When an atom disintegrates, radiation is emitted. If the rate of disintegrations is large, a radioactive source is considered to have a high activity. The unit for the activity of a radioactive source was named after Becquerel (abbreviated Bq) and is defined as:
In a number of countries, the old unit, the curie (abbreviated Ci and named after Marie Curie) is still used. The curie-unit was defined as the activity in one gram of radium. The number of disintegrations per second in one gram of radium is 37 billion. The relation between the curie and the becquerel is given by:
1 Bq = 1 disintegration per sec.