Patent ID: 7772564

Claim:
A particle-optical apparatus equipped with a gas ion source, which gas ion source comprises: a diaphragm wall in which an exit diaphragm with a diameter of at most 20 μm is fitted, an electron source with an electron-emitting surface for generating an electron beam, an opening for admitting a gas at a first side of the diaphragm wall, a vacuum source for causing a vacuum, in the form of a lower gas pressure, at the other side of the diaphragm wall; an acceleration electrode for producing an acceleration field accelerating the electron beam to an energy whereby the electrons of this electron beam ionize the gas in an ionization volume, and an extraction electrode for producing an extraction field extracting the produced gas ions from the ionization volume, characterized in that the electron-emitting surface is located at the gas side of the diaphragm wall, and the diameter of the electron beam at the location of the ionization volume is less than 30 μm.