Investigation of the resolution affecting parameters of a nuclear microprobe using a solenoid lens

The spatial resolution of a nuclear microprobe system may in practice be limited by a number of factors such as beam energy spread, system misalignment and imperfections of the final lens. A superconducting solenoid as final lens promises to have theoretically good ion optical properties. The experimental determination of relevant parameters of the solenoid lens installed at the Bochum heavy ion microprobe and of the incident ion beam is presented. With RBS and STIM the lateral dimension of the focal spot has been determined, while NRRA was used to measure the energy spread of the ion beam. The effect of alternating stray magnetic fields could be detected and was reduced. It was shown that mechanical vibrations present in the beam line setup have negligible influence on the focus quality. The shadow projection method allowed to investigate the effect of minor imperfections of the final lens. Introducing an electrostatic stigmator reduced the astigmatism of the beam spot significantly. A focus of 0.6 mu mx0.7 mu m could be established at a current of 100 pA.