An achromatic proton microbeam of 5000 å width and 1.2 MeV energy

Abstract

The performance of a doublet of achromatic quadrupole lenses has been experimentally tested by means of 1.2 MeV protons from a Van de Graaff accelerator. The lenses have a bore of 1.04 mm diameter, a length of 6.0 cm, a center-to-center separation of 7.0 cm, and a working distance (measured from the center of the second lens) of 10.0 cm. Focus, achromatic operating point, and lens rotation are adjusted to compensate first-order aberrations, and hexapole perturbing fields are adjusted to compensate second-order parasitic aberrations. A beam width of 0.5 μm is determined from the secondary electron signal produced as the beam grazes a 4.2 μm carbon filament. Improvement of the residual gas pressure in the 3.6 m distance between object and doublet is required in order to obtain improved resolution.