Curie

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For other uses, see Curie (disambiguation).

The curie (symbol Ci) is a non-SI unit of radioactivity, named after Marie and Pierre Curie.[1][2] It is defined as

1 Ci = 3.7 × 1010 decays per second.

While its continued use is discouraged by NIST[3] and other bodies, the curie is widely used throughout the US government and industry.

One curie is roughly the activity of 1 gram of the radium isotope 226Ra, a substance studied by the Curies.

The SI derived unit of radioactivity is the becquerel (Bq), which equates to one decay per second. Therefore:

1 Ci = 3.7 × 1010 Bq = 37 GBq = 37.000.000.000 atoms decaying per second

and

1 Bq ≅ 2.703 × 10−11 Ci ≅ 27 pCi

Another commonly used measure of radioactivity is the microcurie:

1 μCi = 3.7 × 104 disintegrations per second = 2.22 × 106 disintegrations per minute

Correspondingly, and more commonly encountered with natural levels of radiation, a picocurie is:

1 pCi = 0.037 disintegrations per second = 2.22 disintegrations per minute

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.

Ingesting even a millicurie is usually fatal (unless it is a very short-lived isotope). For example, the LD-50 for ingested polonium-210 is 240 μCi, about 5.5 nanograms.

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 have an activity of approximately 2×0.1 μCi or 7400 decays (mostly from beta decay and rarely from gamma decay) per second inside the person's body.

Curies as a measure of quantity[edit]

Curies are occasionally used to express a quantity of radioactive material rather than a decay rate, such as when one refers to 1 Ci of caesium-137. This may be interpreted as the number of atoms that would produce 1 Ci of radiation. The rules of radioactive decay may be used to convert this 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 a Curie in moles:

\begin{align}\text{1 Ci}&=\frac{3.7\times 10^{10}}{(\ln 2)N_{\rm A}}\text{ moles}\times t_{1/2}\text{ in seconds}\\ &\approx 8.8639\times 10^{-14}\text{ moles}\times t_{1/2}\text{ in seconds}\\ &\approx 5.3183\times 10^{-12}\text{ moles}\times t_{1/2}\text{ in minutes}\\ &\approx 3.1910\times 10^{-10}\text{ moles}\times t_{1/2}\text{ in hours}\\ &\approx 7.6584\times 10^{-9}\text{ moles}\times t_{1/2}\text{ in days}\\ &\approx 2.7972\times 10^{-6}\text{ moles}\times t_{1/2}\text{ in years} \end{align}

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:

IsotopeHalf lifeMass of 1 CurieSpecific activity (Ci/g)
232Th1.405×1010 years9.1 tonnes1.1×10−7 (110,000 pCi/g, 0.11 µCi/g)
238U4.471×109 years2.977 tonnes3.4×10−7 (340,000 pCi/g, 0.34 µCi/g)
40K1.25×109 years140 kg7.1×10−6 (7,100,000 pCi/g, 7.1 µCi/g)
235U7.038×108 years476 kg2.1×10−6 (210,000 pCi/g, 0.21 µCi/g)
129I15.7×106 years5.66 kg0.00018
99Tc211×103 years58 g0.017
239Pu24.11×103 years16 g0.063
240Pu6563 years4.11 g0.24
226Ra1601 years1.01 g0.99
241Am432.6 years0.286 g3.43
14C5730 years0.22 g4.5
238Pu88 years54 mg18.5
137Cs30.17 years12 mg83
90Sr28.8 years7.2 mg139
241Pu14 years8.3 mg121.2
60Co1925 days883 μg1132
210Po138 days223 μg4484
3H12.32 years103.6 μg9708
131I8.02 days8 μg125000
123I13 hours0.5 μg2000000

The number of Curies present in a sample decreases with time because of decay.

Radiation Related Quantities[edit]

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

QuantityNameSymbolUnitYear
Exposure (X)roentgenResu / 0.001293 g of air1928
Absorbed dose (D)erg•g−11950
radrad100 erg•g−11953
grayGyJ•kg−11974
Activity (A)curieCi3.7 × 1010 s−11953
becquerelBqs−11974
Dose equivalent (H)roentgen equivalent manrem100 erg•g−11971
sievertSvJ•kg−11977
Fluence (Φ)(reciprocal area)cm−2 or m−21962

See also[edit]

References[edit]

  1. ^ curie - Britannica Online Encyclopedia
  2. ^ Paul W. Frame. "How the Curie Came to Be". Retrieved 2008-04-30. 
  3. ^ Nist Special Publication 811, paragraph 5.2.