Curie (unit)
The Curie (symbol Ci) is a non-SI unit of radioactivity, named after Marie and Pierre Curie.[1][2] It is defined as
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
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
- Geiger counter
- Ionizing radiation
- Radiation exposure
- Radiation poisoning
- United Nations Scientific Committee on the Effects of Atomic Radiation
References
- ^ curie - Britannica Online Encyclopedia
- ^ Paul W. Frame. "How the Curie Came to Be". Retrieved 2008-04-30.
- ^ Nist Special Publication 811, paragraph 5.2.