Optimizing the ALCHEMI technique

Systematic row atom location by channelling-enhanced microanalysis (ALCHEMI ) experiments have been optimized theoretically with regard to dynamical in teraction (beam voltage and reflecting planes), foil thickness and orientat ion. 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 b e increased up to the Bragg angle. As the level of dynamical interaction ap proaches that corresponding to the critical voltage, channelling (and the e ase of ALCHEMI experiments) decreases to zero.