The size or weight of a 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.
For example, uranium-238, with a 4.5 billion year half-life, has only 0.00015 curies of activity per pound, while cobalt-60, with a 5.3 year half-life, has nearly 513,000 curies of activity per pound. This "specific activity," or curies per unit mass, of a radioisotope depends on the unique radioactive half-life and dictates the time it takes for half the radioactive atoms to decay.
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 transformations per unit time, 1 μCi = 2,220,000 dpm.
The System International of units (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 amount, one is likely to see a prefix noting a large multiplier used with the Bq as follows:
- 37 GBq = 37 billion Bq = 1 curie
- 1 MBq = 1 million Bq = ~27 microcuries
- 1 GBq = 1 billion Bq = ~27 millicuries
- 1TBq = 1 trillion Bq = ~27 curies