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In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can be the electrons and protons themselves, as well as other elementary particles, like positrons. It may also be an atomic nucleus devoid of electrons, such as an alpha particle, a helium nucleus. Neutrons have no charge, so they are not charged particles unless they are part of a positively charged nucleus. Plasmas are a collection of charged particles, atomic nuclei and separated electrons, but can also be a gas containing a significant proportion of charged particles. Plasma is called the fourth state of matter because its properties are quite different from solids, liquids and gases.
Interaction between charged particle and matter
Energy loss process of charged particle (α, β) in matter
1. Elastic scattering
No ionization is produced, with the incident particle "bouncing off" the target atoms, causing it to change direction. Energy and momentum are conserved.
- Coulomb’s Force: electrical repulsive force is acting on α-particle and nucleus.
- Elastic collision: sum of momentum is conserved before and after.
- Rutherford scattering equation
2. Inelastic collision
- Target atom ionization caused by incoming charge.
- α-particle loses momentum corresponding to ionization event.
- Stopping power: Energy loss due to a number of ionizations along the particle track (per unit length) particle in the target.
- Linear Energy Transfer (LET): absolute value of stopping power.
- Specific Ionization: number of ion pairs produced per unit track length.
- Bragg curve: the graph of specific energy loss along the track of a charged particle. As it loses energy, stopping power increases approximately (scaling with 1/E) then peaks before dropping off again. The peak is known as the Bragg peak.
- Range: distance that heavy charged particle progresses before energy is completely lost by repeated scattering with target atoms. Found using the Bethe-Bloch formula.
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