Characterization of helium bubble size and faceting by electron holography

Iron-9% chromium alloy of controlled purity and microstructure was helium implanted and studied using transmission electron microscopy and electron holography (EH). Fresnel imaging is compared with electron holography for a set of identical helium bubbles of size 2-4 nm. Electron holography reveals that the bubbles are, in fact, faceted and the lateral and projected dimensions of the cavities are measured. Current measurements allow the detection of 1 nm diameter bubbles using EH. The merits of conventional and holographic techniques are discussed and future improvements anticipated. Electron holography seems to be the ideal technique for investigating the morphology of helium bubbles and cavities in general.