Distribution of helium in metal tritides

Abstract

THE first generation of fusion reactors will utilise the reaction between deuterium and tritium nuclei. Various metal tritides, therefore, have potential applications for the transport, storage, purification, isotope enrichment and gettering of tritium. Although many properties of the metal tritides can be estimated¹ from the corresponding hydrides and deuterides, the effects of the radioactive triton decay into a β-particle and ³He atom with a half life of 12.3 yr can only be determined by direct observation. Considering the expected² insolubility of inert ³He atoms in metal lattices, it is surprising that many metal tritides^1,3,4 contain ³He/T atom ratios in excess of 0.10. The physical state of the retained ³He in these tritides is a very interesting and unresolved problem. Weaver and Camp^5,6 suggest the ³He atoms are trapped in the octahedral interstitial sites for tritides with fluorite crystal structures while Bowman, et al.¹ believe the ³He atoms primarily reside in microscopic gas bubbles with dimensions < 1,000Å. Here we describe nuclear magnetic resonance (NMR) studies of the relaxation times for ³He nuclei in LiT, TiT_1.9, and UT₃, which are representative of ionic, transition metal, and actinide tritides. These NMR measurements support the ³He bubble concept and further indicate an increase in the mean bubble dimensions as the samples age.