Critical angles for channeling of low energy ions in tungsten

Abstract

The critical angle ψ for channeling of light and heavy ions in crystals has been studied around the [Formula: see text] direction in a tungsten single crystal by measuring the yield of backscattered ions at room temperature. The ions and their energies E were chosen such that the critical angle was comparable to, or larger than, the ratio between the screening radius a and the atom spacing along the string d. According to Lindhard [Formula: see text] for ψ < a/d and [Formula: see text]. We have studied the variation of the critical angle with E/Z1 for the ions 1H, 2H, 4He, 12C, 20Ne, 40Ar, and 84Kr in the critical angle region 4a/d > ψ > a/d. For 12C, 20Ne, 40Ar, and 84Kr, most of our data is in the transition region between ψ1 and ψ2, but an approach to the ψ2 behavior can be seen at the low energy end. For 1H, 2H, and 4He, however, the energy dependence is rather of the ψ1-type for the whole region studied, that is even for critical angles as large as ψ = 4a/d. The value of the multiplicative constant C2 is also different for the two cases, being close to 1 for the heavy ions but about 2 or larger for the light ions.