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 m isalignment and imperfections of the final lens. A superconducting sol enoid as final lens promises to have theoretically good ion optical pr operties. 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 dimensi on of the focal spot has been determined, while NRRA was used to measu re the energy spread of the ion beam. The effect of alternating stray magnetic fields could be detected and was reduced. It was shown that m echanical vibrations present in the beam line setup have negligible in fluence on the focus quality. The shadow projection method allowed to investigate the effect of minor imperfections of the final lens. Intro ducing 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.