The charged particle experiences Coulomb interactions with the nuclei and the orbital electrons of the atoms in a matter when it travels through the matter. As shown in Figure 4429, these interactions can be divided into three categories depending on the impact parameter b compared with the atomic radius a: a) Interaction with the external nuclear field (bremsstrahlung production) for b << a; b) Interaction with the orbital electrons for b ≈ a (hard collision); c) Interaction with the orbital electron for b >> a (soft collision). On the other hand, the particle experiences a large number of interactions before its kinetic energy is completely lost. Some important parameters of charged particle interaction with matter are stopping power, the range, energy transfer, mean ionization potential, and radiation yield.
Figure 4429. Radiative (a), hard (b) and soft (c) collisions. b is the impact parameter and a is the atomic radius.