Physics and Engineering of Radiation Detection

32 Chapter 1. Properties and Sources of Radiation

electromagnetic interactions. The whole wireless communications on which we are
so dependent nowadays is possible by photons largely in the microwave and radio-
frequency regions. There are many ways to produce photons that are not in the
visible light region of the spectrum. For example whenever a charged particle moves
(such as a changing current in a wire), it creates electromagnetic waves around it that
propagate in space. These waves are considered to be excitations in the underlying
electromagnetic field and a quantum of these excitations is called a photon.
The energy carried by a photon can be absorbed in a number of ways by other
particles with which it interacts. Also, like other particles, a photon can scatter
off from other particles and even experience gravitational pull. We’ll discuss these
interactions in detail in the next chapter.
In terms of radiation exposure and biological damage we are generally concerned
with high energy photons, such asγ-rays, x-rays, and ultraviolet rays. Having high
energies, these photons can penetrate deeper and cause more damage than the low
energy photons such as visible light.

Basic Properties of Photons

Rest mass = Zero

Electrical charge = Zero

Energy = hν=hcλ

Momentum = hλ

Examples : visible light, x-rays,γ-rays

An important property of photons is that they carry momentum even though
they have no rest mass. The momentumpγof a photon with energyE, frequency
ν, and wavelengthλis given by,

pγ=

E

c

=

hν

c

=

h

λ

(1.6.1)

A.1 SourcesofPhotons.......................

Photons play very important roles not only in physics but also in engineering, med-
ical diagnostics, and treatment. For example laser light is used to correct vision, a
process called laser surgery of the eye. In medical diagnostics, x-rays are used to
make images of internal organs of the body. In the following we will look at some
of the most important of sources of photons that have found wide applications in
various fields.