Coupling optics for a combined electron positron scanning microscope

This paper discusses the construction of a positron microbeam combined with a scanning electron microscope (SEM). The setup uses the positron beam at the Delft research reactor. In the SEM it will be possible to focus the positron beam into a probe of 100 nm. To position the brightness enhanced positron beam on the optical axis of the SEM a double focusing deflector is used. A transport lens between the remoderation section and the deflector has to be used to bring the beam in the right position in front of the deflector. The aberrations of the transport lens, deflector and lenses in the SEM, together with the dispersion of the deflector, cause an increase in opening angle and spot size of the positron beam and thus a decrease in effective brightness. Calculations show that the influence of the deflector can be kept small. The loss in brightness of the beam and thus the loss of positrons in the probe is less than a few percent for energies above 1000 eV, and about 28% for the lowest possible energy of 500 eV.