| 摘要 |
PROBLEM TO BE SOLVED: To stabilize the emission electron quantity of an electron gun by providing a dose monitor detecting the ionization quantity of accelerated electrons and a control circuit controlling the grid voltage setting signal applied to an electron gun power supply based on the detected ionization quantity inputted from the dose monitor. SOLUTION: A control circuit 11 receives the detected ionization quantity of emitted electrons 5 into the atmosphere from a dose monitor 4 and outputs a grid increase signal or a grid decrease signal corresponding to the difference between the detected ionization quantity and a set ionization quantity to a grid voltage setting line 10 when the detected ionization quantity becomes lower or higher than the set ionization quantity predetermined in the control circuit 11, i.e., the control circuit 11 applies a feedback to the grid voltage setting signal based on the output fluctuation from the dose monitor 4. The grid voltage setting signal applied to an electron gun source 8 is increased or decreased by the difference between the detected ionization quantity and the set ionization quantity, and the electron emission quantity of an electron gun 1 is invariably kept constant.
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