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Physical Review E, Vol. 69, p. 36109-1, (2004)

R. P. Taleyarkhan,(1) J. S. Cho,(2) C. D. West, R. T. Lahey, Jr.,(3) R. I. Nigmatulin,(4) and R. C. Block(3)

1. Purdue University, West Lafayette, Indiana 47907, USA
2. Oak Ridge Associated Universities, Oak Ridge, Tennessee 37830, USA
3. Rensselaer Polytechnic Institute, Troy, New York 12180, USA
4. Russian Academy of Sciences, 6 Karl Marx Street, Ufa 450000, Russia
Received 13 May 2003; published 22 March 2004

ABSTRACT

Time spectra of neutron and sonoluminescence emissions were measured in cavitation experiments with
chilled deuterated acetone. Statistically significant neutron and gamma ray emissions were measured with a calibrated liquid-scintillation detector, and sonoluminescence emissions were measured with a photomultiplier tube. The neutron and sonoluminescence emissions were found to be time correlated over the time of significant bubble cluster dynamics. The neutron emission energy was less than 2.5 MeV and the neutron emission rate was up to ~4x10^5 n/s. Measurements of tritium production were also performed and these data implied a neutron emission rate due to D-D fusion which agreed with what was measured. In contrast, control experiments using normal acetone did not result in statistically significant tritium activity, or neutron or gamma ray emissions.

INTRODUCTION

The intense implosive collapse of bubbles, including acoustic cavitation bubbles, can lead to extremely high compressions and temperatures, and to the generation of light flashes attributed to sonoluminescence ~SL!. The modeling and analyses of the basic physical phenomena associated with such a process have been discussed elsewhere [1 and the key phenomena are depicted schematically in Fig. 1. Figure 1(a) shows the start of bubble implosion, during the compression phase of the impressed acoustic pressure field, when the gas/vapor Mach number is much less than unity. As the interfacial Mach number approaches unity a compression shock wave is formed in the gas/vapor mixture and, as shown schematically in Fig. 1(b), this shock wave (dashed line) moves toward the center of the bubble and, in doing so, intensifies. Figure 1(c) shows the situation just after the shock wave has bounced off itself at the center of the bubble which highly compresses and heats a small core region near the center of the bubble. At this point we normally have a SL light pulse, and if we have a suitable (e.g., deuterated) liquid in which the bubble temperatures, density and their duration are large enough, we may also have conditions suitable for nuclear emissions (i.e., nuclear fusion). Interestingly, these emissions and the pressurization process continue until a short time later when the interface comes to rest [see Fig. 1(d)]. Figure 1(e) shows the onset of bubble expansion during the rarefaction phase of the impressed acoustic pressure field, and Fig. 1(f) shows that a relatively weak shock wave is formed in the liquid surrounding the bubble during bubble expansion. As will be described later, for sufficiently violent implosions of relatively large bubbles this shock wave is normally heard by the experimenters after it reaches the wall of the test section in which the experiment is being performed

Our aim was to study the ultrahigh compression effects and temperatures in vapor bubbles nucleated in highlytensioned liquids by means of fast neutrons, whereby the bubble radius increases from an initial radius (R0) of tens of nanometers to a maximum radius (Rm) in the millimeter range. This results in a related volumetric expansion ratio which is huge [1] compared to that obtainable in conventional SL experiments (where Rm ~10R0).

Such an approach,with its vastly increased energy concentration potential during implosions, gives rise to very much higher peak temperatures and densities within the imploding bubbles, possibly leading to DD fusion and detectable levels of nuclear particle emissions in suitable deuterated liquids. Indeed, we have previously presented evidence [1] for neutron emission and tritium production during cavitation experiments with chilled deuterated acetone. Comments received [2–6] on the previously published results [1] suggested the need for improved instrumentation and data gathering; for enhancing our understanding of the timing and rates of neutron and gamma ray emission activity; for improving the efficiency for the detection of neutron emissions during bubble implosions; and for addressing the potential for significant chemical effects from cavitation in the tritium measurements. Specifically, in the data resented previously [1], time spectra were obtained only for the period corresponding to the first implosion of the nucleated bubbles. In this paper, we report results of investigations using improved and additional instrumentation. These additional data fully support our previous results and provide complementary evidence of nuclear emissions from cavitating chilled deuterated acetone that are indicative of deuterium-deuterium (DD) fusion.

