What is the SI unit of the rate of radioactive decay?

The SI unit of the rate of radioactive decay is the becquerel, abbreviated Bq. One becquerel corresponds to one nuclear disintegration, or radioactive decay event, per second.

The unit measures activity: how frequently unstable nuclei decay in a sample. It does not directly measure the energy absorbed by a person or the biological harm caused by radiation. A sample with an activity of 1,000 Bq undergoes about 1,000 decay events per second, although radioactive decay remains random for individual atoms.

The becquerel is named for French physicist Henri Becquerel, whose discovery of spontaneous radioactivity helped launch nuclear physics. He shared the 1903 Nobel Prize in Physics with Marie Curie and Pierre Curie. The becquerel became the SI unit for activity in 1975, replacing the older curie in formal SI usage.

The curie is still encountered in historical and medical contexts and is much larger: one curie equals 3.7 × 10^10 Bq. The sievert measures biological radiation effect, while the gray measures absorbed radiation energy; neither is a unit of decay rate.