General Notes About Radiation

Types of Radiation

There are only two types of radiation; these are
(1) electromagnetic radiation
and
(2) particle radiation.

The general characteristics of these types of radiation are given below.

Electromagnetic Radiation

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 Electromagnetic radiation travels at approximately 300,000 km/s (the velocity of light) in the vacuum of space. In materials which are transparent to electromagnetic radiation, the velocity of propagation is slightly less than the speed of light in a vacuum.

Electromagnetic radiation has a variety of names, but they are all manifestations of a transverse wave which propagates through space a series of alternating, self induced electric and magnetic fields.

Notice in the figure to the left that the designations of many types of electromagnetic radiation overlap.

For example, radio waves, microwaves, light, X-rays, infrared and ultra-violet radiation and gamma rays are all examples of electromagnetic radiation. Notice that the designated wavelength for X-rays and gamma rays overlap.

All electromagnetic radiation is of the same physical type. Electromagnetic radiation is an electromagnetic phenomena, one kind differing from another only in its frequency (and consequently wavelength) and the amount of energy carried by the radiation. In fact most electromagnetic radiation is a mixture of many frequencies (wavelengths) with the strongest wavelength depending upon the temperature of the radiation source.

An object at about 6000K appears bluish-white. This "white-light", when passed through a dispersing medium such as a glass prism, will produce the famous optical spectrum first discussed in detail by Sir Isaac Newton (1642-1727).

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Neutron stars, with surface temperatures in excess of 100,000K, radiate with maximum intensity in the X-ray part of the electromagnetic spectrum, whereas a household toaster (when it's working) emits most strongly in the infrared part of the electromagnetic spectrum.

Thermal Electromagnetic radiation

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 Other than man-made sources of electromagnetic radiation such as radio, TV, clinical and industrial X-rays and gamma rays from radioactive decay (and a few notable cosmic sources), virtually all electromagnetic radiation, including the light from the stars, is thermal electromagnetic radiation.

Every object in the universe emits electromagnetic radiation. The spectrum of the electromagnetic energy, that is, its intensity as a function of its emitted wavelength, is uniquely characterized by the object's temperature.
The graph above shows the characteristic spectrum of an ideal emitter at 6000K. The shape of this curve has been derived using Planck's Black Body Function1 and closely resembles the spectrum of the sun. Perfect emitters are referred to as "black bodies", signifying that they are both perfect emitters and perfect absorbers of electromagnetic radiation.

While the energy distribution of the emitted radiation is given by the Planck Function(shown above), the total amount of emitted radiation per second (emitted power) is given by the Stefan-Boltzmann Law and the wavelength of maximum intensity if given by Wien's Displacement Law, the latter two of which are discussed in the accompanying assignment.

1Max Planck (1858-1947). Received the Nobel Prize in Physics in 1918 for his derivation of this law, which today is know by physicists as Planck's Black Body Function.

Electromagnetic radiation as a wave

Electromagnetic radiation, when propagating through space and interacting with other electromagnetic radiation, displays characteristics which are absolutely unique to waves, such as interference and diffraction. This behavior defines the radiation's wavelengths and frequencies. The ability to be polarized further characterizes electromagnetic radiation as being a transverse wave.

Electromagnetic radiation as a particle

When interacting with matter, behaviour such as the photoelectric effect clearly characterizes electromagnetic radiation as an energetic zero-mass particle.

These localized particle-like features of electromagnetic radiation are called electromagnetic quanta2 or because they were first investigated as an optical (light) phenomena, they are often simply referred to as photons.

2In modern physics the photon is defined as the quantum of the electromagnetic field.
Quanta being the plural form of quantum.

Particles or Waves?

The question then arises; " is electromagnetic radiation a particle phenomena or a wave phenomena?" Strange as it may seem the answer is an ambiguous "it is both a particle and a wave!"

In general, electromagnetic radiation behaves as a wave when moving through space, but it behaves as a particle when it interacts with matter.

Since our interest is mainly in how radiation interacts with matter, especially in biological systems, we must treat electromagnetic radiation as if it were made up of a stream of discrete energetic particles (quanta).

Particle Radiation

A graphic representation of various subatomic particles

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  1. Beta particles. These are high velocity electrons with an electrical charge of -1. Beta radiation is a by-product of the natural radioactive decay of radioactive substances and is also abundant in the high velocity stream of electrically charged particles with are emitted by the sun. An electron's mass is about 1/2000th the mass of a proton.

  2. Alpha particles. These are helium ions (helium atoms which are stripped of their electrons and hence have an electrical charge of +2). Alpha radiation is a by-product of the natural radioactive decay of radioactive substances.

  3. Cosmic rays. Cosmic rays are made of a complex "soup" of atomic particles and exotic subatomic particles. Primary cosmic rays are mostly protons (hydrogen nuclei) which rain down on the Earth from outer space. They have an electrical charge of +1. Secondary cosmic rays are produced when primary cosmic rays shatter the atoms of the Earth's upper atmosphere and produce a complex shower of subatomic debris and electromagnetic radiation.

How is radiation detected?

Both electromagnetic and particle radiation move energy from their source to some distant point where the energy is ultimately deposited in whole or in part.

It is the deposition of energy that allows one to detect the presence of radiation.

Radiation that does not deposit any energy can never be detected, since it never interacts with anything, and therefore has no effect on the Universe!

In general, electromagnetic radiation is much more easily detected than particle radiation. This is because electromagnetic radiation readily interacts with electrons thereby making its presence known.

Charged particles are easier to detect than neutral particles because their charge causes them to interact with electrons. Neutrons, which have zero charge on the other hand, tend to pass straight through matter.

Because neutrons have no charge they are able to penetrate the electron shells surrounding an atom's nucleus and also evade the effects of the nucleus' positive electrical field; therefore, low energy neutrons are able to interact with the nucleus of other atoms more easily than electrons or protons or other charged particles.

Student Assignment (Particle Radiation)

Student Assignment (Electromagnetic Radiation)

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Prepared by the YES I Can! Science Team,