| 摘要 |
PURPOSE:To effectively avoid a generation of a spurious signal due to the return beam, by absorbing the return beam from a target emitted toward beyond a position not less than 2/3 times the width of an electron beam scanning region of the target. CONSTITUTION:Width of an electrode plate 8 having a limiting aperture (a), arranged at the rear end of a grid electrode G2 arranged in such a manner as to be at a face-to-face position to a target 4, and axially extended in such a manner as to close the grid electrode, is formed into a dimension not more 2/3 times the width of an electron beam scanning region of the target 4. Accordingly, all of return beams 10 emitted toward other than the scanning region of the target 4 pass without collision against the electrode plate 8, thus to avoid the beam emitted again toward a direction of the target 4. |
