While its continued use is discouraged by NIST and other bodies, the curie is widely used throughout the US government and industry.
- 1 Ci = 3.7 × 1010 Bq = 37 GBq = 37.000.000.000 atoms decaying per second
- 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.
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)
- N = 3.7 × 1010 / λ,
where λ is the decay constant in (s−1).
We can also express a Curie in moles:
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.
Radiation Related Quantities
The following table shows radiation quantities in SI and non-SI units.
|Exposure (X)||roentgen||R||esu / 0.001293 g of air||1928|
|Absorbed dose (D)||erg•g−1||1950|
|Activity (A)||curie||Ci||3.7 × 1010 s−1||1953|
|Dose equivalent (H)||roentgen equivalent man||rem||100 erg•g−1||1971|
|Fluence (Φ)||(reciprocal area)||cm−2 or m−2||1962|
- Geiger counter
- Ionizing radiation
- Radiation exposure
- Radiation poisoning
- Radiation burn
- United Nations Scientific Committee on the Effects of Atomic Radiation