Identification and elimination of internal and external parasitic multipole components in nuclear microprobe lens systems

The grid-shadow technique has proved to be an important tool for the characterization of the quadrupole lenses used for focusing the ion beam in nuclear microprobes. A technique is described which can identify and distinguish between beam aberrations, seen in the grid-shadow patterns, caused by parasitic multipoles in the lenses from those caused by stray external fields. Also presented is an on-line method to correct for the internal parasitic fields in the lenses by observing the grid-shadow patterns, thereby improving the spatial resolution in the nuclear microprobe. (orig.)