National Physical Laboratory

Radiation units

radiation3


Measurement of radiation

Absorbed dose

The measure of the energy deposited in a medium by radiation is called the absorbed dose.

The absorbed dose is expressed in the unit J/kg, which is called the gray (Gy) 

Equivalent dose

The equivalent dose is a measure of the radiation dose to tissue taking into account different relative biological effects of the different types of ionising radiation.

The equivalent dose describes the health hazard caused by radiation and is expressed in units of sieverts (Sv) . The dose is often given in thousandths of sieverts, i.e. millisieverts (mSv) or in millionths i.e. microsieverts (µSv).

Equivalent dose is calculated by multiplying the absorbed dose with the quality factor, which is 1 for gamma rays and beta particles.

For beta, gamma and X-rays, 1 Gy is the same as 1 Sv, but neutrons and alpha rays are more damaging and for these 1Gy is worth between 5 Sv and 20 Sv.

Bequerel

The unit of radioactivity is the Becquerel (Bq), this is equal to one nuclear disintegration per second.

In the field of ionizing radiation, the SI unit of activity is designated the Becquerel rather than the reciprocal second, and the SI units of absorbed dose and dose equivalent are designated in the gray and sievert, respectively, rather than the joule per kilogram. These names were introduced because of the dangers to human health that might arise from mistakes involving the reciprocal second and joules per kilogram.

Electronvolt

Atomic properties like ionization energy are often quoted in electronvolts (eV). This is not an SI unit as its value is obtained experimentally.

 


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