A study of the diffusion of helium in metals

Abstract

A greatly improved apparatus for the detection and measurement of micro-quantities of helium and neon was constructed and used to investigate the behaviour of helium in lithium/aluminium alloy, gold, beryllium and meteoritic iron. The work described involved neutron transmutation techniques for the production of helium in lithium/aluminium alloy and beryllium, and took advantage of the fact that helium is produced in meteoritic iron when bombarded with cosmic rays. The work on gold utilised the alpha- decay scheme of radon as a source of helium. The diffusion of helium in lithium/aluminium alloy appears to takeplace by a substitutional mechanism according to the equation. D = 42. exp. (formula) where D is the diffusion coefficient expressed in square centimetres per second. The conditions under which this equation is obeyed, however, are shown to be very restricted and highly dependent on the physical state of the metal and its previous annealing history. Accumulation of the gas into bubbles is believed to be the cause of the marked decrease in evolution of helium observed when annealing times longer than a few hours are used. The values obtained for the diffusivity of helium in gold are believed to be invalid on account of the presence of voids along the line of the cold- weld used to sandwich helium-rich layers of this metal together. The validity of this particular technique for gag/metal systems is reviewed in some detail. No values are presented for the diffusivity of helium in beryllium or meteoritic iron but the conclusion is that bubble-formation is a significant factor in diminishing the evolution of helium from these metals.