The quantities of radioactive substances and ionising radiation doses are measured using three different units:
- the Becquerel (Bq) expresses activity, i.e. the number of transformations within a radioactive substance per second.
- the Gray (Gy) expresses the dose physically absorbed, i.e. the quantity of ionising radiation absorbed by the tissues in the human body or by an object. Gray is a unit typically used in nuclear medicine.
- the Sievert (Sv) expresses the effective dose, i.e. the biological impact of ionising radiation (α, β, γ, etc.).