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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


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

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 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}

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 1.1×107 (110,000 pCi/g, 0.11 µCi/g)
238U 4.471×109 years 2.977 tonnes 3.4×107 (340,000 pCi/g, 0.34 µCi/g)
40K 1.25×109 years 140 kg 7.1×106 (7,100,000 pCi/g, 7.1 µCi/g)
235U 7.038×108 years 463 kg 2.2×106 (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 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.

Quantity Name Symbol Unit Year
Exposure (X) roentgen R esu / 0.001293 g of air 1928
Absorbed dose (D) erg•g−1 1950
rad rad 100 erg•g−1 1953
gray Gy J•kg−1 1974
Activity (A) curie Ci 3.7 × 1010 s−1 1953
becquerel Bq s−1 1974
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

See also[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.