Curie (unit)

Last revised by Daniel J Bell on 5 May 2021

The curie (symbol Ci) was the unit for radioactive decay in the cgs system. One curie was defined as the radioactivity of one gram of pure radium-226; this is equivalent to 3.7 x 1010 decays per second. It was officially replaced by the becquerel in 1975. 


One curie was too large to be useful for most everyday applications and therefore the millicurie (mCi) and even the nanocurie (nCi) were regularly employed; 1 nCi still represented 37 disintegrations per second or 37 Bq.

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

History and etymology

The curie is named after Marie Skłodowska-Curie (1867-1934) and her husband, Pierre Curie (1859-1906) 2, French physicists, who performed much of the early fundamental work on radioactivity. 

In 1975 the International Commission on Radiation Units and Measurements (ICRU) recommended that the curie be replaced by the new unit, the becquerel. The new unit was named after Antoine Henri Becquerel (1852-1908), the French physicist, who discovered radioactivity, and ironically shared the Nobel Prize for Physics in 1903 with the Curie couple 3!

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