Curie (unit)
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]
| Curie | |
|---|---|
 A sample of radium, the element which was used in the original definition of the curie. | |
| General information | |
| Unit of | Activity |
| Symbol | Ci |
| Named after | Pierre Curie |
| Conversions | |
| 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[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 Skłodowska–Curie, after initially accepting this, changed her mind and insisted on one gram of radium. According to Bertram Boltwood, Marie Skłodowska–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. However, millicurie quantities of electromagnetic emitting radionuclides are routinely used in nuclear medicine.
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
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
The following table shows radiation quantities in SI and non-SI units:
| 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 | 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 | |
| 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
References
- 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. Archived from the original on 20 March 2012. Retrieved 3 July 2015.
- United States Atomic Energy Commission (1951). Semiannual Report of the Atomic Energy Commission, Volume 9. p. 93.
- "Resolution 7 of the 12th CGPM". International Bureau of Weights and Measures (BIPM). 1964. Archived from the original on 2021-02-19.
- Delacroix, D. (2002). "Radionuclide and Radiation Protection Data Handbook 2002". Radiation Protection Dosimetry. Nuclear Technology Publishing. 98 (1): 147. Archived from the original on 2016-03-05.
- "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 (Report). NIST. 28 January 2016. Retrieved 22 March 2016.