Some Statistical Distributions of Interest

This can be understood as the spatial distance over which particles interact scales with the associated particle s wavelength and the fact that wavelength is inversely proportional to particle mass. Another way of expressing this is that more massive bodies experience a lesser uncertainty in their position at a given energy (Heisenberg uncertainty principle). This explains why  

A Field Emission Scanning Electron Microscope (FE-SEM) operated at 15 keV produces images with a spatial resolution of 1 nm (electrons have a mass of 9.109 X 10 g). 

A He Ion Microscope (HIM) operated at an accelerating voltage of 45 keV with a point source can relay images to 0.25 nm (He has a mass 8000 times that of an electron). 

That being said, the spatial dimensions over which a charged particle beam can be focused (the minimum spot size) is also a function of the space charge effects and the aberrations (primarily spherical and chromatic) suffered. This explains why  

As the current of an ion beam is increased, so too is the minimum spot size (this explains why imaging in the microprobe mode, whether in SIMS, SEM, or HIM, must be carried out at very low ion currents). 

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