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.

Cerenkov Radiation

Cerenkov radiation refers to the blue light of nuclear reactor core when the reactor is operating.

This image shows the Cerenkov blue radiation from the reactor core of Missouri-Rolla Nuclear Reactor.

When the reactor is in operation, many fission products emit high-energy beta particles. These particles travel at speed greater than that of light in water. Water molecules line up along the path of beta particles, and as they return to their normal random orientations, energy is released in the form of blue light. This type of radiation is called Cerenkov radiation, name after the Russian scientist who studied it.

©cchieh@uwaterloo.ca