millicurie

The millicurie survives mainly on paperwork: radiopharmacy order forms, sealed-source certificates and licence documents in the United States are still written in it, while the scientific literature and most of the world use the megabecquerel. The conversion is exact and worth carrying in both directions, 1 mCi = 37 MBq, so the technetium-99m activities used in nuclear imaging, typically written as 10 to 30 mCi in American practice, are the same few-hundred-megabecquerel quantities described on the megabecquerel page.

The unit's size is not arbitrary. The curie was set to approximate the activity of one gram of radium-226, which makes the millicurie the activity of about one milligram, and radium was handled by the milligram: the earliest clinical inventories were literally ledgers of milligrams of radium distributed among needles and tubes, so for that one nuclide the milligram and the millicurie were the same number to within about one percent. The specific activity of radium-226 works out to 0.989 curies per gram, which is how close the 1953 definition landed to its historical target.

An activity in millicuries decays like any other, following the half-life arithmetic in the site's decay solvers, which is why a stated activity only means something alongside its assay date.