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The unit of activity (of a radioactive substance) in SI. Symbol, Bq.

A substance is radioactive if the nuclei of its atoms are unstable, spontaneously changing into nuclei of another element while emitting radiation. One becquerel is one such spontaneous nuclear transition per second.

The CGPM adopted the becquerel in 1975 (15th CGPM, Resolution 8) on the advice of the International Commission on Radiation Units and Measurement and the International Commission on Radiological Protection. It replaced the curie, whose temporarily continued use was sanctioned. One curie = 3.7 × 1010 Bq. The primary reason for replacing the curie with the becquerel was to make SI coherent.

photograph of Becquerel

The unit is named for the French physicist Antoine Henri Becquerel (1852 – 1908), who discovered radioactivity.

K. Lidén.
SI Units in Biomedical Dosimetry.
Biomedical Dosimetry. Proceedings of an International Symposium on Advances in Biomedical Dosimetry Held by the International Atomic Energy Agency in Vienna, 10-14 March 1975.
Vienna: International Atomic Agency Agency, 1975.

Describes the rationale behind the ICRU's recommendation that the CGPM adopt the becquerel.

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