Sample ofcobalt-60 that emits 1 μCi (microcurie) of radioactivity; i.e. 37,000 decays per second.
Thecurie (symbolCi) is a non-SI unit ofradioactivity originally defined in 1910. According to a notice inNature at the time, it was to be named in honour ofPierre Curie,[1] but was considered at least by some to be in honour ofMarie Skłodowska-Curie as well,[2] and is in later literature considered to be named for both.[3]
It was originally defined as "the quantity or mass ofradium emanation in equilibrium with one gram ofradium (element)",[1] but is currently defined as 1 Ci =3.7×1010decays persecond[4] after more accurate measurements of the activity of226Ra (which has a specific activity of3.66×1010 Bq/g[5]).
While its continued use is discouraged by theNational Institute of Standards and Technology (NIST)[7] and other bodies, the curie is still widely used throughout government, industry and medicine in the United States and in other countries.
At the 1910 meeting, which originally defined the curie, it was proposed to make it equivalent to 10 nanograms of radium (a practical amount). But Marie Curie, after initially accepting this, changed her mind and insisted on one gram of radium. 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".[2]
The power emitted in radioactive decay corresponding to one curie can be calculated by multiplying thedecay energy by approximately 5.93 mW / MeV.
Aradiotherapy machine may have roughly 1000 Ci of a radioisotope such ascaesium-137 orcobalt-60. This quantity of radioactivity can produce serious health effects with only a few minutes of close-range, unshielded exposure.
Radioactive decay can lead to the emission of particulate radiation or electromagnetic radiation. Ingesting even small quantities of some particulate emitting radionuclides may be fatal. For example, themedian lethal dose (LD-50) for ingestedpolonium-210 is 240 μCi; about 53.5 nanograms.
The typical human body contains roughly 0.1 μCi (14 mg) of naturally occurringpotassium-40. A human body containing 16 kg (35 lb) of carbon (seeComposition of the human body) would also have about 24 nanograms or 0.1 μCi ofcarbon-14. Together, these would result in a total of approximately 0.2 μCi or 7400 decays per second inside the person's body (mostly from beta decay but some from gamma decay).
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 particularradionuclide, 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 thespecific activity of that radionuclide.
The activity of a sample decreases with time because of decay.
The rules ofradioactive decay may be used to convert activity to an actual number of atoms. They state that 1 Ci of radioactive atoms would follow the expression
^Note that NUBASE2020 uses thetropical year to convert between years and other units of time, not theGregorian year. The relationship between years and other time units in NUBASE2020 is as follows:1 y = 365.2422 d = 31 556 926 s
