Curie

The Curie (symbol Ci) is a non-SI unit of radioactivity, named after Marie and Pierre Curie. It is defined as

1 Ci = 3.7 × 1010 decays per second.

Its continued use is discouraged.

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.