ABSTRACT
As pointed out in Section 2.2, electrons, capable of exciting and ionizing molecules, have energies in the range of 5 to 10 eV. They are produced from fast electrons by energy degradation. As they penetrate solids and liquids, they generate ions, radicals, and excited molecules. The ionization results from inelastic collisions of fast electrons with the medium, and in the process the electrons lose energy. An empirical relationship expressing the energy of the electrons to the depth of electrons is due to Grun:1
where RG is Grun range in μm and E0 is the electron energy in keV. This correlation is valid for a wide variety of materials, for polymers (e.g.,
polystyrene) and metals (e.g., aluminum). Thus, as the energy of the electrons increases, so does their penetration depth, and the amount of energy for high-energy electrons dissipated is small and constant over a large depth.2 The stopping power of a material, which is the energy loss per unit path of an incident electron, depends on the density of the medium, and if it is a multi-component system, it also depends on the relative concentrations of the individual components and on their molecular weights. This is important in cases where the organic medium contains pigments, which slow down the incident electrons without yielding useful species.3 However, the “slowed down” electrons react more readily with organic species.4 The reactive species are dispersed randomly throughout the entire thickness of the material (see Chapter 2, Figure 2.5).
