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, un-shielded exposure.

The typical human body contains roughly 0.1 μCi of naturally occurring potassium-40.

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).

The use of Curies to measure quantity is unique in that 1 Ci of a nuclide will be fewer Curies at any later time.

See also [edit]

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

References [edit]