PURPOSE: To make sensitivity ratios between incident particles with different mass numbers constant by maintaining the acceleration rates of incident particles constant irrespective of a high voltage applied to a secondary-electron multiplier by applying a high voltage in the vicinity of a diode which receives the incident particles.

CONSTITUTION: Since voltage applied to a secondary-electron multiplier is 1W 3KV, which is about 100 times as high as the energy of ionized particles, the speed or energy of the ionized particles right before they reach an initial-stage diode 8 is dominated by the voltage applied to the secondary-electron multiplier. Therefore, by constantly applying a high voltage of several KVs, which is equal to the voltage applied to the secondary-electron multiplier, to a metallic plate 15, the energy of incident particles right before they reach the diode 8 becomes almost constant for each mass number irrespective of the high voltage applied to the secondary-electron multiplier. As a result, sensitivity ratios between incident particles having different mass numbers become unchanged even when the voltage applied to the secondary-electron multiplier is varied.