Radiation

Radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.

Electromagnetic radiation

Electromagnetic waves can, like all waves, be characterized by their

amplitude, wavelength (λ), frequency (ν) and speed. The amplitude is the

intensity of the wave. The wavelength is the distance between identical points on

adjacent cycles. The frequency is the number of complete wave oscillations per

unit time. The speed of the wave is equal to the product of the frequency and the

wavelength, and its magnitude depends upon the nature of the material through

which the wave travels and the frequency of the radiation. In a vacuum, however,the speed for all electromagnetic waves is a constant, usually denoted by c, and in which case:

c = λν

For X rays, wavelength is usually expressed in nanometres (nm)

(1 nm = 10^-9 m) and frequency is expressed in Hertz (Hz) (1 Hz = 1 cycle/s = 1 s–1).

When interactions with matter are considered, electromagnetic radiation is

generally treated as series of individual particles, known as photons. The energy

of each photon is given by:

E = hν

where the constant h is known as Planck’s constant. In diagnostic radiology, the

photon energy is usually expressed in units of keV, where 1 electronvolt (eV) is

the energy received by an electron when it is accelerated across of a potential

difference of 1 V.

Particulate radiation

In diagnostic radiology, the only particulate radiation that needs to be

considered is the electron. This has a rest mass of 9.109 × 10–31 kg and a rest

energy of 511 keV.

Ionizing and non-ionizing radiations

Radiation is classified as ionizing or non-ionizing, depending on its ability

to ionize matter:

●●Non-ionizing radiation cannot ionize matter.

●● Ionizing radiation can ionize matter, either directly or indirectly:

——Directly ionizing radiation: Fast charged particles that deposit their

energy in matter directly, through many small Coulomb (electrostatic)

interactions with orbital electrons along the particle track.

——Indirectly ionizing radiation: X or gamma ray photons or neutrons that

first transfer their energy to fast charged particles released in one or a

few interactions in the matter through which they pass. The resulting

fast charged particles then deposit their energy directly in the matter.

The minimum energy required to ionize an atom, i.e. to remove an electron,

is known as the ionization potential. For elements, its magnitude ranges from a

few electronvolts for alkali metals to 24.5 eV for helium. For water, it is 12.6 eV.

Electromagnetic radiation of frequency higher than the near-ultraviolet region

of the electromagnetic spectrum is ionizing, whereas electromagnetic radiation

with energy below the far-ultraviolet region (e.g. visible light, infrared and

radiofrequency) is non-ionizing.