Effect of diffraction condition on the mean free path of inelastic scattering, which is the parameter that needs to be known prior to thickness determination by EELS, was studied using platelet haematite particles. Abnormal collection angle dependence of the mean free path was observed if the energy-loss spectrum was acquired with incident electrons parallel to a zone axis, while it was absent with the electron beam off the zone axis. As a result, large deviation of the measured mean free path occurred even if the spectra were obtained from the same specimen with the same collection angle, e.g. the deviation was 24% for a sample with approximate thickness of 498 nm at the collection angle 12.1 mrad. It was justified that the abnormal dependence was owing to the dynamical diffraction effect, which strongly modified the intensity ratio of elastic and inelastic scattering.
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