EXPERIMENTAL SYSTEM
The experimental test apparatus [Fig. 2(a)] was kept similar to that used for the results reported previously [1]. The test liquid was placed in an approximately cylindrical Pyrex glass test section and driven acoustically with a leadzirconate- titanate (PZT) piezoelectric driver ring attached to the outside surface of the test section. This induced an acoustic standing wave in the test section with a pressure antinode of amplitude ~+/-15 bar. A new liquid scintillator (LS) detection system was set up for pulse-shape discrimination (PSD), and was used for the detection of neutron and gamma ray signals with the instrumentation shown in Fig. 2(b). As compared to the data acquisition system used earlier [1], the new system [7] includes fast multichannel scaling (MCS) capability that could be used to obtain time spectra of the neutron and SL signals over the entire time span of experimentation. A multichannel analyzer (MCA), operated in the pulse height mode, was used to obtain pulse height data with and without gating, and also for gating of gamma ray signals.

In the experimentally observed sequence of events [Fig. 2(c)], neutrons from a pulse neutron generator (PNG) nucleated bubbles in the tensioned liquid when the cavitation threshold was exceeded at the time of the PNG neutron burst. Thereafter, the vapor bubbles grew until increasing pressure in the liquid during the second half of the acoustic cycle caused them to collapse. If the collapse was robust enough(~i.e., an implosion occurred), the bubble emitted a SL flash which was detected by a photomultiplier tube (PMT) [7]. If the vapor contains sufficient deuterium (D) atoms, and the conditions are appropriate for DD fusion, nuclear particles (neutrons and gamma rays) would be emitted and seen in the response of the LS detector. Subsequent to the first implosion the bubble cloud may undergo periodic growth and energetic collapses at the 19.3 kHz frequency of the forcing acoustic pressure field. This process is repeated until the bubbles condense, and there can be neutron and gamma ray emissions during the subsequent implosions, however, the yield can differ from that during the initial implosion. Shown in Fig. 2(d) are typical photographic images of bubble clouds taken 1 ms apart in acetone at ~3 °C. It is seen that the bubble clouds persist in the pressure antinode of the test section for ~5 ms prior to condensing, and reach bubble cloud sizes in the range of ~6 mm in diameter. The liquid was first degassed, as reported on previously[1], by acoustically cavitating the liquid under vacuum (~10 kPa) with neutrons for ~2 h. This process is important since gassy vapor bubbles do not exhibit the desired intense implosive collapse characteristics as clearly evidenced from shock traces measured by microphone signals. Subsequent to degassing the liquid, the PNG was operated at ~;200 Hz (i.e., at a rate 100 times smaller than the acoustic driving frequency) during which neutrons were emitted over a time span of ~15 ms [6–7 ms full width at half maximum (FWHM)]. The PNG burst was initiated when the liquid tension was greatest (i.e., at 215 bar). For these conditions a bubble cloud was formed and rapidly expanded. Later, when the impressed acoustic pressure increases, it imploded emitting a burst of closely spaced SL flashes over an ~15– 20 ms time interval (each of ~5 ms duration).

