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Response to Questions Regarding Nuclear Emissions in Cavitation Experiments
By Rusi Taleyarkhan et al.
Science

Science 6 September 2002:
Vol. 297. no. 5587, p. 1603
DOI: 10.1126/science.297.5587.1603a
[Received] 15 April 2002; accepted 19 August 2002

Technical Comments

Response to Questions Regarding Nuclear Emissions in Cavitation Experiments

We appreciate the opportunity to clarify issues and respond to the comments of Saltmarsh and Shapira on our study (1).

First, Saltmarsh and Shapira suggest that the number of neutrons detected appears smaller than that deduced from the tritium data cited (1) and that the detector efficiency does not match predictions of the SCINFUL numerical code. The neutron emissions reported (1), however, when appropriately corrected for losses (2), are in the range of ~3 × 10⁵ to 4 × 10⁵ neutron/s, which is compatible (within one standard deviation of uncertainty) with the derived production rate of about 7 × 10⁵ atom/s from the tritium data. Saltmarsh and Shapira used the neutron detection rates (1) that were not corrected for losses in the acetone, glass, etc. (2).

The detection efficiency for 2.5-MeV and 14-MeV neutrons was actually measured in (1). Despite the availability of direct experimental evidence (3) for both independent detectors, Saltmarsh and Shapira relied on theoretical calculations for detector efficiency in which the effect of discriminator thresholds and counting efficiency for different-energy neutrons were not included. The SCINFUL code predictions of detector efficiency presented by Saltmarsh and Shapira do not match the count rates measured (2) from a Pu-Be source of known strength with either their detector or ours; we thus conclude that, for our detector at least, the computer model is not accurate. Statistically significant increases in neutron emission--on the order of 10 or more standard deviations of effective change--were consistently measured (1) only in the 2.5-MeV range during tests with chilled cavitated deuterated acetone, and not for tests with natural acetone. Strict control of system geometry was maintained for tests with the baseline and control liquids (deuterated and natural acetone, respectively). The corrected value of ~3 × 10⁵ neutron/s agrees well with the derived tritium emission data; the difference, a factor of ~2, is within detection uncertainties.

Saltmarsh and Shapira also argue that the light output values shown in the online supplemental data for (1) are inconsistent with expected values for NE-213 liquid scintillation detectors. Using published light output curves, a 14-MeV proton recoil edge channel number of 110 implies that the "theoretical" Compton edge for cesium-137 emissions, which lies just below the 2.5-MeV neutron energy cutoff, should be around channel 10; Saltmarsh and Shapira suggest that, because the number of neutron counts below channel number ~20 in (1) is close to zero and because channel numbers between 30 to 40 were used in (1) as corresponding to the cutoff range for 2.5-MeV neutrons, the neutron data in (1) are questionable. However, we carefully calibrated our detector for determining the channel range for 2.5-MeV neutrons using sharp Compton edges from cesium-137 and cobalt-60 sources and for higher energies using a Pu-Be isotope source and 14-MeV pulsed-neutron generator source. It is significant to note that with our multichannel analyzer (MCA) and system settings, there was an offset in pulse height, such that zero pulse height corresponded to approximately channel 21. When this shift is taken into account, the ratio of our observed pulse height for 14 MeV to that for 2.5-MeV proton recoils is ~6 to 8. The various channels corresponding to edges for cesium-137, cobalt-60, Pu-Be, and 14-MeV neutrons exhibited a linear variation of light output with energy. This is in line with, and well within, the spread of 20 to 50% uncertainties of experimental data conducted with several detectors of different size, shape, and age (3-7). Therefore, we see no incompatibility--although, in retrospect, we should have noted in (1) that correcting the neutron spectra for the ~21-channel offset is necessary for comparisons with light output from similar detectors.

Finally, Saltmarsh and Shapira, examining our coincidence data, argue that our report has not provided any evidence of real (i.e., nonrandom) coincidences. The coincidence measurements in (1) were conducted in two modes of operation. In Mode 1, no false SL signals occur; therefore, there were no false coincidences. However, real coincidences were monitored only for chilled, cavitated deuterated acetone [figure 5A in (1)]. In Mode 2, with a higher bias voltage to the photomultiplier tube (PMT), false SLs occur during PNG operation, and this will indeed lead to false coincidences. Some readers--Saltmarsh and Shapira in particular--have misinterpreted the statement about the fraction of false SLs occurring during PNG firing: The quoted value (1) of 30% for false SLs corresponds only to the SL events recorded by our MCA for the first 100 ms after PNG firing. The rate of false SLs is actually more than 10 times smaller than the value of ~0.3/s used by Saltmarsh and Shapira; if this correction is made, the number of random coincidences occurring in Mode 2 operation (during PNG firing) amounts to less than 100, which is in line with our reported measurements of 60 to 70 (attributed to false coincidences occurring during PNG operation).

With cavitation turned on, we recorded around 30 to 45 true coincidences out of 100 total coincidences in a typical run. This was only observed during cavitation of deuterated acetone with testing at ~0°C, an effect that disappeared for tests at ~20°C and was not observed for natural acetone at any temperature. The random coincidences during the period of bubble implosion are estimated at ~ <3 (1). Therefore, true coincidences were indeed observed during cavitation of chilled deuterated acetone during both Mode 1 and Mode 2 operation.

In addition to observing coincidences in a multitrace digital oscilloscope, we also obtained time-correlation data with conventional particle counting systems by taking time spectra using an MCA. The MCA data clearly showed (1) very statistically significant nuclear emissions (increases of 100% or more above background) precisely during the time interval corresponding to bubble implosion only for tests with deuterated acetone at ~0°C, an effect that was not observed for tests with deuterated acetone at higher temperatures nor for tests with the control fluid, natural acetone. Thus, two sets of measurements, oscilloscope and MCA, provided confirmation of time-correlated coincidence events.

Saltmarsh and Shapira conclude their comment by expressing doubt on our assertion that we have provided evidence of D-D fusion during acoustic experiments with deuterated acetone. We maintain, however, that our study (1) does indeed provide compelling evidence for fusion. That evidence includes the observation that statistically significant tritium activity increased only in chilled (~0°C) cavitated deuterated acetone; comparable-scale evidence for statistically significant neutron emissions (time correlated with SL emissions) in chilled cavitated deuterated acetone; the absence of neutron emissions and tritium production in irradiated control tests with natural acetone; and confirmatory HYDRO code (1) simulations that predict hot (~10⁶ to 10⁷ K) and highly compressed conditions within the bubbles imploding in these experiments.

R. P. Taleyarkhan
Oak Ridge National Laboratory
Oak Ridge, TN 37831, USA
R. C. Block^*
Rensselaer Polytechnic Institute
Troy, NY 12180, USA
C. D. West^*
Oak Ridge National Laboratory
R. T. Lahey, Jr.
Rensselaer Polytechnic Institute

REFERENCES

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