Measuring ionising radiation - Grays and Sieverts

The human senses cannot detect radiation or discern whether a material is radioactive.

However, a variety of instruments can detect and measure radiation reliably and accurately.

The amount of ionising radiation, or 'dose', received by a person is measured in terms of the energy absorbed in the body tissue, and is expressed in gray. One gray (Gy) is one joule deposited per kilogram of mass.

Equal exposure to different types of radiation expressed as gray do not however necessarily produce equal biological effects. One gray of alpha radiation, for example, will have a greater effect than one gray of beta radiation. When we talk about radiation effects, we therefore express the radiation as effective dose, in a unit called the sievert (Sv).

Regardless of the type of radiation, one sievert (Sv) of radiation produces the same biological effect.

Smaller quantities are expressed in 'millisievert' (one thousandth) or 'microsievert' (one millionth) of a sievert. We will use the most common unit, millisievert (mSv), here.