LIQUID SCINTILLATION (LS) DETECTOR CALIBRATION
Careful calibration of the response of the LS detector was conducted using gamma rays from Cs-137 and Co-60 sources, 14 MeV neutrons from a PNG, and emissions from a plutonium-beryllium (Pu-Be) isotope source. The results of these calibrations are shown in Fig. 3(a). The pulse height spectra for Cs-137 and Co-60 are also shown on a stretched scale in the lower plot; a relatively sharp Compton edge results from the monoenergetic ~0.67 MeV gamma ray emission from Cs-137, whereas, a somewhat broader edge results for Co-60 due to the two closely-spaced 1.17 and 1.33 MeV gamma rays. Note also that the Pu-Be source exhibited the well-known 4.4 MeV Compton edge associated with the deexcitation 12C* gamma ray. The pulse height response of the LS detector matched the well accepted light output for EJ-301/NE-213 type detectors [8,9]. The ~2.5 MeV proton recoil edge is known [10] to lie between the Compton edges for gamma rays from Cs-137 and Co-60. Moreover, the ratio of light output for 14 MeV neutrons to that for 2.5 MeV neutrons was found to be ~11.7 as expected [10]. Using a Pu-Be source with a known neutron emission rate, the net efficiency for the detection of fast (mainly <4 MeV) neutrons was estimated [11] to be ~6x10^-4. Figure 3(b) displays time spectra of gamma and neutron signals with and without pulse shape discrimination (PSD). For PSD the discriminator settings were chosen to reject more than 95% of the gamma rays. As expected [9], the fractional counts associated with the gamma rays was found to be ~45% of the total counts in the time spectrum for the Pu-Be source. In addition, the LS detector was carefully calibrated with monoenergetic neutrons at the RPI LINAC, and these calibrations agreed very well with the in situ ORNL calibrations discussed above.

EXPERIMENTAL OBSERVATIONS FOR C3H6O AND C3D6O
We conducted experiments with standard acetone, C3H6O (100 atom% pure), and deuterated acetone, C3D6O (certified 99.92 atom% D-acetone), filtered before use through 1 mm filters. Degassing and other aspects of the forcing pressure amplitude were kept the same as for the conditions reported previously [1]. The pressure amplitude at the acoustic pressure antinode in our test section was maintained at a nominal value of ~+/-15 bar.

NEUTRON AND SL SPECTRA DATA ACQUISITION
We used well-established pulse shape discrimination (PSD) techniques in experiments with and without cavitation to check for neutron production and time correlations with SL emission data. Time spectra were obtained using 1000 channel MCS boards with 5 ms dwell times, such that the neutron counts during PNG operation, which occur within the first 25 ms after the trigger signal is transmitted to the PNG, are well separated from the signals during ubsequent events all the way to 5000 ms, which is just before the next PNG trigger pulse was transmitted. It was verified that, for identical settings without cavitation, the collective PNG neutron output of counts over 50 s duration was stable and varied by only ~+/-1% from measurement to measurement. Time spectra data were collected and statistically significant counts were obtained. Representative neutron and SL time spectra are shown in Figs. 4 through 7. Figure 4 shows the MCS cumulative neutron count spectra vs time after the PNG fires for tests with C3D6O. These spectra were accumulated for 10 000 sweeps, each of 5 ms duration. Figure 4(a) is with cavitation while Fig. 4(b) is without. It can be seen that excess neutrons were only detected when there were cavitation bubbles. It is interesting to note in Figs. 4(a) and 7(a) that there is a significant peak during the implosion associated with the first sonic cycle of period 52 ms, which occurs about 20 ms after the PNG neutrons emanate, and then there appears a time span of low neutron counts extending to channel ~100 (~500 ms) which is ~10 acoustic cycles. Starting at channel ~100 (500 ms) the neutron counts start to grow significantly to a peak near channel ~180 (~900 ms) and then asymptotically decrease to a lower level around channel ~500 (2 500 ms), reaching values about 10 times smaller than in the peak channels. In comparison, the non-cavitation spectrum shown in Fig. 49(b), which was obtained by shifting the phase of the PZT driver, showed a peak lasting ~15 ms from the PNG source neutrons followed by rapidly decreasing neutron counts to a relatively constant background level which persisted out to the end of the sweep at channel 1000 (5 ms). It should be noted that these data trends are consistent with previously reported experimental observations(www.rpi.edu/;laheyr/SciencePaper.pdf).

