltmarsh, and an unpublished rebuttal of the rebuttal by the original authors. I have summarized the results of the 3 papers. My own interpretations and opinions are marked with <>.
Evidence of fusion resulting from acoustically forced cavitation bubbles using deuterated acetone has recently been reported by Taleyarkhan et. al. The crux of method was to induce bubbles with a pulsed neutron generator in an acoustically forced environment. The acoustic field oscillates between tension and compression with a period of approx. 52 usec. The neutron burst is timed when the solution is acoustically under tension to initiate the generation of small bubbles. While the solution is under tension, the bubbles grow in size. When the solution becomes compressed, the bubbles implosively collapse. The claim is that ultra high compression temperatures formed from the collapsing bubbles are sufficient to yield a D-D fusion reaction.
The D-D reaction has 2 outcomes. The first leads to the production of He-3 and a 2.5 MeV neutron. The second leads to the production of tritium (H-3) and a proton. Taleyarkhan et al. sampled the solution for tritium and a measured neutrons generated to support the fusion claim.
When the bubbles collapse, light flashes attributable to sonoluminescence (SL) are generated and detected with a PMT. Neutrons should be detected in coincidence with the light flashes using a 2"x2" (5cmx5cm) organic liquid scintillation detector. The detection mechanism of fast neutron detectors is that as the neutron collides with hydrogen of the organic detector material, energy is imparted into a recoil proton which generates a scintillation flash. Pulse shape discrimination is often used to differentiate between gamma events and neutron events.
The highlights of the original Taleyarkhan et al. experiment are:
(1) The experiment was tested using both deuterated acetone and normal acetone. For each solution the system was tested with and without cavitation and with and without the use of the neutron generator.
(2)For the case of using deuterated acetone with the full experimental setup, the tritium activity increased from 53.4 +/- 2.3 counts to 68.9 +/- 2.6 counts (1 sigma). All other setups did not produce increased tritium.
(3) An increase in the total neutron counts was observed. The total neutrons detected in 100 seconds increased from 14,566 to 15,115 counts when cavitation was turned on. (The presumption is that the high background is from firing the neutron generator)
(4) Coincidences between the neutron detector and the SL flashes detected with the PMT were measured with a digital storage oscilloscope. The scope was triggered with the SL flash and coincident neutron traces recorded. An increase in the counts is observable in the graphical data representation when the cavitation experiment was turned on. The time period in which coincidences were observed was approximately +/- 2 usec from observation of the light flash.
(5) The analysis of bubble implosion dynamics and a modeling evaluation was also discussed.
<Personal Note: SL events have been known for a long time. Historical thinking has been that the temperature and pressures are not anywhere close to sufficient to generate nuclear fusion. I do not have the background to evaluate this statement nor any analysis of bubble dynamics. Nonetheless, this work presents convincing arguments of fusion, from both tritium production and neutron detection measurements.>
The findings are, of course, being disputed. The highlights of the rebuttal by Shapira and Saltmarsh are as follows:
(1) The original authors set up the reaction chamber. The method employed neutron counting but did not verify tritium production.
(2) The detector was a similar liquid scintillation detector but much larger. It had dimensions of 320 sq. cm by 18 cm (approx. 7"x7"x7"). The center of the detector was located 30 cm from the reaction chamber and was reported to have an intrinsic efficiency of approx. 0.3 and an overall efficiency of approx. 3E-03. The efficiency was estimated from first theoretical interactions and was not verified with a source. Shapira and Saltmarsh did not employ pulse shape discrimination to reject gamma interactions within the detector
(3) Shapira and Saltmarsh report seeing a slight but statistically significant increase in the total neutron/gamma counts following firing of the neutron generator when acoustic cavitation was turned on. This observation was discounted as the total observed counts was far less than what would be expected to be consistent with the observed tritium production rate.
(4) They measured and recorded the time dependence of both the PMT and neutron detectors relative to the neutron generator pulse. They also looked for looked for coincidences. Timing was initiated by the neutron/gamma signal and Shapira and Saltmarsh looked for coincident SL flashes within a 20 usec window for each neutron/gamma signal. Coincident SL flashes above the expected random rate were not found.
(5) Shapira and Saltmarsh also looked at coincidences between neutrons and SL flashes at a considerable length of time from the initial neutron generator pulse. Further coincidences were expected from a resonating expansion and implosion of the gas bubbles. No additional coincidences were found.
