When the primary electron beam strikes the sample, the beam electrons lose energy by repeated random scattering via collisions with orbital electrons and by interactions with the nucleus of the sample’s atoms.  The collisions result in a teardrop-shaped irradiated volume in the specimen known as the interaction volume, which, depending on the sample composition, extends from less than 100 nm to approximately 5µm into the sample.  As the primary beam electrons transfer their energy to the sample, the sample then releases this acquired energy by giving off a variety of ‘secondary’ signals.  These signals are either electrons (secondary, backscattered, Auger), or electromagnetic radiation (cathodoluminescence, characteristic x-rays, continuum x-ray radiation).  These ‘secondary’ signals are used to evaluate the sample.

​

There is another type of beam instrument called a ‘Transmission Electron Microscope’ (TEM).  This instrument examines samples that are less then 1µm in thickness and thus the electron beam can pass through the sample.  Additional secondary signals are then available to characterize the sample (see left). In the case of the EPMA, or SEM, the samples are infinitely thick compared to the depth of beam penetration.