# Curie

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

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:

Isotope Half life Mass of 1 Curie
238U 4.471×109 years 2.977 tonnes
40K 1.25×109 years 140 kg
129I 15.7×106 years 5.66 kg
99Tc 211×103 years 58 g
239Pu 24.11×103 years 16 g
14C 5730 years 0.22 g
226Ra 1601 years 1.01 g
137Cs 30.17 years 12 mg
90Sr 28.8 years 7.2 mg
3H 12.32 years 103.6 μg
60Co 1925 days 883 μg
210Po 138 days 223 μg
131I 8.02 days 8 μg
123I 13 hours 0.5 μg

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

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 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