GB1251333A - - Google Patents

Abstract

1,251,333. Electron beam irradiation apparatus. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. 25 Oct., 1968 [31 Oct., 1967], No. 49528/67. Heading H1D. An electron gun for emitting a sheet electron beam through a window on to an object to be irradiated, comprises a tubular anode 9, Fig. 1, provided with the window 12, an electron emitter filament 1 and a slotted intermediate tubular focusing and accelerating electrode 5 in which the slot is fitted with a wire mesh 7 to prevent the field between the electrode 5 and the anode 9 from penetrating into the space between the electrode 5 and the filament 1. The anode 9 is earthed, the electrode 5 held at a negative potential of 25-300 kV. and the filament 1 and a further slotted tubular electrode 3 held at a potential of 200- 2000 V. negative with respect to the electrode 5. The accelerating section within the electrode 5 is operated at a low voltage gradient of e.g. 500 volts per cm. and the second accelerating section between the mesh 7 and the window 12 at a high voltage gradient of e.g. 30 kV. per cm. The wire mesh 7 may have about 20 wires of 0À01 inch diameter per inch and the wires may be inclined at an angle of about 20 degrees to the longitudinal axis of the beam so as to prevent striations in the final beam. The slotted tube 3 may be of magnetic material with a filament feed wire (14, Fig. 2) together with the filament forming a single turn induction loop whereby a transverse magnetic field (15) is produced across the slot in the electrode 3, (3a) deflects low energy electrons longitudinally by an angle sufficient to eliminate shadows from the filament supports 2. The gun of Fig. 1 may be modified to produce angularly spaced multiple beams (Fig. 3). In the modification of Fig. 4 the tube 3 is replaced by focusing electrodes 16 mounted on the filament support plate 17. Each focusing electrode 16 may be constructed from a thin metal plate having a high resistivity or each electrode 16 may be segmented into a longitudinal array of plates (Fig. 5) interconnected by resistors such that when the filament 1 and the electrodes 16 are supplied with A.C. the instantaneous voltage applied to each pair of electrodes matches that applied to the adjacent region of the filament. A potential difference may be maintained between the electrodes 16 and the filament 1. In further modifications the electrodes 16 of Fig. 4 may be replaced by a single resistive planar mesh electrode or by a longitudinal array of mesh electrodes electrically interconnected mesh electrodes, an additional mesh electrode may be incorporated into the gun of Fig. 4. In the gun of Fig. 7 two beams are produced in diametrically opposite directions through the windows 12 in anode 9. A mesh electrode 27 is provided between the emitter filament 1 and each mesh window 7 in the tubular electrode 5. In the multiple beam gun of Fig. 8, radial electrodes 32, 33 split the electron stream emitted by the filament 1 into three separate beams. The electrodes 32, 33 may be constructed from a number of electrically inter-connected plates.