Isothermal Diffusion and Trapping of Helium in a Silver-Lithium-Alloy after Neutron Irradiation

Abstract

The release of helium from neutron-irradiated Ag-9.6 at.-% Li-alloy is measured in the range between 510 and 784 °C. Analytical expressions are developed taking into account a recoil modified concentration profile in a surface layer. The isothermal kinetics can be described to some extent with two parameters, the effective diffusion coefficient p2D and the absorption coefficient μ of helium in traps. From foil-experiments the untrapped fraction p of gas after irradiation is found to be 0.2 ≦ p ≦ 1 for a helium concentration of 1.1·10-5 at.-%. The activation energy of diffusion is found to be 1.7 eV, which indicates a vacancy mechanism. The absorption coefficient μ is of the order of some 10-4 [sec-1] and nearly independent of temperature. It is discussed whether the diffusion to the traps or the capture process itself with a negative activation entropy is rate determining. The method seems to be applicable to incomplete degassing of other gas-metal-systems described in the literature.