Curie

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:

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 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.

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)

and so,

N = 3.7 × 1010 / λ,

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 curieSpecific activity (Ci/g)
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 years140 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 years5.66 kg 0.00018
99Tc 211×103 years58 g 0.017
239Pu 24.11×103 years16 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 years0.22 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 days883 μg 1,132
210Po 138 days223 μg 4,484
3H 12.32 years 104 μg 9,621
131I 8.02 days8 μg 125,000
123I 13 hours0.5 μg 2,000,000

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

Radiation related quantities
QuantityNameSymbolUnitYearSI 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·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

See also

References

  1. 1 2 Rutherford, Ernest (6 October 1910). "Radium Standards and Nomenclature". Nature. 84 (2136): 430–431. Bibcode:1910Natur..84..430R. doi:10.1038/084430a0.
  2. 1 2 Frame, Paul (1996). "How the Curie Came to Be". Health Physics Society newsletter. Retrieved 3 July 2015.
  3. Delacroix, D (2002). Radionuclide and Radiation Protection Data Handbook 2002. RADIATION PROTECTION DOSIMETRY Vol. 98 No 1: Nuclear Technology Publishing. p. 147.
  4. "SI units for ionizing radiation: becquerel". Resolutions of the 15th CGPM (Resolution 8). 1975. Retrieved 3 July 2015.
  5. "Nist Special Publication 811, paragraph 5.2". NIST. Retrieved 22 March 2016.
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