What is radiation?
Radiation is energy travelling through space.
Sunshine is one of the most familiar forms of radiation. It
delivers light, heat and suntans. While enjoying and
depending on it, we control our exposure to it.
Beyond ultraviolet radiation from the sun are higher-energy
kinds of radiation which are used in medicine and which we all get
in low doses from space, from the air, and from the earth and
Collectively we can refer to these kinds of radiation
as ionising radiation. It can cause
damage to matter, particularly living tissue. At high levels
it is therefore dangerous, so it is necessary to control our
While we cannot feel this radiation, it is readily detected and measured, and exposure can easily be monitored.
Living things have evolved in an environment which has significant levels of ionising radiation.
Furthermore, many people owe their lives and health to such
radiation produced artificially. Medical and dental X-rays
discern hidden problems. Other kinds of ionising radiation
are used to diagnose ailments, and some people are treated with
radiation to cure disease.
Ionising radiation, such as occurs from uranium ores and nuclear
wastes, is part of our human environment, and always has been so.
At high levels it is hazardous, but at low levels such as we all
experience naturally, it is harmless. Considerable effort is
devoted to ensuring that those working with nuclear power are not
exposed to harmful levels of radiation from it. Standards for
the general public are set about 20 times lower still, well below
the levels normally experienced by any of us from natural
Background radiation is that ionizing radiation which is
naturally and inevitably present in our environment. Levels
of this can vary greatly. People living in granite areas or
on mineralised sands receive more terrestrial radiation than
others, while people living or working at high altitudes receive
more cosmic radiation. A lot of our natural exposure is due
to radon, a gas which seeps from the Earth's crust and is present
in the air we breathe.
Radioactivity in material
Apart from the normal measures of mass and volume, the amount of
radioactive material is measured
in Becquerel (Bq), which enables us to
compare the typical radioactivity of some natural and other
materials. A Becquerel is one atomic decay per second, so a
household smoke detector with 30,000 Bq contains enough americium
to produce that much disintegration per second. A kilogram of
coffee or granite might have 1000 Bq of activity and an adult human
7000 Bq. Each atomic disintegration produces some ionizing
Ionising radiation - alpha, beta and
Ionising radiation comes from the nuclei of atoms, the basic
building blocks of matter. Most atoms are stable, but certain atoms
change or disintegrate into totally new atoms. These kinds of atoms
are said to be 'unstable' or 'radioactive'. An unstable atom has
excess internal energy, with the result that the nucleus can
undergo a spontaneous change. This is called 'radioactive
decay'.We all experience radiation from natural sources every day
An unstable nucleus emits excess energy as radiation in the form
of gamma rays or fast-moving sub-atomic particles. If it
decays with emission of an alpha or beta particle, it becomes a new
element. One can describe the emissions as gamma, beta and
alpha radiation. All the time, the atom is progressing in one
or more steps towards a stable state where it is no longer
Alpha particles consist of two
protons and two neutrons, in the form of atomic nuclei. Alpha particles are doubly charged (arising from the charge of the two protons). This charge and the relatively slow speed and high mass of alpha particles means that they interact more readily with matter than beta particles or gamma rays and lose their energy quickly. They therefore have little
penetrating power and can be stopped by the first layer of skin or
a sheet of paper. But inside the body they can inflict more
severe biological damage than other types of radiation.
Beta particles are fast-moving
electrons ejected from the nuclei of many kinds of radioactive
atoms. These particles are singly charged (the charge of an electron), are lighter and ejected at a much faster speed than alpha particles. They can penetrate up to 1 to 2 centimetres of water or human
flesh. They can be stopped by a sheet of aluminium a few
Gamma rays, like light, represent energy
transmitted in a wave without the movement of material, just like
heat and light. Gamma rays and X-rays are virtually identical
except that X-rays are generally produced artificially rather than
coming from the atomic nucleus. But unlike light, these rays
have great penetrating power and can pass through the human body.
Mass in the form of concrete, lead or water is used to shield us
The effective dose of all these kinds of radiation is measured in a
unit called the Sievert, although most doses experienced are much lower than a Sievert, so figures are given in millisieverts (mSv), which are one-thousandth of a Sievert.