# Curie

Curie
A sample of radium, the element which was used in the original definition of the curie.
Unit information
Unit of Specific activity
Symbol Ci
Named after Pierre Curie
Unit conversions
1 Ci in ... ... is equal to ...
rutherfords    37000 Rd
SI derived unit    37 GBq
SI base unit    s−1

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 named in honour of Pierre Curie,[1] but was considered at least by some to be in honour of Marie Curie as well.[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 3.66×1010 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.

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 Curie, after initially accepting this, changed her mind and insisted on one gram of radium. 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 power in milliwatts emitted by one curie of radiation can be calculated by taking the number of MeV for the radiation times approximately 5.93.

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 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).

## 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 Bq / λ,

where λ is the decay constant in s−1.

We can also express activity in moles:

{\displaystyle {\begin{aligned}{\text{1 Ci}}&={\frac {3.7\times 10^{10}}{(\ -\ln 1/2)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{aligned}}}

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, ordered by half-life:

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
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

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

Quantity Unit Symbol Derivation Year SI equivalence
Activity (A) curie Ci 3.7 × 1010 s−1 1953 3.7×1010 Bq
becquerel Bq s−1 1974 SI
rutherford Rd 106 s−1 1946 1,000,000 Bq
Exposure (X) röntgen R esu / 0.001293 g 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
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