# Curie

The curie (symbol Ci) is a non-SI unit of radioactivity, named after Pierre Curie,[1] but probably also after Marie Curie. 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.[2])

In 1975 the General Conference on Weights and Measures gave the becquerel (Bq), equal to one reciprocal second, official status as the SI unit of activity.[3] 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 NIST[4] 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.'[5]

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 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 predicable 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:

\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:

Isotope Half life Mass of 1 curie Specific 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 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
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 1132
210Po 138 days 223 μg 4484
3H 12.32 years 104 μg 9621
131I 8.02 days 8 μg 125000
123I 13 hours 0.5 μg 2000000

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

Quantity Name Symbol Unit Year
Exposure (X) roentgen R esu / 0.001293 g of air 1928
Absorbed dose (D) erg•g−1 1950
gray Gy J•kg−1 1975
Activity (A) curie Ci 3.7 × 1010 s−1 1953
becquerel Bq s−1 1975
Dose equivalent (H) roentgen equivalent man rem 100 erg•g−1 1971
sievert Sv J•kg−1 1977
Fluence (Φ) (reciprocal area) cm−2 or m−2 1962