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A method is described for measuring the intensity distribution of the electron source in a scanning transmission electron microscope (STEM) fitted with an objective lens aberration corrector. The method is applied to a Cs-corrected 300kV field emission gun TEM/STEM, which is found to have an effective source size of 0.56Å full width at half maximum (FWHM) under optical conditions suitable for high resolution STEM imaging. This corresponds to a probe intensity distribution at the specimen plane of 0.72Å FWHM using a probe-forming aperture of 25mrad and including the measured residual lens aberrations.
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Dwyer et al. (2008) studied this question.
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