curie
Radioactivityexact by definition
The curie is the older unit of radioactivity, fixed since 1953 at exactly 3.7e10 becquerels. That number was chosen to approximate the activity of one gram of radium-226, which later measurement put slightly differently, so the modern curie is an exact defined value rather than a physical property of radium. It is not an SI unit but remains in regulatory and industrial use, particularly in the United States.
Watch out: The curie and the becquerel differ by more than ten orders of magnitude, so mixing them is one of the more dangerous unit errors in the field. One curie is 37 gigabecquerels; one becquerel is about 27 picocuries. A number that looks reassuringly small in curies can be alarming in becquerels and the other way round.
| 1 Ci | 37,000,000,000 Bq |
Named for Pierre and Marie Curie, who isolated polonium and radium in 1898. Marie Curie remains the only person to have won Nobel Prizes in two different sciences, physics in 1903 and chemistry in 1911.
About the curie
The curie was defined at a time when radium was the reference substance for everything radioactive, and the intent was that one curie should be the activity of one gram of radium-226. Later measurements gave a slightly different figure, and rather than let the unit drift the 1953 agreement froze it at exactly \(3.7 \times 10^{10}\) decays per second. So the curie today is an exact number with a historical origin, not a property of any sample you could weigh.
The practical difficulty is the size gap. The becquerel is one decay per second, chosen so that the unit is defined by the physics rather than by a substance, but it is so small that environmental readings run to thousands and medical doses to hundreds of millions. The curie is so large that ordinary work happens in micro and millicuries. Regulatory documents in the United States are still written in curies, international standards in becquerels, and conversions between the two show up in shipping papers, licence limits and drinking water standards. The picocurie per litre used for radon in air, where 4 pCi/L is the common action level, equals 148 Bq/m³.