Measurement of Coulomb interactions in an electron beam monochromator

Electron energy loss spectroscopy in the TEM with sub-0.1-eV energy resolution requires the incorporation of an electron beam monochromator in the condenser column. At high beam currents, the statistical Coulomb interactions in the monochromator broaden the energy distribution (Boersch effect) and blur the apparent source, thus limiting the energy resolution and spatial resolution of the microscope. We have built a monochromator (Wien filter type) with a length of 50 mm and measured the blurring of the energy dispersed beam for various beam currents. At low beam currents, the observed blur agrees reasonably well with the theoretical size of the blur. We calculate the theoretical magnitude of the Boersch effect in the filter, and we conclude that the combined action of the Boersch effect and the blur does not impede an energy resolution better than 0.1 eV as long as the total beam current is below ≈30 nA.