Detrapping of interstitial helium in metal tritides: NMR studies

Measurements of NMR line shapes and relaxation times were made for 3H nuclei and their β-decay products 3He in aged samples of titanium tritide (Ti 3H/sub x/). Data were obtained on samples aged 1 y (x = 1.7) and 8 y (x = 1.4), where x is the initial concentration. These data indicate that 3He atoms are trapped interstitially in young crystals; whereas, in contrast, the older samples are found to contain helium-gas bubbles. Additionally, the younger sample was found to exhibit considerably more lattice distortion than the older sample. This is the first direct evidence for a detrapping mechanism which has been supported by a growing body of indirect evidence. A theory is described which explains the detrapping by a percolative mechanism: When enough 3He atoms have been created to exceed a relevant critical percolation density, the helium atoms, which are considerably ''oversized'' for available interstices, are able to cooperatively strain the lattice beyond its elastic limit, hence irreversibly-deforming the lattice. The subsequent rapid diffusion of helium would then allow heterogeneous bubble nucleation with release of helium from the crystal by blistering and/or release along grain boundaries