We interpret the spectrum of Fig. 4(a) as cavitation induced neutron production from D-D reactions in the periodically imploding bubble cloud. On the other hand, the noncavitation spectrum of Fig. 49b) shows the PNG source neutron peak followed by a rapidly decaying counting rate to channel ~10 (~50 ms); this decaying count rate is attributed to neutron capture of the PNG source neutrons and possibly the ~5% of gamma rays which may also be counted when the LS detector is operated in the PSD mode. It is also worth noting that the counting rate in Fig. 4(a) from channels ~500 to 1000 remains greater than the background counts in the corresponding region of Fig. 4(b); we interpret this as a small amount of cavitation-induced neutrons from the periodically imploding bubble cloud.

Figures 4(c) and 4(d) present the corresponding time spectra for the cases of cavitation on and off with gating on gamma rays (i.e., we rejected the counts associated with neutrons). As noted therein, the emission spectra do not display the trends observed for neutrons and provide confidence that the spectrum presented in Figs. 4(a) and 4(b) are quite distinct and represent neutron emissions. The aspect of gamma ray counts with and without cavitation for deuterated and natural acetone is covered in greater detail in a later section. As can be seen in Fig. 7(a), the increase of counts (at an overall rate of up to ~250 cps) starts about 30 ms after PNG neutron emanation (i.e., during the initial implosion of the bubble cloud which was formed) and the cumulative number of counts, between a 30 ms to 5000 ms time span, was found to be comparable to the counts collected during PNG firing; these counts during bubble implosion varied from run to run between a factor of ~0.3 to ~1.5 times the total number of neutron counts that were measured for in ~15 ms time bins corresponding to PNG firing, and, as such, they provide an additional useful benchmark for estimating the neutron emission rate due to bubble implosion in C3D6O. The counts during PNG neutron emission were essentially constant from run to run (i.e., well within 1 SD). Assuming Poisson statistics, the change in counts with cavitation that were recorded after PNG operation, for tests with chilled C3D6O, represents a very significant increase of more than 60 SD’s [13] above background. Figures 5(a) and 5(b) show, respectively, the neutron count spectrum for the control liquid C3H6O with and without cavitation. These spectra are very similar to Fig. 4(b) and show no evidence of cavitation-induced neutron production.

Figures 6 display representative results for the sonoluminescence (SL) emission spectrum using C3D6O; the spectrum for C3H6O was found to be similar. As expected, there were no significant SL emission signals unless cavitation bubble implosions took place. Interestingly, however, unlike in Fig. 4(a), there is not a large ‘‘dead time’’ evident in Fig. 6(a). This indicates that the implosions for the first ten or so acoustic cycles after the bubble cloud implosion were not energetic or numerous enough to induce significant DD fusion, but were able to produce SL light flashes. A plot (for the first 100 ms) of the results for neutron and SL activity is shown in Figs. 7(a) and 7(b), respectively. The time correlation of the neutron and SL emissions is evident during the initial implosion of the bubble cloud. A composite plot of the raw neutron and SL data with cavitation off and cavitation on over the entire 5000 ms sweep time is shown in Fig. 7(c). The strong time correlation between neutron and SL emissions is evident. This is also seen with binning in Figs. 7(d) and 7(e) that, in the time interval 0.5 ms to 2.0 ms, the SL emissions are also strongly time-correlated with neutron emissions, a result which indicates that D-D neutron emissions, subsequent to the initial neutron-induced bubble nucleation in chilled C3D6O, also occur during subsequent, sufficiently energetic, bubble cloud implosions. The binning process used is depicted in the accompanying table to Fig. 7(d0. Figure 7(e) displays an aggregate of all neutron counts in the two-channel bins corresponding to neutron peak (and corresponding SL counts) occurrence versus the channels in between the peaks. As is clearly seen, the aggregate counts per channel in the peak channels are very significantly greater (by factor of ~50) than for the channels in between the peaks. The corresponding SL counts are also higher by ~500%. In contrast, if the neutron pulses had been random they would be spread out over the total time duration with no such correlation being observed.

