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The 'curie' (symbol 'Ci') is a unit of radioactivity, defined as
: 1 Ci = 3.7×10¹⁰ decays per second.
This is roughly the activity of 1 gram of the radium isotope ²²⁶Ra, a substance studied by the pioneers of radiology, Marie and Pierre Curie (for whom the unit is named). The curie has since been replaced by an SI derived unit, the becquerel (Bq), which equates to one decay per second. Therefore:
: 1 Ci = 3.7×10¹⁰ Bq
and
: 1 Bq = 2.70×10⁻¹¹ Ci