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.
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.
|Velocity of electron|
with Ek (MeV)
|Ek ||v (m/s)
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 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.