We also measured the energy (i.e., pulse height) spectra of the neutron counts with and without cavitation for experiments with C3H6O and C3D6O to determine the energy level of the neutrons that are emitted. Representative results, shown in Figs. 8(a), 8(b), and 8(c), were taken with the Spectrum Techniques UCS-20™ MCA used in collecting the neutron counts. It should be noted that a statistically significant increase (.25– 50 SD) in neutron emission occurred [14] only when there was cavitation and only when chilled C3D6O was the test liquid. In Fig. 8(c)we see that such an increase of counts takes place sharply at and below the ~;2.5 MeV PRE, indicating the emission of neutrons that are peaked in energy at ~2.5 MeV. No such change was noted in the higher energy channels, and none for conditions without cavitation, and also none for tests with the control liquid, C3H6O as seen from Fig. 8(b). The neutron emission rate from our MCS/MCA data acquisition system (which varied between ~60 and ~225 cps from run to run), when divided by the calibrated efficiency for fast (<4 MeV) neutron detection gives a net emission rate varying from ~(1–4)x10^5 n/s (at ~2.5 MeV), which, as will be discussed below, is well within experimental uncertainties for the inferred neutron emission rate of ~(3.5– 5)x10^5 n/s from the tritium data.

GAMMA RAY SPECTRA DATA
D-D fusion neutrons generated by imploding cavitation bubbles immersed in liquid C3D6O can be expected to interact with the various surrounding materials and structures resulting in gamma ray emissions (principally from the capture of thermalized neutrons by hydrogen in the liquid scintillator and boron in the Pyrex glass test section).

Therefore, to evaluate whether a measurable change in gamma ray emission could be detected, we conducted experiments with and without cavitation for chilled C3H6O and C3D6O, obtaining the time and energy spectrum of counts in both the gamma ray and neutron regions. The results, shown in Fig. 9, indicate that for cavitation in chilled C3D6O the gamma ray counts increased. In various different runs, the increase of gamma ray counts varied from 10% to 20% of the increase in neutron counts. The increase of counts in the neutron region has been noted earlier to be >60 SD. The 10–20% increase in counts in the gamma region time window amounts to a change of >10 SD which is also very statistically significant. This implies that some of the D-D neutrons emitted are captured by the experimental apparatus. No such change was noted for tests with C3H6O. Typical C3D6O gamma ray emission energy spectra with cavitation (less those without cavitation) are shown in Fig. 10. It was found that the increase of gamma ray emissions was mainly (>80%) below 2.2 MeV (as would be expected from the capture of thermalized neutrons by hydrogen and the boron in the Pyrex glass test section). These results confirmed the accompaniment of <2.5 MeV neutrons with a statistically significant emission of gamma rays for cavitation experiments with C3D6O, but not for tests with the control liquid, C3H6O.

CONFIRMATORY TRITIUM EXPERIMENTS
Following the experimental procedures developed previously [1], additional tests [12] were conducted for seven hours to reconfirm [1] that a statistically significant quantity of tritium (T) was generated during cavitation in chilled C3D6O (i.e, at ~0 °C). These tests were conducted with deuterated acetone in a newly fabricated test section, and they confirmed earlier findings [1], and indicated an increase (over background! in tritium counts in the range of 4.5 to 5.9 cpm @with a 1 standard deviation (SD) of about 3 cpm]. This represents an individual difference of up to ~2 SD and a collective change of more than 3 SD. These data (i.e., tritium decay at 4.5 to 5.9 cpm) imply an average neutron production rate of between ~3x10^5 n/s to ~5x10^5 n/s, respectively. Testing in the new chamber with natural acetone (C3H6O) resulted in a change that was well within 1 SD (i.e., no statistically significant change was found), in agreement with previous findings [1]. For reference, these new confirmatory data are shown together with past data [1] in Fig. 11.

