| 摘要 |
PURPOSE:To improve largely the beam detecting efficiency, by providing an electro-conductive mesh, near the last electrode on the outlet side of a decelerating tube, which is kept equal potential to the last electrode. CONSTITUTION:A decelerating tube 18 has many ring electrodes, and near one of them, the last electrode 18n in the outlet side of the beam, an electro- conductive mesh 30 is provided, which is kept equal potential to the electrode 18n. The mesh 30 is constructed, for instance, with thin wire 32 of tantalum, tungsten or aluminum strung on a stainless steel frame 31. If the size of a cell of the mesh 30 is too large, the potential leakage through it becomes large, and the beam is diverged after converging, and if the size of the cell is too small, the beam strikes the mesh, and the material of the mesh is sputtered causing wrong data. Therefore, the size of the cell of the mesh 30 is adequate to be slightly larger than the diameter of the beam BM which passes through it, so, for example, when the diameter of the beam BM is 6-8mmphi, the size of the cell of the mesh is set to about 10mm square in the central portion, and about 5mm square in the peripheral portion. |
