On the monochromatisation of high brightness electron sources for electron microscopy

In electron energy loss spectroscopy the spectral resolution is limited by the initial energy width of the electron source. When incorporating an electron energy monochromator in a transmission electron microscope special care has to be taken to limit the brightness loss of the primary beam, as it determines the performance of a microscope. This article goes through the possible monochromator configurations. It shows the advantages and disadvantages of using lenses, dispersive elements and an energy selection slit, when pursuing both a high-energy resolution and high-spatial resolution. The inclusion of a filter and selection slit in the gun area of the microscope has technical and operational advantages. A new monochromator configuration is proposed consisting of a short Wien filter directly after the electron source. A drift space between the filter and the selection slit makes it possible to align the beam to a fixed Nanoslit in the gun area. Because of the small dimensions of the selection slit, the demagnification of the filtered probe to the specimen does not put extra demands on the condenser system of the microscope.