As shown in Figure 4428, 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. 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).
Figure 4428. Radiative (a), hard (b) and soft (c) collisions. b is the impact parameter and a is the atomic radius.