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. The impact parameter b is defined as the perpendicular distance between the path of a projectile and the center of the field U(r) created by an object that the projectile is approaching as shown in Figure 4426. It is often referred to in Nuclear Physics as well as in Classical Mechanics. These interactions can be divided into three categories depending on the impact parameter b compared to 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 4426. Radiative (a), hard (b) and soft (c) collisions. b is the impact parameter and a is the atomic radius.