Entrapment of helium ions at (100) and (110) tungsten surfaces

Two (100) and two (110) tungsten surfaces have been bombarded with helium ions of selected energies between 5 and 2000 eV. The helium thermal desorption spectra obtained when the targets were subsequently heated at 40 °K/s were used to deduce the fraction of the incident particles trapped in the crystal, and their binding energies. For ion energies ≤400 eV, the fractions trapped were small (~ 1 × 10−3 on the (100) surfaces and ~ 1 × 10−2 on the (110) surfaces) and almost all the trapping occurred in surface-related sites with binding energies ≤ 2.1 eV. For ion energies >500 eV, trapping occurred predominantly in sites characteristic of the bulk material, the sites being created by the incoming ions as soon as their energy was sufficient to produce atomic displacements in the lattice (480 eV). At doses <1 × 1013 ions/cm2, four characteristic binding energies were observed: 2.65, 3.05, 3.35, and 4.15 eV. At higher doses, additional binding states were observed with energies up to 5.4 eV.The results suggest that helium diffuses rapidly in tungsten at room temperature and that it is trapped within the crystals only if it encounters lattice defects. Estimates of the ion penetration depth lead to the tentative conclusion that in the annealed crystals (ion energies < 400 eV) the fractional trap concentration is <1 × 10−9 and the helium incident at a few hundred eV energy becomes trapped at depths up to at least 1 μ.