Nuclear radiation comes from the nucleus of an atom. Substances that give out radiation are said to be radioactive. There are three types of nuclear radiation:
Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper.
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Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.
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Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead, or many metres of concrete.
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Alpha, Beta and Gamma Decay