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Search for 3He and 4He in Arata-Style Palladium Cathodes II: Evidence for Tritium Production

W. Brian Clarke, Brian M. Oliver, Michael C. H. McKubre, Francis L. Tanzella, Paolo Tripodi

Fusion Science and Technology

Volume 40 / Number 2 / September 2001 / Pages 152-167


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Measurements have been made of 3He, 4He, and 3H in a sample containing 2.7% of the gas from the interior of an Arata-style hollow palladium electrode charged with ~5 g Pd-black that had undergone electrolysis in D2O as a cathode for 90 days and then as an anode for a further 83 days. There is no evidence for the much larger amounts of 4He observed by Arata and Zhang in similar experiments. However, a very large concentration has been found of 3He, 2.3 ± 0.5 × 1012 atoms/cm3 standard temperature and pressure that apparently can all be attributed to the decay of tritium produced during electrolysis. No direct production of 3He can be specified, a result that is also different from the conclusions of Arata and Zhang. The 3He and tritium measurements and the results of a gas analysis using a Finnigan-type mass spectrometer show that at the end of the anodic electrolysis, the electrode void contained 5.8 ± 0.7 × 1013 atoms tritium in the gas phase as HT, DT, and T2, and 1.7 ± 0.3 × 1015 atoms tritium in the aqueous phase as HTO, DTO, and T2O. At this stage, the gas phase pressure was ~18.8 atm in a free volume of 0.6 cm3, and the total mass of water was ~5.7 mg. The gas phase tritium value is viewed as a lower limit for gaseous tritium produced inside the electrode because some of that tritium must have been removed into the D2O electrolyte during the anodic episode.

The 3He and 4He measurements were also made in the two samples of the Pd-black and in sections cut from the walls of both Pd electrodes. The H2O electrolyzed samples did not show any evidence of unusually high 3He and/or 4He, but all the D2O electrolyzed samples showed clear evidence of 3He from tritium decay. A stepwise temperature heating experiment performed with a 24.9-mg sample of the D2O Pd-black showed that the diffusion process for 3He can be described by an equation of the form D = D0 exp(-U/kT) with an activation energy U of 1.1 eV. It is also apparent that the 3He from tritium is quantitatively retained in the Pd-black at room temperature.

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