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Types and units

Types of radiation

Radiation takes five main forms.

Alpha particles consist of two protons and two neutrons and have a net positive charge. Beta particles consist of a stream of fast electrons and have a net negative charge.

Gamma rays are an intense form of energy - like light. You could see them if your eyes were tuned to their very short wavelengths.

People who work with radioactive materials may also meet two other forms of radiation: X-rays and neutron particles. None of these forms of radiation can be sensed by man, but all of them can be detected by simple instruments.

Unstable atoms persist in this process of radioactive decay until they reach a stable, non-radioactive form. This may take them billions of years or less than a second.

We cannot 'speed up' this process, nor slow it down, nor stop it. But it is also predictable. This has enabled safe handling procedures for radioactive materials to be established.

Units of radioactivity

Radioactivity and radiation are measured in three different units - the gray, the bequerel and the sievert.

Radiation varies in type and intensity: a small amount of one type may do more damage to the body than a large amount of another type. The sievert (Sv) is the unit calculated to take account of these differences.

Sieverts are used to measure and compare the doses of radiation different people receive from different sources. The sievert is a very large unit and it is usual to measure doses in smaller fractions of it. The most common fractions are one-thousandth and one-millionth of a sievert (millisieverts [mSv] and microsieverts [mSv] respectively).