Patent Number: 
Section: abstract

An electron gun that serves to reduce the quantity of electron stimulated desorption and accomplishes vacuum evacuation efficiently with a sufficient degree of vacuum. An electron source 1 and an extraction electrode 6 are provided for emitting an electron beam 7 from the electron source 1. A first vacuum chamber 16 containing the electron source 1 is connected to a second vacuum chamber 9 via an aperture 8 provided in the extraction electrode 6. Each vacuum chamber is differentially evacuated with an independent vacuum evacuation means, and the generation of electron stimulated desorption gas 11 is reduced by securing a wide route of vacuum evacuation around the electron source 1 and intercepting the procession of back scattered electrons 12 emitted from the area with the electron beam 7 on the extraction electrode 6 by using a shielding electrode 22 given a prescribed potential.