An electron multiplier includes a series of discrete electron emissive surfaces or a continuous electron emissive resistive surface configured to provide an electron amplification chain; and a housing surrounding the series of electron emissive surfaces or the continuous electron emissive resistive surface and separating the environment inside the housing from the environment outside the housing. The housing includes an electron-transparent, gas-impermeable barrier configured to allow electrons to pass through into the housing to reach a first discrete electron emissive surface of the series of discrete electron emissive surfaces or a first portion of the continuous electron emissive resistive surface.