• Argon;
  • beam loss;
  • differential pumping;
  • direct simulation Monte Carlo;
  • DSMC;
  • environmental scanning electron microscope;
  • ESEM;
  • gas flow;
  • helium;
  • hydrogen;
  • neon;
  • nitrogen;
  • optimum beam transfer;
  • orifice properties;
  • oxygen;
  • pressure-limiting aperture;
  • scattering cross-sections;
  • water vapour


The gas density of argon along the axis of a pressure-limiting aperture in an environmental scanning electron microscope is found by the direct simulation Monte Carlo method. The aperture is made on a thin material plate, producing the sharpest possible transition region between the specimen chamber and the differentially pumped region downstream of the gas flow. The entire regime from free molecule to continuum flow has been studied, which covers any size of aperture diameter and any pressure from vacuum to one atmosphere. The amount of electron beam transmitted without scattering at any point along the aperture axis is found in the range of accelerating voltage between 1 and 30 kV for argon. The electron beam transmission is further computed for helium, neon, hydrogen, oxygen, nitrogen and water vapour. This study constitutes the basis for the design and construction of an environmental scanning electron microscope having an optimum electron beam transfer, which is the primary requirement for an optimum performance instrument.