# Interaction of Radiation with Matter - Light Charged Particles

Particles with mass comparable to those of electrons are light charged particles. Essentially, they are high-speed positron and electrons. Muons are 200 times more massive than electrons, but muons interact with material mainly by ionization.

## Velocity of High-energy Electrons

Newtonian physics applied to estimate the velocity of high-energy electrons gives velocities larger than that of light, the limiting speed. Thus, Einstein's theory of relativity must be applied.

Velocity of electron
with Ek (MeV)
Ek v (m/s)
0.01 4.2e7
0.1 1.2e8
0.5 2.1e8
1.0 2.4e8
2.0 2.7e8
5.0 2.86e8
10.0 2.93e8
A simple method in agreement with the theory of relativity is to consider the relative mass as the sum of rest mass and kinetic energy, (0.51 + Ek) MeV,

m = (0.51 + Ek) MeV The velocity of the electron is then v = (1 - 0.51/m)1/2 c There are many electrons in a material, and fast-moving electrons go through a medium with considerable deflection as depicted in the diagram shown. Thus, the ranges for beta particles are poorly defined due to range straggling, low intensities for a spread of thickness as shown in the diagram.

## Mechanisms of Interaction Between High-energy Electrons and Matter

Ionization, Bremsstrahlung radiation, and annihilation with positrons are the three mechanisms by which electrons lose energy in a medium.

Coulomb interactions between fast moving electrons and molecular electrons excite and ionize the molecule, producing ion pairs.

When a fast-moving electron is accelerated or decelerated, a photon is emitted, and such photons are called bremsstrahlung radiation (braking radiation). Their properties are similar to those of X-rays.

Annihilation of electron and positron has been discussed, but it is known that an electron and a positron combine into a short-live atomic-like system called positronium which decay into two gamma photons.