Becquerel (SI unit)

Last revised by Daniel J Bell on 4 May 2021

The becquerel (symbol: Bq) is the SI unit of radioactivity and is defined as one nuclear disintegration per second 1; it officially replaced the curie (Ci), the unit in the superseded cgs system, in 1975.

On we primarily express all radioactive doses in becquerels. However as the curie remains in common use in some parts of the world we always provide the curie equivalent in brackets after the becquerel dose, e.g. 37 MBq (1 mCi)

One becquerel is a very small unit and is invariably used with a prefix, e.g. megabecquerel (MBq), gigabecquerel (GBq), terabecquerel (TBq) and, even petabecquerel (PBq) are all used.

As for all other eponymous SI units when the unit is written out in full it is not capitalized, but when shortened to its symbol it is capitalized.

  • 100 MBq equates to 0.002703 Ci, which shows what an extremely small unit one becquerel is
  • 1 millicurie equates to 37 MBq

The unit was proposed in 1975 by the International Commission on Radiation Units and Measurements (ICRU). It is named after Antoine Henri Becquerel, the French physicist, who discovered radioactivity, and shared the Nobel Prize for Physics in 1903 with Marie Skłodowska-Curie and her husband Pierre Curie 1,2. Ironically the former unit that the becquerel replaced was the curie!

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