Influence of channelling in conventional helium-3 activation analysis

Abstract

Channelling effect results when energetic charged particles enter a crystal with small angles relative to its lattice rows or planes; the ions suffer small-angle collisions with lattice atoms that tend to channel their paths inside the crystal. The channelling effect offers advantages and drawbacks to charged-particle activation analysis. To investigate whether this effect should be expected in routine analytical bombardments, a germanium single-crystal ingot was cut into numerous slices, perpendicular to its [1,1,1] axis. Each of these slices was mounted on a 0·2° precision goniometer, and bombarded at different angles with 7·8 ±0·2-MeV helium-3 ions. To ensure the validity of the results over a wide area in charged-particle activation, the alignment was much more precise than in ordinary bombardments, so that observation of channelling could reasonably be expected. As activation analysis is based on counting of induced radioactivity, this was preferred to measurement of prompt nuclear reaction products. As the channelling critical angle in our investigations was 0·56°± 0·01°, the bombardment angles used (beam with respect to [1,1,1] axis of sample) were –1·0°, –0·8°, –0·6°, –0·4°, –0·2°, 0°, 0·2°, 0·4°, 0·6°, 0·8°, 1·0°, 4·0°, 4·4°, 4·8°, 5·0°, 5·2°, 5·6° and 6·0°. The products of the germanium bombardment, viz., arsenic-71, arsenic-72, arsenic-74, arsenic-76 and selenium-73, were identified and the individual radioactivities, as well as their sum, were plotted against irradiation angle. Results indicate that the induced radioactivity is independent of angle, i.e., channelling effect is not observed. Because of the conditions chosen for these experiments it can be concluded that the channelling effect cannot influence most conventional charged-particle activation analyses.