  In this paper, we have analysed the depth resolution that can be achieved by
on-axis transmission Kikuchi diffraction (TKD) using a Zr-Nb alloy. The results
indicate that the signals contributing to detectable Kikuchi bands originate
from a depth of approximately the mean free path of thermal diffuse scattering
(lambdaTDS) from the bottom surface of a thin foil sample. This existing
surface sensitivity can thus lead to the observation of different grain
structures when opposite sides of a nano-crystalline foil are facing the
incident electron beam. These results also provide a guideline for the ideal
sample thickness for TKD analysis of 6lambdaTDS, or 21 times the elastic
scattering mean free path (lambdaMFP) for samples of high crystal symmetry. For
samples of lower symmetry, a smaller thickness 3lambdaTDS, or 10lambdaMFP is
suggested.
