Charged particle

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

It is the process of changing in direction of travelling particle due to the correlation with atom. Conservation of Energy is valid and momentum is preserved.

Rutherford scattering equation
• 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

Inelastic collision takes most of the part in energy loss process of charged particle inside of the matter.

Stopping power
• Atom’s ionization caused by α-particle is called inelastic collision.
• α-particle loses momentum corresponding to ionization.
• Stopping power: Energy loss due to per unit length particle in matter. It is a power that interrupts the progress of heavy particle in matter.
Range equation R = Range, E = energy of heavy particle, S = stopping power
• Linear Energy Transfer (LET): absolute value of stopping power.
• Specific Ionization: the 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 value approximately increases along the 1/E then stops. Like this, the peak before its maximum range of stopping power called Bragg peak.
• Range: distance that heavy charged particle progressed until energy is completely lost by repeating ionizing and scattering with atom. In case of the particle that has certain energy, gets certain range. Range is defined as a relation of strength of α-ray and distance.