Penetration of Heavy Ions of keV Energies into Monocrystalline Tungsten

The penetration of radioactive alkali metal (${\mathrm{Na}}^{24}$) and inert gas (${\mathrm{Ar}}^{41}$, ${\mathrm{Kr}}^{85}$, ${\mathrm{Xe}}^{125}$, and ${\mathrm{Xe}}^{133}$) ions into oriented monocrystals of tungsten has been measured in the energy range 0.25 to 160 keV using an electro-chemical stripping technique. For energies ≤ 20 keV, the ion bombardments were performed in ultrahigh vacuum with the crystal surfaces free of oxides and adsorbed gases. The resulting integral penetration distributions clearly confirm the theoretically predicted channeling of the incident ions along the more open directions in the crystal lattice. Maximum channel penetrations (in the $〈111〉$ and $〈100〉$ directions) were observed to be about one order of magnitude larger than those measured for "amorphous tungsten." Extremely penetrating "tails" on the penetration distributions have also been observed, extending to >5 mg/${\mathrm{cm}}^2$ as compared to median penetrations of 0.01 to 0.2 mg/${\mathrm{cm}}^2$. It has been suggested to us that these are associated with excitation of the channeled atoms through multiple collisions, but definite confirmation of this mechanism has not yet been obtained.