Patent ID: 7425708

Claim:
A secondary electron detector unit for a scanning electron microscope defining an optical axis along which a primary electron beam impinges on a sample, said secondary electron detector unit comprising: a head body; a scintillator mounted in said head body; a microporous plate defining an output side directed toward said scintillator and an input side opposite said output side and having an active input area on said input side and an active output area on said output side; said active input area being arranged asymmetrically with respect to said optical axis of said scanning electron microscope; said microporous plate being sealed in said head body to restrict gas flow from one of said sides of said microporous plate to the other one of said sides thereof; a throttling aperture device having a small opening at said optical axis; said throttling aperture device being mounted in said head body; said microporous plate being arranged between said throttling aperture device and said scintillator; and, at least one electrode arranged at said input side of said microporous plate or between said microporous plate and said throttling aperture device.