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The curie (symbol Ci) is a unit of measurement. It measures radioactivity. It is defined as

1 Ci = 3.7×10¹⁰ decays per second.

This is roughly the activity of 1 gram of the radium isotope ²²⁶Ra. Radium was studied by the pioneers of radiology, Marie and Pierre Curie so the unit is named after them. The curie has since been replaced by an SI derived unit, the becquerel (Bq), which is decay per second. Therefore:

1 Ci = 3.7×10¹⁰ Bq or 37 Gigabecquerel (Gbq)

and

1 Bq = 2.70×10⁻¹¹ Ci

In 1962, the International Commission on Radiological Units suggested the usage of this unit named after Marie Curie.