Multipole lenses and their application in nuclear microprobe lens systems

Abstract

In principle, aberrations of nuclear microprobe lens systems can be corrected with multipole lenses. This paper discusses multipole lenses and how they may be used to correct divergence dependent aberrations in a quadrupole probe forming lens system. Also discussed is the grid shadow method which provides a practical means for experimental study of the properties of the multipole lenses as well as a potential means for adjustment of the multipoles to achieve correction. Results are presented showing the effect of sextupole, octupole and duodecapole lenses in simplified lens systems that consisted of a single magnetic quadrupole and a single magnetic multipole. Further results are presented that show the grid shadow method applied to the magnetic quadrupole Russian quadruplet of the Melbourne Proton Microprobe. In this case the system was found initially to suffer from large parasitic aberration, probably octupole, which was later significantly reduced by replacement of some damaged pole tips.