| 摘要 |
PROBLEM TO BE SOLVED: To shorten a settling time when a voltage of a high-voltage DC power source is switched by preparing at least two high-voltage DC power sources corresponding to a plurality of high voltages, and selecting either one high voltage by a voltage selecting means which is supplied to a secondary electron detecting means. SOLUTION: Either one of high voltages V5 or V6 from two high voltage DC power sources 5 and 6 on a ground voltage is selected by a voltage selecting means 10. The voltage selecting means 10 is provided with switch means S1, S2, and S3, and, when the high voltage V5 or V6 of the high voltage DC power sources 5 and 6 is switched/selected by the switch means S1-S3, a ground voltage is selected momentarily. During the ground voltage selecting period, the output side of the high voltages V5 and V6 is temporarily grounded, an accumulated electric charge in a noise removing capacitor C is discharged to be restored to an initial state, and next high voltages V5 and V6 are supplied to a photomultiplexer 30 of a secondary electron detecting means.
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