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 × 1010 decays per second after more accurate measurements of the activity of 226Ra (which has a specific activity of 3.66 x 1010 Bq/g.)
- 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) 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.
Curie 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)
- N = 3.7 × 1010 / λ,
where λ is the decay constant in (s−1).
We can also express activity in moles:
Here are some examples:
|Isotope||Half life||Mass of 1 curie||Specific activity (Ci/g)|
|232Th||×1010 years 1.405||9.1 tonnes||×10−7 (110,000 pCi/g, 0.11 µCi/g) 1.1|
|238U||×109 years 4.471||2.977 tonnes||×10−7 (340,000 pCi/g, 0.34 µCi/g) 3.4|
|40K||×109 years 1.25||140 kg||×10−6 (7,100,000 pCi/g, 7.1 µCi/g) 7.1|
|235U||×108 years 7.038||463 kg||×10−6 (2,160,000 pCi/g, 2.2 µCi/g) 2.2|
|129I||×106 years 15.7||5.66 kg||0.00018|
|99Tc||×103 years 211||58 g||0.017|
|239Pu||×103 years 24.11||16 g||0.063|
|240Pu||6563 years||4.4 g||0.23|
|226Ra||1601 years||1.01 g||0.99|
|241Am||432.6 years||0.29 g||3.43|
|14C||5730 years||0.22 g||4.5|
|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|
|60Co||1925 days||883 μg||1,132|
|210Po||138 days||223 μg||4,484|
|3H||12.32 years||104 μg||9,621|
|131I||8.02 days||8 μg||125,000|
|123I||13 hours||0.5 μg||2,000,000|
The following table shows radiation quantities in SI and non-SI units:
|Activity (A)||curie||Ci||3.7×1010 s−1||1953||non-SI|
|Exposure (X)||röntgen||R||esu / 0.001293g of air||1928||non-SI|
|Fluence (Φ)||(reciprocal area)||m−2 (cm−2)||1962||SI (non-SI)|
|Absorbed dose (D)||erg·g−1||1950||non-SI|
|Dose equivalent (H)||röntgen equivalent man||rem||100 erg·g−1||1971||non-SI|
- Geiger counter
- Ionizing radiation
- Radiation exposure
- Radiation poisoning
- Radiation burn
- United Nations Scientific Committee on the Effects of Atomic Radiation
- Rutherford, Ernest (6 October 1910). "Radium Standards and Nomenclature". Nature. 84 (2136): 430–431. Bibcode:1910Natur..84..430R. doi:10.1038/084430a0.
- Frame, Paul (1996). "How the Curie Came to Be". Health Physics Society newsletter. Retrieved 3 July 2015.
- Delacroix, D (2002). Radionuclide and Radiation Protection Data Handbook 2002. RADIATION PROTECTION DOSIMETRY Vol. 98 No 1: Nuclear Technology Publishing. p. 147.
- "SI units for ionizing radiation: becquerel". Resolutions of the 15th CGPM (Resolution 8). 1975. Retrieved 3 July 2015.
- "Nist Special Publication 811, paragraph 5.2". NIST. Retrieved 22 March 2016.