The Heidelberg proton microprobe

Abstract

All proton microprobes in operation up to now are based on the same principle: a collimated beam is focussed by a magnetic lens. The small dimension and the special shape of the collimators are most characteristic of the Heidelberg proton microprobe. The collimators are responsible for the small halo of the beam. The two singlets allow fast adjustment of the setup. The small aperture of the lens — 5 mm in diameter — makes high precision of the mechanical parts necessary. It is demonstrated that deterioration of the pole pieces of the order of 30 μm causes a dramatic change of the beam focus. A system for imaging the target surface will partially replace the optical microscope. To produce the pictures a fast scanning procedure as known from scanning electron microscopes is used. For this the intensity of the secondary electrons emitted after proton bombardment from the target surface is displayed on a screen (as a function of x and y coordinates) simultaneously with an x/y scan of the beam over the sample. The good quality of the pictures is demonstrated. Problems arising with this method are discussed. Due to the frequencies (∼ 1 Hz) for producing pictures from the sample surface, the beam can be focussed rapidly and accurately. The Ca distributions in sea urchin eggs (egg diameter 50–100 μm) are shown as an example for scans through targets with variable thicknesses. The analysis of the data takes into account the energy loss of the proton penetrating the target and the absorption of X-rays in the sample itself.