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The evolution of the lattice spacing and damage of tritium-charged tantalum during aging is studied by x-ray diffraction. The lattice spacing is found to increase only very weakly with the atomic helium concentration cH, built up upon tritium decay, while the lattice damage monitored by peak broadening increases signiAcantly.Kith the combination of the lattice-parameter rneasurements with recent dilatometric measurements it is concluded that already at very low cH, 2x10 tantalum self-interstitials generated in association with helium bubble formation induce an evolution of a dislocation network. This conclusion is shown to be consistent with the observed peak broadening.

A computer-controlled diffractometer has been built which permits intensity measurements to be made in any direction in reciprocal space in the diffraction plane with step sizes down to 0.01" of arc. Three examples illustrate the performance and application of the instrument: (a) perfect silicon, (b) gadolinium gallium garnet with growth striations and (c) niobium with low-angle grain boundaries.