Dilatometric measurements of helium densities in bubbles arising from tritium decay in tantalum

The swelling rates of the bulk tritides ${\mathrm{TaT}}_{0.42}$ and ${\mathrm{TaT}}_{0.103}$ were measured at room temperature using the technique of strain gauges. Such swelling is expected in tritides because of the decay of tritium to ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and the subsequent precipitation of gas bubbles. Observations were made for up to 10 months. Almost-linear swelling was found in the first months for both tritides, indicative of a constant ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ density in the bubbles. The slopes of the linear parts of the expansion curves normalized to the T concentration were almost identical in the two cases considered. From this a ${}_{}{}^3{}_{}{}^{}\mathrm{Ta}$ atom volume ratio, $v_{\mathrm{He}/\mathrm{\Omega }\mathrm{\le }0.52\pm 0.03}$ in the bubbles was derived, which is in good agreement with recent but less direct spectroscopic measurements of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ densities in bubbles formed after room-temperature implantation into other metals. Corrections for the presence of self-interstitials and their clusters and due to the elastic relaxation of the bubbles were considered. Calculated bubble pressures were in the vicinity of 5 GPa, which is close to the expected threshold pressure for athermal bubble growth.