A high performance quadrupole quintuplet lens system for the CSIRO-GEMOC nuclear microprobe

A new probe forming system for the CSIRO-GEMOC nuclear microprobe, based on a quadrupole quintuplet lens configuration is described. The lens system uses a novel quadrupole lens design with a 30 mm pole-tip extension, which allows a working distance of 85 mm. Each quadrupole has cut-outs at 45 degrees, in the yoke positioned between poles, to allow unrestricted access for detectors into a target chamber at 135 degrees to the beam. Four cut-outs are used to maintain four-fold symmetry. Each lens has a bore of 14 mm. The extra degrees of freedom in the quintuplet configuration allow optimisation to reduce spherical aberration, to minimise spherical and parasitic aberration cross-terms (involving theta phi), and to attain an orthomorphic demagnification of 67 using a short overall system length (object to target) of 4.7 m. The system also has built-in redundancy; two quadrupole positions are very insensitive to parasitic multipole fields. (C) 1999 Elsevier Science B.V. All rights reserved.