GB848982A - Improvements in x-ray microscopes - Google Patents

Abstract

848,982. X-ray tubes. GENERAL ELECTRIC CO. Sept. 27, 1957 [Oct. 1, 1956], No. 30335/57. Class 39(1). An X-ray shadow microscope has an asymmetric electrostatic lens system for focusing the electron beam on to the target which is arranged to cause multiple cross-over of the beam, and whose principal plane is positioned between the lens mid-plane and the target. The microscope of Fig. 1 (not shown), is substantially identical with that shown and described in Specification 780,682, except for the objective or focusing lens assembly shown in more detail in Fig. 2, and which comprises three elements 17, 18, 19. The target window 20 is secured to the last element 19, flush with the inner surface. With the object of reducing the focal length, the lens is designed so that multiple crossing of the optical axis A by the electron rays occurs. This is achieved by suitably controlling the potentials in the centre of the element apertures, and also by the spacing and geometry. The spacing S between elements 18 and 19 must be less than the spacing Zo between elements 17 and 18, and the thickness t and aperture radius R 1 of element 18 are such that S / t/2 > 0À5 and t/2 /R 1 > 1. Equations are also given for deriving the potentials at the centres of the apertures, in terms of the applied voltages and the lens dimensions. The arrangement shown has three cross-over points, and it is stated that a focal length of as low as 1 mm. can be obtained.