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 positro n beam at the Delft research reactor. In the SEM it will be possible t o focus the positron beam into a probe of 100 nm. To position the brig htness 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 righ t position in front of the deflector. The aberrations of the transport lens, deflector and lenses in the SEM, together with the dispersion o f the deflector, cause an increase in opening angle and spot size of t he positron beam and thus a decrease in effective brightness. Calculat ions show that the influence of the deflector can be kept small. The l oss in brightness of the beam and thus the loss of positrons in the pr obe is less than a few percent for energies above 1000 eV, and about 2 8% for the lowest possible energy of 500 eV.