Curies

The curie (symbol Ci) is a non-SI unit of radioactivity, named 'in honour of' Pierre Curie, according to his widow, the famed radiation researcher Marie Curie.

It was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium (element)" but is currently defined as: 1 Ci = 3.7 × 10¹⁰decays per second after more accurate measurements of the activity of ²²⁶Ra (which has a specific activity of 3.66 x 10¹⁰ Bq/g.)

In 1975 the General Conference on Weights and Measures gave the becquerel (Bq), defined as one nuclear decay per second, official status as the SI unit of activity. Therefore:

and

While its continued use is discouraged by National Institute of Standards and Technology (NIST) and other bodies, the curie is still widely used throughout the government, industry and medicine in the United States and in other countries.

The curie is a large amount of activity, and was intentionally so. According to Bertram Boltwood, Marie Curie thought that 'the use of the name "curie" for so infinitesimally small (a) quantity of anything was altogether inappropriate.'

The typical human body contains roughly 0.1 μCi (14 mg) of naturally occurring Potassium-40. A human body containing 16 kg of Carbon (see composition of the human body) would also have about 24 nanograms or 0.1 μCi of Carbon-14. Together, these would result in a total of approximately 0.2 μCi or 7400 Bq inside the person's body.

Units of activity (the curie and the becquerel) also refer to a quantity of radioactive atoms. Because the probability of decay is a fixed physical quantity, for a known number of atoms of a particular radionuclide, a predictable number will decay in a given time. The number of decays that will occur in one second in one gram of atoms of a particular radionuclide is known as the specific activity of that radionuclide.

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