Types and sources of radiation
Radiation occurs when energy is emitted by a source and then travels through a medium, such as air, until it is absorbed by matter. Radiation can be described as two basic types, ionizing and non-ionizing radiation.
- Natural background radiation
- Artificial sources of radiation
People use and are exposed to non-ionizing radiation sources every day. This form of radiation does not carry enough energy to ionize atoms or molecules.
Microwave ovens, global positioning systems, cellular telephones, television stations, FM and AM radio, baby monitors, cordless phones, garage-door openers, and ham radios all make use of non-ionizing radiation. Other forms include the earth’s magnetic field, as well as magnetic field exposure from proximity to transmission lines, household wiring and electric appliances. These are defined as extremely low-frequency (ELF) waves.
Some types of radiation have enough energy that they can knock electrons out of their orbits around atoms, upsetting the electron/proton balance and giving the atom a positive charge. Electrically charged molecules and atoms are called ions. The radiation that can produce ions is called ionizing radiation.
There are many types of ionizing radiation. The following are some of the relevant ones:
Alpha radiation: Alpha particles consist of two protons and two neutrons, and since they have no electrons, carry a positive charge. Due to their size and charge, alpha particles are barely able to penetrate skin and can be stopped completely by a sheet of paper.
Beta radiation: Beta radiation consists of fast moving electrons ejected from the nucleus of an atom. Beta radiation has a negative charge and is about 1/7000th the size of an alpha particle and so is more penetrating. However, it can still be stopped by a small amount of shielding, such as a sheet of plastic.
Gamma radiation: Gamma radiation is a very penetrating type of radiation. It is usually emitted immediately after the ejection of an alpha or beta particle from the nucleus of an atom. Because it has no mass or charge, it can pass through the human body, but will be absorbed by denser materials such as concrete or lead.
X-rays: X-rays are a form of radiation similar to gamma radiation but they are produced mainly by artificial means rather than from radioactive substances.
Neutron radiation: Neutron radiation occurs when neutrons are ejected from the nucleus by nuclear fission and other processes. The nuclear chain reaction is an example of nuclear fission, where a neutron being ejected from one fissioned atom will cause another atom to fission, ejecting more neutrons. Unlike other radiations, neutron radiation is absorbed by materials with lots of hydrogen atoms, like paraffin wax and plastics.
People are constantly exposed to small amounts of ionizing radiation from the environment as they carry out their normal daily activities; this is known as background radiation. We are also exposed through some medical treatments and through activities involving radioactive material.
People are constantly exposed to small amounts of ionizing radiation from the environment as they carry out their normal daily activities; this is known as natural background radiation.
Radiation has always been present and is all around us. Life has evolved in a world containing significant levels of ionizing radiation. Our bodies are adapted to it.
The following section outlines sources of natural background radiation. For information on dose levels from these sources, visit the Canadian Nuclear Safety Commission's (CNSC's) Radiation Doses page and fact sheet on natural background radiation.
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) identifies four major sources of public exposure to natural radiation:
- cosmic radiation
- terrestrial radiation
The earth's outer atmosphere is continually bombarded by cosmic radiation. Usually, cosmic radiation consists of fast moving particles that exist in space and originate from a variety of sources, including the sun and other celestial events in the universe. Cosmic rays are mostly protons but can be other particles or wave energy. Some ionizing radiation will penetrate the earth's atmosphere and become absorbed by humans, which results in natural radiation exposure.
The doses due to natural sources of radiation vary depending on location and habits. Regions at higher altitudes receive more cosmic radiation. The following map shows how levels of cosmic radiation vary with elevations above sea level and longitude and latitude.
Annual outdoor effective dose from cosmic radiation for North America (in microsieverts)
Source: Gratsky et al., 2004
The composition of the earth's crust is a major source of natural radiation. The main contributors are natural deposits of uranium, potassium and thorium which, in the process of natural decay, will release small amounts of ionizing radiation. Uranium and thorium are “ubiquitous”, meaning they are found essentially everywhere. Traces of these minerals are also found in building materials so exposure to natural radiation can occur from indoors as well as outdoors.
Most of the variation in exposure to natural radiation results from inhalation of radioactive gases that are produced by radioactive minerals found in soil and bedrock. Radon is an odourless and colourless radioactive gas that is produced by the decay of uranium. Thoron is a radioactive gas produced by the thorium. Radon and thoron levels vary considerably by location depending on the composition of soil and bedrock. Once released into the air, these gases will normally dilute to harmless levels in the atmosphere but sometimes they become trapped and accumulate inside buildings and are inhaled by occupants. Radon gas poses a health risk not only to uranium miners, but also to homeowners if it is left to collect in the home. On average, it is the largest source of natural radiation exposure. More information on radon gas and the means to control it can be found on Health Canada's website.
Trace amounts of radioactive minerals are naturally found in the contents of food and drinking water. For instance, vegetables are typically cultivated in soil and ground water which contains radioactive minerals. Once ingested, these minerals result in internal exposure to natural radiation.
Naturally occurring radioactive isotopes, such as potassium-40 and carbon-14, have the same chemical and biological properties as their non-radioactive isotopes. These radioactive and non-radioactive elements are used in building and maintaining our bodies. Natural radioisotopes continually expose us to radiation. The table below identifies the amount of radioactivity from potassium-40 contained in about 500 grams of different food products. A becquerel is a unit of radioactivity, equal to one transformation (decay) per second.
|Food||Becquerel (Bq) per 500 grams|
|Brazil nut||103||Brazil nuts also naturally contain radium-226 (between 19 and 130 Bq per 500 grams.)|
Source: Handbook of Radiation Measurement and Protection, Brodsky, A. CRC Press 1978
The human body also contains several radioactive isotopes. The table below contains a list of some of the isotopes naturally found in the body.
|Isotopes||Amount of radioactivity in Bq|
2 ICRP-23 (1975)
3 Environmental Radioactivity from Natural, Industrial and Military Sources, Eisenbud, M and Gesell T. Academic Press, Inc. 1997
4 UNSCEAR 2000
5 ICRP-30 (1980)
Atmospheric testing: The atmospheric testing of atomic weapons from the end of the Second World War until as late as 1980 released radioactive material, called fallout, into the air. As the fallout settled to the ground, it was incorporated into the environment. Much of the fallout had short half-lives and no longer exists, but some continues to decay to this day. People and the environment receive smaller and smaller doses from the fallout every year.
Medical sources: Radiation has many uses in medicine. The most well known use is X-ray machines, which use radiation to find broken bones and diagnose disease. X-ray machines are regulated by Health Canada and provincial authorities. Another example is nuclear medicine, which uses radioactive isotopes to diagnose and treat diseases such as cancer. These applications of nuclear medicine, as well as the related equipment, are regulated by the CNSC. The CNSC also licenses those reactors and particle accelerators that produce isotopes destined for medical and industrial applications.
Normally, there is little that we can do to change or reduce ionizing radiation that comes from natural sources like the sun, soil or rocks. This kind of exposure, while never entirely free of risk, is generally quite low. However, in some cases, natural sources of radioactivity may be unacceptably high and need to be reduced, such as radon gas in the home.
The ionizing radiation that comes from man-made sources and activities is controlled more carefully. In these settings, a balance is struck between the benefits that the radiation provides to society and the risks it imposes on people and the environment. Dose limits are set in order to restrict radiation exposures to both workers and members of the public. In addition, licensees are required to keep all radiation doses as low as reasonably achievable (ALARA), social and economic factors being taken into account. Also, there has to be a net benefit to the use of radiation. For example, smoke detectors are permitted to use radioactive isotopes because smoke detectors save lives.
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