Types of radiation
Radiation is emitted by certain types of atoms, known as unstable atoms, or radioisotopes, which contain too much energy.
Over time, these atoms change, or decay, into stable atoms by releasing their excess energy in the form of radiation. There are different types of radioactive atoms which emit different forms of radiation.
The radiation from unstable atoms may include one or more of the following forms:
The time which radioactive materials take to decay and lose their excess energy is measured in half-lives.
One half-life is the average time for half of the atoms in a quantity of a radioactive material to decay. After two half-lives, only one-quarter of the original atoms will remain. After three half-lives, only one-eighth of the original atoms will remain. As time goes on, more and more of the unstable atoms will change into the stable decay product.
A long half-life means that an atom decays slowly and emits radiation slowly. A short half-life means that an atom decays quickly and emits radiation quickly.
- Alpha particles
Alpha particles can travel only a few centimetres in the air and can be stopped by a sheet of paper or a layer of skin.
However, if a material that emits alpha particles is taken into the human body by being ingested or inhaled, it can irradiate the surrounding cells. Alpha-emitting materials can therefore be hazardous if taken into the human body in some way.
An example of an alpha-emitter is Uranium-238. Uranium is a naturally occurring radioactive material composed of unstable atoms that has been part of the Earth's crust since it first formed. Uranium-238 has a very long half-life—approximately 4.5 billion years. It is found in all rocks, but is particularly concentrated in certain types of granites and sedimentary rocks. In nature, Uranium-238 is found mixed with Uranium-235, the type of uranium that is used as fuel in nuclear reactors to produce energy. Many of the atoms formed directly and indirectly by the breakdown of Uranium-235 and Uranium-238 also emit alpha particles.
- Beta particles
Beta particles can travel metres in the air and several millimetres into the human body.
They can be stopped by a small thickness of light material such as aluminium or plastic sheeting. Low-energy beta particles cannot penetrate the skin, but if taken into the body they will irradiate internal tissues. Beta-emitting materials can therefore be hazardous.
An example of a beta-emitter is carbon-14. Carbon-14 is a naturally occurring radioisotope found in the Earth's atmosphere. It can also be manufactured by humans. It has a half-life of 5700 years. The natural amounts of Carbon-14 that have been taken into organic materials are often used in archaeological studies to determine the ages of human remains and other organic artefacts from ancient civilisations.
- Gamma rays
Gamma rays are the most penetrating type of radiation.
They can be stopped by a thick barrier of lead, concrete or water. They can pass through the human body and irradiate human tissues. Gamma-emitting materials can therefore be hazardous.
An example of a gamma-emitter is Technetium-99m. Technetium-99m is a human-made radioisotope which is used extensively in nuclear medicine. It has a half-life of six hours.
These forms of radiation have different levels of energy and can be managed in different ways.
Understanding the precise type of radioactive material being dealt with is important in minimising the exposure to radioactivity when using radioactive materials, and when disposing of radioactive waste.
For more information about radiation and radioactive waste, see Radioactive waste management.