GB576369A - Improvements in and relating to electron discharge devices and lens systems therefor - Google Patents

Abstract

576,369. Electron microscopes. BRITISH THOMSON-HOUSTON CO., Ltd. March 27, 1944, No. 5605. Convention date, March 31, 1943. [Class 39 (i)] [Also in Group XXXV] An electron lens system for an electron microscope, Fig. 1, comprises one or more permanently magnetised cylindrical elements 7, 8, 9 supported within a metallic, e.g. brass cylinder 6 so as to form therewith a unitary assembly. The lens elements are held in spaced relation by brass cylinders 19, 20 and may be provided with. restricted end portions, as shown. They are preferably made of an alloy of cobalt, nickel and aluminium of the type described in Specifications 423,897 and 424,746, [both in Group II]. In a modification, Fig. 4, each lens element may be formed of a radiallymagnetised disc 89 together with a yoke 90 of U-shaped cross-section, both of which may be provided with circumferential teeth, in which case the yoke is rotatable to vary the magnetic field. Low permeability material, e.g. brass, may be interposed between the yoke and disc and between the teeth. In another modification, Fig. 6 (not shown), the lens elements are in the form of radially magnetised discs and are spaced by magnetic rings. The cylinder 6 may alternatively be of magnetic material, Fig. 3 (not shown), in which case spacers are interposed between the cylinder and the lens elements. The lens elements may be periodically re-magnetised by placing them between two magnetic plates forming part of a magnetic yoke, Fig. 2 (not shown), the latter being energized preferably from a circuit similar to that described in Specification 513,080, [Group XXXV]. The lens assembly is inserted in a suitable housing, e.g. a metal cylinder 1, Fig. 1, which is shielded by'alternate layers of copper and iron 2, 3, 4, 5. The electron gun 21 is similar to that described in Specification 576,003 and comprises a hairpin cathode 26 energized by a transmission line 24, 25, and apertured anodes 38, 39, 40, 41. The cathode is adjustable radially by means of screws 30. An apertured metal cap 37 limits the electron beam.' The specimen insertion chamber 43 is closed by a cover 46, and may be fed with preconditioned air through a valve 51 before the insertion of a specimen. The specimen carrier 54 comprises two parts 55, 56, the latter being pivoted on the former so that it can be raised by a hook or rod (not shown) and a specimen inserted in the collar 57. The specimen may be positioned by a manipulator 60, which is preferably a modification of that described in Specification 575,267, [Group XX]. A fluorescent screen 65 is provided. The Specification as open to inspection under Sect. 91 comprises the following matter. A permanently magnetised cylinder may be provided with a yoke consisting of a ring and a member of L-shaped cross-section. The annular spaces between these members may be filled with low permeability material to modify the magnetic field. In another form, a permanently magnetised disc may be provided with a coaxial insert of low permeability to " equalise " the magnetic field. In a further arrangement, the lens element may be of toroidal shape and may be provided with metallic rings or inserts of suitable permeability. This subject-matter does not appear in the Specification as accepted.