Soft Collision between Charged Particle and Atom
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As shown in Figure 4427, 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 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).

Radiative, hard and soft collisions. b is the impact parameter and a is the atomic radius

Figure 4427. Radiative (a), hard (b) and soft (c) collisions. b is the impact parameter and a is the atomic radius.

 

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