Effect of diffraction condition on mean free path determination by EELS

Effect of diffraction condition on the mean free path of inelastic scatteri ng, which is the parameter that needs to be known prior to thickness determ ination by EELS, was studied using platelet haematite particles. Abnormal c ollection 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 spect ra 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 a t the collection angle 12.1 mad. It was justified that the abnormal depende nce was owing to the dynamical diffraction effect, which strongly modified the intensity ratio of elastic and inelastic scattering.