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Curie (unit)

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Curie
A sample of radium, the element which was used in the original definition of the curie.
General information
Unit ofActivity
SymbolCi
Named afterPierre Curie and Marie Curie
Conversions
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 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 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

and

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

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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:

Nuclide Isotopic mass (Da)[8] Number of atoms in 1 gram Half-life[9][a] Specific activity (Ci/g) Mass of 1 curie
209Bi 208.9803986 2.8816773×1021 2.01×1019 years 8.51×10−17 11.7 billion tonnes
190Pt 189.9599498 3.1702160×1021 4.83×1011 years 3.90×10−9 257 tonnes
147Sm 146.9149044 4.0990673×1021 1.066×1011 years 2.28×10−8 43.8 tonnes
232Th 232.0380536 2.5953246×1021 1.405×1010 years 1.10×10−7 (0.110 μCi/g) 9.12 tonnes
238U 238.0507876 2.5297714×1021 4.468×109 years 3.36×10−7 (0.336 μCi/g) 2.98 tonnes
40K 39.96399817 1.50689146×1022 1.248×109 years 7.18×10−6 (7.17 μCi/g) 140 kg
235U 235.0439281 2.5621342×1021 7.038×108 years 2.16×10−6 (2.16 μCi/g) 463 kg
129I 128.9049836 4.6717672×1021 1.614×107 years 1.72×10−4 (172 μCi/g) 5.82 kg
99Tc 98.90624968 6.0887363×1021 2.111×105 years 1.71×10−4 58.4 g
239Pu 239.0521616 2.5191744×1021 2.411×104 years 6.20×10−2 16.1 g
240Pu 240.0538117 2.5086628×1021 6561 years 0.227 4.41 g
14C 14.00324199 4.30053323×1022 5700 years 4.48 223 mg
226Ra 226.0254082 2.6643645×1021 1600 years 0.989 1.01 g
241Am 241.0568273 2.4982245×1021 432.6 years 3.43 292 mg
238Pu 238.0495582 2.5297845×1021 87.7 years 17.1 58.4 mg
137Cs 136.9070893 4.3987063×1021 30.04 years 86.9 11.5 mg
90Sr 89.9077279 6.6981347×1021 28.91 years 138 7.27 mg
241Pu 241.0568497 2.4982243×1021 14.329 years 104 9.66 mg
3H 3.016049281320 1.996698393×1023 12.32 years 9.62×103 104 μg
228Ra 228.0310686 2.6409299×1021 5.75 years 273 3.67 mg
60Co 59.93381554 1.00479849×1022 5.2714 years 1.13×103 884 μg
210Po 209.9828737 2.8679200×1021 138.376 days 4.49×103 223 μg
131I 130.9061264 4.6003506×1021 8.0249 days 1.24×105 8.05 μg
123I 122.9055898 4.8998103×1021 13.2232 hours 1.93×106 519 ng
212Pb 211.9918959 2.8407410×1021 10.627 hours 1.39×106 719 ng
223Fr 223.0197342 2.7002726×1021 22.00 minutes 3.83×107 26.1 ng
212Po 211.9888680 2.8407816×1021 294.4 nanoseconds 1.81×1017 5.53 ag
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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 3.7×1010 s−1 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

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References

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  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.
  8. ^ Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
  9. ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
  1. ^ Note that NUBASE2020 uses the tropical year to convert between years and other units of time, not the Gregorian year. The relationship between years and other time units in NUBASE2020 is as follows: 1 y = 365.2422 d = 31 556 926 s
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