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While its continued use is discouraged by NIST and other bodies, the curie is widely used throughout the US government and industry.
Another commonly used measure of radioactivity is the microcurie:
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 cause about 2×3700 or 7400 rays (mostly beta but some gamma) per second inside the person's body.
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:
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|
|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.