Curie

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

and

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

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:

Failed to parse(unknown function '\begin'): {\begin{aligned}{\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{aligned}}

where N_A is Avogadro's number and t_1/2 is the half life. The number of moles may be converted to grams by multiplying by the atomic mass.

Here are some examples:

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

See also

• Geiger counter
• Ionizing radiation
• Radiation exposure
• Radiation poisoning
• United Nations Scientific Committee on the Effects of Atomic Radiation

References