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Units of measurement for radioactivity

To measure radioactivity and its effects, three different phenomena must be taken into account: the activity emitted by the radioactive source, the absorbed dose, and the effect of that radiation on the organism or the environment.
The activity of a radioactive source is measured in becquerels, abbreviated Bq. One becquerel corresponds to one nuclear decay per second.
It is in fact a very small unit.
For example, a person who weighs 60 kg has an activity of about 6,000 Bq of radioactive potassium-40 in their skeleton.
The dose absorbed by a target is measured in grays, abbreviated Gy. One gray corresponds to the absorption of one joule of energy by one kilogram of matter (inert or living) exposed to ionizing radiation.
It is used to express high doses, such as those administered in radiation therapy for the local destruction of malignant cells. These doses total tens of grays, which would be a lethal level if it were delivered to the entire body.
The biological effect of a given dose of radiation on a living organism is not a measurable physical quantity. Such effect depends on the energy delivered to the tissues, the type of radiation and the type of tissue concerned. The sievert, abbreviated Sv, is the legal unit of equivalent radiation dose used to quantify this effect.
It is defined as a dose of radiation (measured in grays) weighted by dimensionless factors that take its biological effect into account.
The population of France is exposed to an average radiation dose of 3.5 mSv per year.