(6) Shapira and Saltmarsh concluded that there was no evidence of real coincidences between light flashes and neutron generation and that the overall total neutron production rate was at least 3 orders of magnitude less than what would have been expected from the reported tritium production rate.
<Personal Opinion: There are significant differences in the Shapira and Saltmarsh report in comparison to the original report. First of all, PSD was not employed, so that the detector would be sensitive to a high background gamma-ray flux that would be expected in the vicinity of a neutron generator. Secondly, Shapira and Saltmarsh used a 20 usec window to look for coincidences when the original work published coincidences within +/- 2 usec. Thirdly, the coincidence window was triggered with the neutron/gamma detector pulse and not the infrequent SL light flash.
It would have been difficult for Shapira and Saltmarsh to find the SL flash from a triggering a coincidence measurement from the neutron/gamma pulse when there was a large excess of neutron/gamma pulses from the local background and when the efficiency for observing a neutron from the real event is small and when the window for observing coincidences is 10x greater than it should be.
The neutron detector should have been shielded from background with a collimator, gated on the PMT signal, used pulse shape discrimination and used a narrow window of acceptance.
These types of detection electronics can be tricky. The systems usually have multiple NIM units of timing circuitry (each with several controls). The measurement is complicated by a large pulse from the neutron generator that initially saturates the detector followed by a sea of residual gamma and neutron background activity that varies with time. I’m a firm believer in performing some independent measurements to grossly substantiate a result and in spending considerable effort tracking down even minor discrepancies.
An additional point is that Shapira and Saltmarsh did not determine their efficiency empirically. Their paper does not indicate verification of operation nor verification of their theoretical estimate of the efficiency. Shapira and Saltmarsh did not analyze the solution for tritium, yet they used their results of neutron counting to discount the reported production of tritium. The increase in tritium activity is the real smoking gun. It is completely independent of any quirkiness that may result from a difficult measurement of low-level neutrons in a high and variable background. Shapira and Saltmarsh did report an increase in the total neutron production, but discounted the result without investigating the cause of the increased count rate.>
The highlights of the rebuttal of the Shapira and Saltmarsh rebuttal by the original authors (Taleyarkhan et al.) is as follows:
(1) Taleyarkhan et al. directly analyzed the instrument setup and data collected by Shapira and Saltmarsh.
(2) A Pu-Be source was counted and, combined with modeling, the efficiency of the Shapira and Saltmarsh detector was estimated to be about 1E-05. Using this new efficiency, reasonably good agreement was found between the previously reported tritium production rate and the slight increase in total neutron counts.
(3) Taleyarkhan et al. re-examined the data from the time period at a considerably time from the initial neutron generation pulse. (Events from a resonance of bubble generation and collapse) They were able to post-process the data, using a methodology not well described, to include a pulse shape discrimination filter so that the counts from the scintillation detector were only those counts attributable to neutron events. They then examined recorded neutron events within +/- 1 usec of the 19 recorded SL light flashes. They also reviewed the recorded neutron events in the time period immediately adjacent to the +/-1 usec window. They reported a total of 476 neutron counts from within the +/- 1 usec window and 375 counts from an adjacent window.
Taleyarkhan et al. then concluded that: (a) Using the new efficiency, a reasonably good agreement was found between the slight increase in total neutron counts of the Shapira and Saltmarsh system and the count-rate originally reported.; (b) Using the new efficiency, a reasonably good agreement was found between the previously reported tritium production rate and the slight increase in total neutron counts; (c) The large size of the Shapira and Saltmarsh system suffered severe electronic saturation during the time period approximately 27 usec after the neutron generator pulse and the Shapira and Saltmarsh system was not able to make valid measurements in this time period; and (d) it was impossible for Shapira and Saltmarsh to detect coincidences with their system in the configuration in which they were operating. The overall conclusion was that their original experiment was valid.
<Personal note: The evidence of Taleyarkhan et al. from both the original report and the rebuttal is strong and supports the conclusion that this is fusion events occurred.
When the experiments are repeated to verify the results, follow up work should measure tritium levels, which is the most foolproof indicator of fusion. Solutions can be archived for verification. Furthermore, all experimental systems should be well tested to ensure that the instruments yield measurements that are appropriate to support the conclusion.>