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Questions Regarding Nuclear Emissions in Cavitation Experiments
By M. J. Saltmarsh and Dan Shapira
Science

Science 6 September 2002:
Vol. 297. no. 5587, p. 1603
DOI: 10.1126/science.297.5587.1603a
[Received] 28 March 2002; Accepted 2 July 2002

Technical Comments

Questions Regarding Nuclear Emissions in Cavitation Experiments

Taleyarkhan et al. (1) claimed evidence for D-D (deuterium-deuterium) fusion in cavitation experiments with deuterated acetone. A number of inconsistencies in that study, however--involving data on neutron yield, the reported response of the detector used, and coincidences between sonoluminescence (SL) and scintillator pulses--cast doubt on that claim.

As pointed out by Taleyarkhan et al. (1), if the tritium observed were due to D-D fusion, it would need to be accompanied by a similar neutron yield. However, the reported neutron yield (4 × 10⁴ to 7 × 10⁴ neutron/s) was a factor of 10 to 20 lower than the reported tritium yield (7 × 10⁵ atoms/s) (1), a discrepancy that contradicts the claim that the tritium was due to D-D fusion. As discussed below, the effects cited in (1) as possible explanations for that discrepancy are not sufficient. Moreover, increasing the mismatch in the data, the claimed neutron yield was calculated based on an estimated detection efficiency for 2.5-MeV neutrons, which is a factor ~100 below levels that would be consistent with the reported detector set-up.

The detector used for the singles measurements [note 22 in (1)] was a liquid scintillator 5 cm thick and 5 cm in diameter. In describing the threshold setting of this detector, Taleyarkhan et al. stated that the cutoff was seen at channels 15 to 20 [figure 2.4(b) in supplement 1, online supplemental data for (1)] and that the 2.5-MeV edge lay around channel 40 [note 26 in (1)]. Thus, the effective threshold was set at or below a pulse height corresponding to 50% of the 2.5-MeV neutron edge. Using the standard benchmarked code SCINFUL (2), we calculated the detection efficiency for 2.5-MeV neutrons under those conditions to be 18.9% or greater. Taleyarkhan et al. also reported that the distance of the detector from the center of the test chamber was 5 to 7 cm. Choosing 6 cm as the mean, the solid-angle factor would be  × 2.5²/(4 ×  × 6²) = 4.3 × 10², which implies a net detection efficiency of 8 × 10³ for the experimental geometry--inconsistent with the reported efficiency of 1 × 10⁴ to 2 × 10⁴ [note 26 in (1)]. These simple calculations suggest that the reported neutron yield should be reduced by almost two orders of magnitude.

Taleyarkhan et al. argued that at least part of the neutron-tritium difference may have been due to three factors: (i) "neutron energy losses by scattering in the test chamber"; (ii) "reduced detection efficiency for large-angle knock-ons from 2.5-MeV neutrons"; and (iii) possible nonuniformities in T concentration in the acetone [(1), p. 1872]. An upper limit for effect (i) can be derived using the total reaction cross-sections for 2.5-MeV neutrons to calculate the probability of scattering before leaving the acetone, assuming that all such scatterings result in loss of the neutron. The detector apparently viewed the interaction region at an angle of ~45° [figure 1 in (1)], so that the distance traveled in the acetone was 4.5 cm; those values suggest an upper limit to the losses of about 48%, and decrease the effective efficiency by a factor of two.

Effect (ii) is already accounted for by the choice of detection threshold and is correctly treated in the SCINFUL code. Effect (iii) is difficult to evaluate quantitatively, but we note that the acetone was agitated by cavitating bubbles for 12 hours, during which any dissolved tritium would have been dispersed through the volume. Taleyarkhan et al. proposed no mechanism that would concentrate any tritium produced by the reactions in a small fraction of the acetone volume, while leaving the larger fraction (75%) of tritium already present as a contaminant fully dispersed.

The detector response reported by Taleyarkhan et al. is inconsistent with the accepted response for liquid scintillators. The reported pulse heights corresponding to 14-MeV and 2.5-MeV neutrons were at channels 110 and 40, respectively [note 26 in (1)]. From published light curves (3), the pulse heights corresponding to these energies must be in the ratio of 10:1. Assuming that the electronics used by Taleyarkhan et al. exhibited an approximately linear response, the reported pulse heights could thus only be explained by a 32-channel offset in the data; however, the spectra presented [figure 2.4(a) and (b) in supplement 1, online supplemental data for (1)] clearly show that any offset is less than 5 channels. Absent a reasonably consistent accounting of the detector response, the neutron data reported cannot be reliably interpreted.

Finally, Taleyarkhan et al. reported coincidences between SL and scintillator pulses. They acknowledged that an independent experiment showed that "the coincidences observed may be random in nature"; indeed, as we argue below, the data reported in (1) would also have been dominated by random coincidences.

In discussing this issue, Taleyarkhan et al. stated that "the influence of random coincidences between SL and scintillator flash signals in the region of bubble collapse was estimated to be insignificant." Their calculation that only 0.03 to 1.6 random events would be seen in a typical 1600 second run with a 20-µs coincidence window [note 30 in (1)] appears to support this statement. However, their coincidence experiment did not distinguish between events occurring during the period of bubble collapse and events occurring at other times, in particular during the PNG (pulsed neutron generator) pulse. For the period of the PNG pulse, Taleyarkhan et al. (1) reported an average count rate of 500/s (p. 1872), a pulse width 12 µs (p. 1869), and a repetition frequency of 200 Hz (caption to figure 5); thus, the instantaneous rate during this period was 500/(12 × 10^-6 × 200) counts/s = 2 × 10⁵ counts/s. The number of SL pulses seen during the time of bubble collapse was 1/s [note 30 in (1)]; the number seen during the PNG pulse was 30% of this, or 0.3/s [figure 3(a) in supplement 1, online supplemental data for (1)]. Substituting these two rates into the expression used in note 30 of (1), we find that the number of random coincidences to be expected in a 1600-s run is (20 × 10^-6 s) × (2 × 10⁵/s) × (0.3/s) × (1600 s) ~ 2000.

This calculation overestimates the expected number of random coincidences by about a factor of two, because the coincidence time used (20 µs) is longer than the PNG pulse width (12 µs). Even allowing for that overstatement, however, ~1000 random coincidences would still be expected per 1600 s run--about ten times the rate reported by Taleyarkhan et al. Furthermore, the time structure for those randoms would be very similar to the shape of the PNG pulse, i.e., a peak with a width [full width at half maximum (FWHM)] ~4 to 6 µs. [(1), p. 1869]. Because the observed coincidence rates were dominated by randoms, any measurement of true coincidences must include a sufficiently accurate estimate of random rates to be meaningful.

The experimental evidence cited by Taleyarkhan et al. for D-D fusion rests on three basic observations: excess tritium, excess neutrons, and coincidences between neutrons and sonoluminescence light. As we have detailed here, however, the study presented significant internal inconsistencies in the measurements of neutron singles and neutron/SL coincidences, as well as a very large quantitative mismatch between the tritium and neutron data. These inconsistencies cast serious doubt on the claimed evidence for D-D fusion in these experiments.

M. J. Saltmarsh
Dan Shapira
Physics Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831, USA
E-mail: shapirad@ornl.gov

REFERENCES

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