curie

Sample of cobalt-60 that emits 1 μCi (microcurie) of radioactivity; i.e. 37,000 decays per second. The curie (symbol Ci) is a non-SI unit of radioactivity originally defined in 1910. According to a notice in Nature at the time, it was to be named in honour of Pierre Curie, but was considered at least by some to be in honour of Marie Skłodowska-Curie as well, and is in later literature considered to be named for both.

It was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium (element)", but in 1964 was redefined as 1 Ci = 3.7×10 decays per second after more accurate measurements of the activity of Ra (which has a specific activity of 3.66×10 Bq/g).