The SEM image included in this brief paper shows the presence of spherical voids, bubbles, in a thin palladium foil as compared to known helium appearance in other metals. The foil was loaded with deuterium via the use of intense cavitation while immersed in D₂O. During the experiment a large signature of anomalous heat (>100 watts) in excess of the acoustic energy input was observed. The original experiment was performed in 1996 at SRI International (Menlo Park, CA) under contract to the Electric Power Research Institute (Palo Alto, CA) as part of a program to demonstrate anomalous nuclear heating via the techniques employed (see EPRI report). In May 1997 using a scanning electron microscope we were able to inspect a fracture cross section of the foils which revealed the internal structure.

The SEM effort revealed spherical voids or bubbles which appear to decorate grain boundaries. Strings of bubbles line up in an orderly fashion roughly following grain boundaries. The bubble/voids of interest measured here between 100nm and 1000nm are consistent with similar helium bubbles seen in various metals and widely reported in the scientific literature. Larger voids may have an alternative origin such as a micro nuclear explosions as referenced in the brief paper presented elsewhere on this web site.

Phenomena which create helium bubbles in metals are well known and have an extensive literature. They are created when metals are subjected to intense radiation, implanted with helium, or in the case palladium having been used to store tritium for a long period during which ³He is produced by tritium decay. Helium in metal has very low mobility and cannot escape. It does however tend to accumulate in the spherical voids as seen in the following image.

SEM Image provided by Sandia Lawrence Laboratories showing characteristics of helium in vanadium metal.

Sandia Labs image of helium in Niobium as a
result of helium ion Implantation.

References suggest the pressure of helium in such voids ranges between 0.5Gpa and 2Gpa (at which pressure the helium in the bubbles may approach that in a metallic state). The very high pressure in the bubbles is presumed based on calculations of the energy required to distort the metal lattice. Looking at the bubble distribution in the lattice as apparent from the image above we can estimate that there are in the neighborhood of 10⁹ bubbles in the target foil. Each bubble is estimated to contain approximately ~10¹⁰ atoms of helium. This would suggest the total foil contains ~10¹⁹ atoms of helium entrained in voids (see detail below). One might logically imagine that much more helium is distributed in the lattice not visible to our technique allowing for a higher total helium concentration. Perhaps the total helium produced by the reactions in the lattice is 10²⁰ - 10²¹ atoms in the 3 gram target foil. This number compares quite closely to the work of Arata who reports observing >10²¹ atoms of helium in 3-5 gm samples of palladium from experiments which loaded palladium to a high D:Pd ratio and produced anomalous heat. Arata assumes a nuclear process is responsible for the creation of helium isotopes in the lattice. Of great interest in the work of Arata is the observation of a ³He to ⁴He ratio of 1:4 rather than the normal abundance of 1:800,000 (Journal of the High Temperature Society of Japan Jan 1997).

The observation of helium bubbles in the metal helps link several other interesting observations from this work. Helium loading and helium bubbles in the lattice help to account for the physical damage the metal suffers during the experiments. As the literature reports helium loaded metals are substantially weakened and suffer dramatic damage. In addition in earlier work in association with the Naval Research Laboratory similar Pd foils from our experiments were observed via x-ray diffraction and were shown to be characteristically that of a stable highly loaded lattice. In work using helium mass spectroscopy at the US Bureau of Mines Helium lab, SRI's MS lab, and analytical services of Rocketdyne /Rockwell we have measured on many occasions 10¹⁷ -10¹⁸ atoms of helium in the reactor gases from our experiments. The levels of helium observed were 50ppm -500ppm considerably beyond the atmospheric concentration of ~5ppm, isotopic studies proved the helium was not of any known origin.

A review of noble gases in metals is available in a NATO Advanced Research Workshop paper edited by Donnelly and Evans - Nato ASI Series B 1991. Another paper providing an overview is Noble Gas Inclusions in Materials by Fleischer and Norton in Heterogeneous Chemistry reviews vol. 3 1996.