Diffusivity of 3He atoms in perfect tungsten crystals

The diffusivity of 3He atoms in perfect crystals of tungsten has been studied employing the atom-probe field-ion microscope (FIM) technique. Tungsten FIM specimens were implanted in situ with 300-eV 3He+ ions, to a fluence of 3×1015 ions cm−2, at specimen temperatures which ranged from 60 to 110 K. The 3He+ ion beam was analyzed magnetically and the ion source was connected to the atom probe through a differentially pumped aperture. At an implantation energy of 300 eV no radiation damage was produced by the implanted 3He atoms. Thus, the state of a tungsten specimen after an implantation consisted of 3He atoms with an initial depth distribution that was determined solely by the range profile of the low-energy ions. Isothermal annealing experiments between 90 and 110 K were employed to study the kinetics of recovery of the implanted 3He atoms; at 60 K the 3He atoms are immobile. This data, in combination with a suitable diffusion model, was used to determine—for the first time—the diffusivity [D3He(T)] and the enthalpy change of migration of 3He atoms (Δhm3He ) in tungsten. The quantity D3He (T) is given by the Arrhenius expression: D3He(T)=(5.4±10.63.8) ×10−3 exp(−0.28 eV/kT)cm2 s−1. The value of Δhm3He is approximately the same as for Δhm4He (0.24–0.32 eV). D3He (T) is compared with the diffusivity values of 1H and self-interstitial atoms in tungsten—the self-interstitial atoms are considerably more mobile than 3He or 4He atoms at the same temperature, as are the 1H atoms.