If an atomic nucleus does not change or break down, we say that it is stable. Most nuclei are stable. However, some atomic nuclei are unstable and may change or break down. When this happens, they emit particles or rays, including:
The table shows some features of the four main types of radiation:
|Type of radiation||Nature||Symbol in a nuclear equation||Electric charge|
|alpha, α||Two protons and two neutrons bound together (identical to a helium nucleus)||+2|
|beta, β||A fast-moving electron from a nucleus||-1|
|neutron, n||A neutral subatomic particle from a nucleus||0|
|gamma, γ||Electromagnetic radiation||0|
Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, and gamma radiation travels very large distances.
All types of radiation become less intense as they travel further away from the radioactive material. This is because the particles or rays become more spread out. The thicker the substance, the more the radiation is absorbed.
Alpha, beta and gamma radiations penetrate materials in different ways.
Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper or a human hand.
Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.
Gamma radiation is the most penetrating. It can penetrate air, paper or thin metal. It may only be stopped by many centimetres of lead or many metres of concrete.