The size, weight, or quantity of material does not indicate how much radioactivity is present. A large quantity of material can contain a very small amount of radioactivity, or a very small amount of material can have a lot of radioactivity.

In the United States, the “amount” of radioactivity present is traditionally determined by estimating the number of curies (Ci) present. The more curies present, the greater amount of radioactivity and emitted radiation.

Common fractions of the curie are the millicurie (1 mCi = 1/1,000 Ci) and the microcurie (1 μCi = 1/1,000,000 Ci). In terms of disintegrations per unit time, 1 μCi = 2,220,000 dpm.

The SI system uses the unit of becquerel (Bq) as its unit of radioactivity. One curie is 37 billion Bq. Since the Bq represents such a small quantity, usually a prefix noting a large multiplier is used with the Bq as follows:

• 37 GBq = 37 billion Bq = 1 curie
• 1 MBq = 1 million Bq = ~ 27 microcuries (27 μCi)
• 1 GBq = 1 billion Bq = ~ 27 millicuries (27 mCi)
• 1TBq = 1 trillion Bq = ~ 27 curies (27 Ci)