Secondary electrons

Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation (the primary radiation). This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential.

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Secondary electrons are also the main means of viewing images in the scanning electron microscope (SEM). The range of secondary electrons depends on the energy. Plotting the inelastic mean free path as a function of energy often shows characteristics of the "universal curve"[1] familiar to electron spectroscopists and surface analysts. This distance is on the order of a few nanometers in metals and tens of nanometers in insulators [2,3]. This small distance allows such fine resolution to be achieved in the SEM.

References

1. A. Zangwili, Physics At Surfaces (Cambridge University Press, 1988), p.21.
2. H. Seiler, J. Appl. Phys. 54(11), R1 (1983).
3. J. Cazaux, J. Appl. Phys. 85(2), 1137 (1999).