Ionizing radiation consists of electromagnetic waves with sufficient energy to cause electrons to become detached from atoms and molecules in the matter they pass through, changing their structure – a process known as ionization. As a result, they become electrically charged (Figure 1) 1.
As part of our environment, from both natural sources present in the earth (terrestrial) and from space (cosmic), we are continually exposed to ionizing radiation. In addition, man-made sources also contribute to our exposure.
There are a number of types of ionizing radiation that each have a different power of penetration and cause different degrees of ionization in material.
The most widely known types of ionizing radiation are X-rays, used in radiological equipment for medical purposes e.g. in diagnosis and treatment. X-rays occur together with alpha (α), beta (β) and gamma (γ) radiation produced by the unstable nuclei of atoms.
Ionizing radiation penetrates according to its type and energy. While alpha particles can be blocked by a sheet of paper, beta particles require a few millimeters of e.g. aluminum to block them while high energy gamma radiation requires dense materials to block it, e.g. lead or concrete.
Ionizing radiation occurs naturally, for instance from the radioactive decay of natural radioactive substances such as radon gas. The rate at which a radionuclide decays (becomes less radioactive) is defined by its “half life”, i.e. the time it takes for a radioactive material to decay by 50%. Depending on the radionuclide, this can vary from fractions of a second to millions of years.
Radiation is very easy to measure in materials, even at very low levels, because of the ionization it causes. Radioactive material in a medium such as air, water, soil, grass, food, etc. can be collected and the quantity of radioactivity measured and expressed as a concentration.
In the United States, radiation dose is usually measured in units called rem and millirem. In the International System of Units, dose is measured in units called sieverts (Sv) and millisieverts (mSv). One sievert is equal to 100 rem.