Optimizing the ALCHEMI technique

Systematic row atom location by channelling-enhanced microanalysis (ALCHEMI) experiments have been optimized theoretically with regard to dynamical interaction (beam voltage and reflecting planes), foil thickness and orientation. The optimum level of dynamical interaction is w(s) = 2(1/2) + xi(g)/2 xi(2g), the optimum orientation is the symmetry orientation and the optimum foil thickness lies between xi(g)/4 and xi(g)/3 when dynamical interaction is optimized. The corresponding maximum level of channelling is about 0.3-0.5, depending on absorption. Experimental measurements shaw good agreement with theory. It is shown experimentally that beam convergence may safely be increased up to the Bragg angle. As the level of dynamical interaction approaches that corresponding to the critical voltage, channelling (and the ease of ALCHEMI experiments) decreases to zero.