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Curie (unit)

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A sample of radium, the element which was used in the original definition of the curie.
General information
Unit ofActivity
Named afterPierre Curie and Marie Curie
1 Ci in ...... is equal to ...
   rutherfords   37000 Rd
   SI derived unit   37 GBq
   SI base unit   3.7×1010 s−1
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,[1] but was considered at least by some to be in honour of Marie 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 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[4] after more accurate measurements of the activity of 226Ra (which has a specific activity of 3.66×1010 Bq/g[5]).

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.[6] 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 the National 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 the decay energy by approximately 5.93 mW / MeV.

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.

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, 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 (35 lb) 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[edit]

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.

Here are some examples, ordered by half-life:

Isotope Half-life Mass of 1 curie Specific activity (Ci/g)
209Bi 1.9×1019 years 11.1 billion tonnes 9.01×10−17
232Th 1.405×1010 years 9.1 tonnes 1.1×10−7 (110,000 pCi/g, 0.11 μCi/g)
238U 4.471×109 years 2.977 tonnes 3.4×10−7 (340,000 pCi/g, 0.34 μCi/g)
40K 1.25×109 years 140 kg 7.1×10−6 (7,100,000 pCi/g, 7.1 μCi/g)
235U 7.038×108 years 463 kg 2.2×10−6 (2,160,000 pCi/g, 2.2 μCi/g)
129I 15.7×106 years 5.66 kg 0.00018
99Tc 211×103 years 58 g 0.017
239Pu 24.11×103 years 16 g 0.063
240Pu 6563 years 4.4 g 0.23
14C 5730 years 0.22 g 4.5
226Ra 1601 years 1.01 g 0.99
241Am 432.6 years 0.29 g 3.43
238Pu 88 years 59 mg 17
137Cs 30.17 years 12 mg 83
90Sr 28.8 years 7.2 mg 139
241Pu 14 years 9.4 mg 106
3H 12.32 years 104 μg 9,621
228Ra 5.75 years 3.67 mg 273
60Co 1925 days 883 μg 1,132
210Po 138 days 223 μg 4,484
131I 8.02 days 8 μg 125,000
123I 13 hours 518 ng 1,930,000
212Pb 10.64 hours 719 ng 1,390,000
223Fr 22 minutes 26 ng 38,000,000
212Po 299 nanoseconds 5.61 ag 1.78×1017

Radiation related quantities[edit]

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

Ionizing radiation related quantities
Quantity Unit Symbol Derivation Year SI equivalent
Activity (A) becquerel Bq s−1 1974 SI unit
curie Ci 1953 3.7×1010 Bq
rutherford Rd 106 s−1 1946 1000000 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
Equivalent dose (H) sievert Sv J⋅kg−1 × WR 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1 × WR 1971 0.010 Sv
Effective dose (E) sievert Sv J⋅kg−1 × WR × WT 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1 × WR × WT 1971 0.010 Sv

See also[edit]


  1. ^ a b Rutherford, Ernest (6 October 1910). "Radium Standards and Nomenclature". Nature. 84 (2136): 430–431. Bibcode:1910Natur..84..430R. doi:10.1038/084430a0.
  2. ^ a b Frame, Paul (1996). "How the Curie Came to Be". Health Physics Society Newsletter. Archived from the original on 20 March 2012. Retrieved 3 July 2015.
  3. ^ United States Atomic Energy Commission (1951). Semiannual Report of the Atomic Energy Commission, Volume 9. p. 93.
  4. ^ "Resolution 7 of the 12th CGPM". International Bureau of Weights and Measures (BIPM). 1964. Archived from the original on 2021-02-19.
  5. ^ Delacroix, D. (2002). "Radionuclide and Radiation Protection Data Handbook 2002". Radiation Protection Dosimetry. 98 (1). Nuclear Technology Publishing: 147. doi:10.1093/oxfordjournals.rpd.a006705. PMID 11916063. Archived from the original on 2016-03-05.
  6. ^ "SI units for ionizing radiation: becquerel". Resolutions of the 15th CGPM (Resolution 8). 1975. Retrieved 3 July 2015.
  7. ^ NIST Special Publication 811, paragraph 5.2 (Report). NIST. 28 January 2016. Retrieved 22 March 2016.