To check for possible chemical effects, tests were also conducted using a powerful acoustic horn [12] immersed into C3H6O, and separately into C3D6O, to evaluate whether cavitation of the type produced using conventional means would produce any statistically significant change in the readout for tritium content. The temperature of the acetone was maintained at ~0°C and an acoustic horn operating at an input power level of ~300 W was used to induce robust cavitation at 20 kHz. The results revealed a negligible change in counts in samples taken before and after cavitation, amounting to about 0.1 cpm ~with 1 SD =~3.5 cpm) for C3D6O, and about 1 cpm ~with a 1 SD =~2 cpm) for C3H6O. These changes are well within 1 standard deviation (SD), indicating negligible chemical effects contributing to scintillation activity in the 5 to 18 keV beta decay window characteristic of T decay. These data indicate that the effect of any chemical activity on tritium measurements during cavitation in our experiments is well below that caused by nuclear fusion between deuterium atoms. Therefore, cavitation with SL emanation from deuterated liquids by itself does not lead to an increase of T counts. Indeed, statistically significant quantities of tritium were found to be generated only when we conducted cavitation experiments following the experimental process outlined in Figs. 2 in chilled deuterated acetone (C3D6O); a finding which was also supported by hydrodynamic shock code simulations [1]. Previously we had also conducted tests with cavitation at liquid temperatures of ~20 °C and we did not find any statistically significant change in counts for C3D6O nor C3H6O [1}. Finally, it should be noted that these findings were also supported by hydrodynamic shock code simulations [1].

SUMMARY AND CONCLUDING REMARKS
Large and statistically significant emissions of ~2.5 MeV and below neutrons were noted during cavitation experiments in chilled deuterated acetone. This neutron emission was well separated in time from the neutrons generated by the PNG (used to nucleate the cavitation bubbles) and was time correlated with SL light emissions during bubble implosion events. The neutron output during cavitation in chilled deuterated acetone was found to be between ~1x10^5 n/s and ~4x10^5 n/s. Statistically significant increases in tritium were also measured during cavitation of chilled C3D6O and the amount of tritium produced was consistent with the measured DD neutron emissions. Statistically significant gamma ray emissions were also measured in cavitation experiments with chilled C3D6O. No statistically significant change in neutron or gamma ray emissions and no significant tritium generation were observed when there was no cavitation in C3D6O and in control tests with C3H6O, both with and without cavitation.

ACKNOWLEDGMENTS
Sponsorship of this research by the Defense Sciences Office of the U.S. Defense Advanced Projects Agency is gratefully acknowledged. We wish to acknowledge the one-on-one in-depth technical involvement, review, comments,and suggestions for improvement provided by Dr. Jack Harvey of Physics Division, Oak Ridge National Laboratory (ORNL). Additionally, we are grateful to Professor Lee Riedinger, of UT-Battelle, LLC-ORNL and Dr. Glenn Young of Physics Division, ORNL for their in-depth technical reviews and for arranging and coordinating intensive technical reviews over several months by a multitude of scientific staff at ORNL. We further sincerely thank and acknowledge technical reviews by Professor Mark Embrechts and Professor Y. Danon of Rensselaer Polytechnic Institute, Troy, NY, Professor William Bugg, Physics Department and Physics Division jointly of The University of Tennessee, Knoxville, TN and ORNL, Professor Larry Miller of the University of Tennessee, Knoxville, TN and ORNL, Dr. Larry Dubois of Stanford Research Institute (previously of DARPA), and Ross Tessien, Dr. Felipe Gaitan, and Dr. William Mead of Impulse Devices, Inc. We also thank Dr. M. Murray, ORNL, for in-depth and direct technical assistance for tritium measurements, and E. Bickel of Channel Industries, Inc., Dr. F. Bergamo of Activations Technologies, Inc., Dr. R. Stevens of Spectrum Techniques, Dr. D. Gedcke of Ortec (Ametek), and Dr. D. Neville of Canberra, Inc. for their valuable advice on nuclear data acquisition. The support and oversight received from Dr. W. Madia of UT-Battelle, LLC-ORNL were also valuable and highly appreciated.

