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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


1 Bq ≅ 2.703 × 10−11 Ci

Another commonly used measure of radioactivity is the microcurie:

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

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 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 Curie
238U4.471×109 years2.977 tonnes
40K1.25×109 years140 kg
129I15.7×106 years5.66 kg
99Tc211×103 years58 g
239Pu24.11×103 years16 g
14C5730 years0.22 g
226Ra1601 years1.01 g
137Cs30.17 years12 mg
90Sr28.8 years7.2 mg
60Co1925 days883 μg
210Po138 days223 μg
131I8.02 days8 μg
123I13 hours0.5 μg

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.

Exposure (X)röntgenResu / 0.001293 g of air1928
Absorbed dose (D)erg•g−11950
radrad100 erg•g−11953
Activity (A)curieCi3.7 × 1010 s−11953
Dose equivalent (H)röntgen equivalent manrem100 erg•g−11971
Fluence (Φ)(reciprocal area)cm−2 or m−21962

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.