出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2017/10/02 10:15:42」(JST)
The curie (symbol Ci) is a non-SI unit of radioactivity, named 'in honour of' Pierre Curie,[1] according to his widow, the famed radiation researcher Marie Curie.[2]
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 x 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:
and
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 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.'[2]
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.
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:
and so,
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:
| Isotope | Half life | Mass of 1 curie | Specific activity (Ci/g) |
|---|---|---|---|
| 232Th | 7010140500000000000♠1.405×1010 years | 9.1 tonnes | 6993110000000000000♠1.1×10−7 (110,000 pCi/g, 0.11 µCi/g) |
| 238U | 7009447100000000000♠4.471×109 years | 2.977 tonnes | 6993340000000000000♠3.4×10−7 (340,000 pCi/g, 0.34 µCi/g) |
| 40K | 7009125000000000000♠1.25×109 years | 140 kg | 6994709999999999999♠7.1×10−6 (7,100,000 pCi/g, 7.1 µCi/g) |
| 235U | 7008703800000000000♠7.038×108 years | 463 kg | 6994220000000000000♠2.2×10−6 (2,160,000 pCi/g, 2.2 µCi/g) |
| 129I | 7007157000000000000♠15.7×106 years | 5.66 kg | 0.00018 |
| 99Tc | 7005211000000000000♠211×103 years | 58 g | 0.017 |
| 239Pu | 7004241100000000000♠24.11×103 years | 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:
| Quantity | Name | Symbol | Unit | Year | SI Quantity |
|---|---|---|---|---|---|
| Activity (A) | curie | Ci | 3.7×1010 s−1 | 1953 | 3.7×1010 Bq |
| becquerel | Bq | s−1 | 1974 | SI | |
| rutherford | Rd | 106s−1 | 1946 | 1,000,000 Bq | |
| Exposure (X) | röntgen | R | esu / 0.001293g of air | 1928 | 2.58×10−4 C/kg |
| Fluence (Φ) | (reciprocal area) | m−2 | 1962 | SI | |
| Absorbed dose (D) | Erg | erg·g−1 | 1950 | 1.0×10−4 Gy | |
| rad | rad | 100 erg·g−1 | 1953 | 0.010 Gy | |
| gray | Gy | J·kg−1 | 1974 | SI | |
| Dose equivalent (H) | röntgen equivalent man | rem | 100 erg·g−1 | 1971 | 0.010 Sv |
| sievert | Sv | J·kg−1×WR | 1977 | SI |
| リンク元 | 「mCi」 |