[1] R. P. Taleyarkhan et al., Science 295, 1868 ~2002!; R. Nigmatulin, R. T. Lahey, Jr., and R. P. Taleyarkhan, Science Online, www.sciencemag.org/cgi/content/full/295/5561/1868/ DC1, 2002.
[2] C. Siefe, Science 295, 1808 (2002).
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[4] A. Galonsky, Science 297, 1645 (2002); D. Shapira, M. Saltmarsh, R. P. Taleyarkhan, R. C. Block, C. D. West, and R. T. Lahey, Jr., ibid. 297, 1645 (2002); R. P. Taleyarkhan and S. Putterman (private communication).
[5] B. Levi, Phys. Today 55(4), 16 (2002).
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[7] The Alpha Spectra LS detector has dimensions of 5 cm (diameter) by 5 cm (length). The scintillation liquid was EJ-301™ which is essentially the same as NE-213™. The test cell (a Pyrex flask ~200 mm high365 mm O.D.), with a PZT driver, was designed, fabricated, and set up at ORNL for experiments. The MCA (UCS20™) used was procured from Spectrum Techniques, Oak Ridge, TN, USA; the MCS-PCI™ was procured from Ortec (Ametek), Oak Ridge, TN, USA and the AccuSpec- FMS MCS was procured from Canberra, Inc. SL detection was performed with a (2-ns rise time) PMT.
[8] N. P. Hawkes et al., Nucl. Instrum. Methods Phys. Res. A 476, 190 (2002).
[9] G. F. Knoll, Radiation Detection and Measurement (Wiley, New York, 1989).
[10] J. Harvey and N. Hill, Nucl. Instrum. Methods 162, 507 (1979).
[11] The Pu-Be source used for efficiency and other calibrations emits ~2x10^6 n/s, a large fraction of which has energies below 4 MeV. The efficiency of detection of these neutrons by the LS detector located about 18 cm away was found to be ~2x10^-4. This efficiency was corrected for the actual distance (~7 cm) of the LS detector from the cavitation region, and by a factor of 2 for the attenuation from interaction with acetone atoms, giving a net detector efficiency of ~6 x10x-4, a value which is more than ten times greater than the corresponding value reported earlier [1]. This is primarily at attributed to the improved detector and electronics used along with a lower bias level. The PNG emitted monoenergetic 14.1 MeV neutrons at a rate that is estimated to lie between ~(5–7)x10^5 n/s. The threshold was set at (0.7 MeV (corresponding to ~0.7 MeV proton recoil energy).
[12] A Beckman LS6500™ scintillation counter, calibrated to detect 5- to 18-keV beta ray decays from T, was used. A 1-cm3 sample of test liquid was withdrawn from fluid in the top region in the acoustic chamber after testing and mixed with 15 cm3 of Ecolite™ scintillation cocktail in a borosilicate glass vial. When testing with C3D6O or C3H6O, with or without irradiation or cavitation, we used the same experimental configuration, including placing the chamber under standard vacuum conditions. An acoustic horn system (by Misonix™ Corporation) was operated at 20 kHz and half of the ~660 W nominal power to result in intense cavitation of the acetone liquid. This power level was about 10 times the power level used to drive the cylindrical PZT affixed to the glass chamber. The tip was immersed into the liquid in the test section which was held under vacuum. A cold air line maintained the temperature of the system at ~0 °C.
[13] In Fig. 4, for the time span after PNG operation (from 30 ms to 5000 ms), counts collected with and without cavitation were 9869 and 2699, respectively. Assuming Poisson statistics, 1 SD amounts to [square root of] 9869 +2699 =~112 counts. Therefore, the 7170 count increase above background represents a change of
~64 SD.
[14] In Fig. 8 the data presented were taken with a Spectrum Techniques UCS20™ MCA in which the background counts from the PNG were included. For the specific discriminator settings and for these test runs with deuterated acetone with and without cavitation, 8347 and 5337 neutrons were counted for the channels below the 2.5 MeV cutoff, with 4193 and 4231 counts above the 2.5 MeV cutoff, respectively. Assuming Poisson statistics, 1 SD amounts to ~117 counts. Therefore, the 3010 count increase above background in the <2.5 MeV range represents a change of ~26 SD. Corresponding 1 SD values are shown in Fig. 8 for the various cases cited therein.

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