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 named in honour of Pierre Curie,[1] but was considered at least by some to be in honour of Marie Curie as well.[2]

A sample of radium, the element which was used in the original definition of the curie.
General information
Unit ofSpecific activity
Named afterPierre Curie
1 Ci in ...... is equal to ...
   rutherfords   37000 Rd
   SI derived unit   37 GBq
   SI base unit   3.7×1010 s−1

It was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium (element)" [1] but is currently defined as 1 Ci = 3.7×1010 decays per second after more accurate measurements of the activity of 226Ra (which has a specific activity of 3.66×1010 Bq/g[3]).

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.[4] Therefore:

1 Ci = 3.7×1010 Bq = 37 GBq


1 Bq ≅ 2.703×10−11 Ci ≅ 27 pCi

While its continued use is discouraged by National Institute of Standards and Technology (NIST)[5] 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 in milliwatts emitted by one curie of radiation can be calculated by taking the number of MeV for the radiation times approximately 5.93.

A radiotherapy machine may have roughly 1000 Ci of a radioisotope such as caesium-137 or cobalt-60. This quantity of radioactivity can produce serious health effects with only a few minutes of close-range, unshielded exposure.

Ingesting even a millicurie is usually fatal (unless it is a very short-lived isotope). For example, the median lethal dose (LD-50) for ingested polonium-210 is 240 μCi; about 53.5 nanograms.

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 decays per second inside the person's body (mostly from beta decay but some from gamma decay).

As a measure of quantity

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.

The activity of a sample decreases with time because of decay.

The rules of radioactive 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:

N (atoms) × λ (s−1) = 1 Ci = 3.7 × 1010 Bq

and so,

N = 3.7 × 1010 Bq / λ,

where λ is the decay constant in s−1.

We can also express activity in moles:

where NA is Avogadro's number and t1/2 is the half life. The number of moles may be converted to grams by multiplying by the atomic mass.

Here are some examples, ordered by half-life:

IsotopeHalf lifeMass of 1 curieSpecific activity (Ci/g)
232Th1.405×1010 years9.1 tonnes1.1×10−7 (110,000 pCi/g, 0.11 µCi/g)
238U4.471×109 years2.977 tonnes3.4×10−7 (340,000 pCi/g, 0.34 µCi/g)
40K1.25×109 years140 kg7.1×10−6 (7,100,000 pCi/g, 7.1 µCi/g)
235U7.038×108 years463 kg2.2×10−6 (2,160,000 pCi/g, 2.2 µCi/g)
129I15.7×106 years5.66 kg0.00018
99Tc211×103 years58 g0.017
239Pu24.11×103 years16 g0.063
240Pu6563 years4.4 g0.23
14C5730 years0.22 g4.5
226Ra1601 years1.01 g0.99
241Am432.6 years0.29 g3.43
238Pu88 years59 mg17
137Cs30.17 years12 mg83
90Sr28.8 years7.2 mg139
241Pu14 years9.4 mg106
3H12.32 years104 μg9,621
228Ra5.75 years3.67 mg273
60Co1925 days883 μg1,132
210Po138 days223 μg4,484
131I8.02 days8 μg125,000
123I13 hours518 ng1,930,000
212Pb10.64 hours719 ng1,390,000

The following table shows radiation quantities in SI and non-SI units:

Ionising radiation related quantities
QuantityUnitSymbolDerivationYearSI equivalence
Activity (A) becquerel Bq s−1 1974 SI unit
curie Ci 3.7 × 1010 s−1 1953 3.7×1010 Bq
rutherford Rd 106 s−1 1946 1,000,000 Bq
Exposure (X) coulomb per kilogram C/kg C⋅kg−1 of air 1974 SI unit
röntgen R esu / 0.001293 g of air 1928 2.58 × 10−4 C/kg
Absorbed dose (D) gray Gy J⋅kg−1 1974 SI unit
erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 Gy
rad rad 100 erg⋅g−1 1953 0.010 Gy
Dose equivalent (H) sievert Sv J⋅kg−1 × WR 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1 1971 0.010 Sv

See also


  1. Rutherford, Ernest (6 October 1910). "Radium Standards and Nomenclature". Nature. 84 (2136): 430–431. Bibcode:1910Natur..84..430R. doi:10.1038/084430a0.
  2. Frame, Paul (1996). "How the Curie Came to Be". Health Physics Society Newsletter. Retrieved 3 July 2015.
  3. Delacroix, D (2002). Radionuclide and Radiation Protection Data Handbook 2002. Radiation Protection Dosimetry, Vol. 98 No 1: Nuclear Technology Publishing. p. 147.
  4. "SI units for ionizing radiation: becquerel". Resolutions of the 15th CGPM (Resolution 8). 1975. Retrieved 3 July 2015.
  5. "Nist Special Publication 811, paragraph 5.2". NIST. Retrieved 22 March 2016.
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