What is the SI unit of radioactivity?

The SI unit of radioactivity is the becquerel. It measures a radioactive substance’s activity: the average number of nuclear transformations occurring each second. One becquerel equals one decay per second, written 1 Bq = 1 s⁻¹.

The unit is named after Henri Becquerel, the French physicist who discovered that uranium emits penetrating radiation in 1896. Becquerel shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie for research into radioactivity. The becquerel became the official SI unit of activity in 1975.

A common mix-up is confusing activity with radiation dose. The becquerel describes how many decays a source produces, while the gray measures absorbed energy per kilogram and the sievert estimates biological effect. The curie is also a unit of radioactivity, but it is an older non-SI unit. One curie equals 37 billion becquerels, so the becquerel is often expressed with prefixes such as kilobecquerel, megabecquerel, or gigabecquerel when describing stronger sources.