Range Profiles of 300- and 475-eVHe+4Ions and the Diffusivity ofHe4in Tungsten

Range profiles for 300- and 475-eV ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$ ions implanted in situ in tungsten at 60, 61, 80, 90 K were measured, directly and absolutely, employing an atom-probe field-ion microscope. A mean range ($\overline{x}$) of 40 ± 4 Å and a parent standard deviation ($\sigma$) of 20 to 36 Å was obtained for 300-eV ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$; values of $\overline{x}$ and $\sigma$ of 56 ± 6 Å and 37 to 42 Å, respectively, were determined for 475-eV ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{+}{}_{}{}^{}$. The existence of an isolated and immobile interstitial ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ atom was established and an enthalpy change of migration of 0.24 and 0.32 eV was determined.