Specific activity of a radionuclide can be shown with two different units; curie (Ci), defined back in 1910, and becqeurel (Bq), defined later in 1981. But why did we come up with a new unit while we already had one? And why on earth, 1 Ci is equal to 37 billion Bq?
Curie as a unit was originally defined by the Radium Standards Committee at the International Congress of Radiology and Electricity in Brussels (13-15 September 1910). If this new unit was going to be named after someone, by all means, it was going to be Marie and Pierre Curie, who discovered radium in 1898. On the second day of the congress, it was decided that 1 curie should be equal to the activity of 10-8 grams of radium-226 isotope. However, the next day Marie Curie argued that it would be inappropriate to use the name of curie for such a minuscule quantity and the definition was modified as 1 gram. 1
By the end of 1950s, there was a huge diversity of units among various fields of science, leading to a lack of coordination among different disciplines. In 1960, a group of international organizations, brought together by Metre Convention, published a new system of units. The logic behind the “International System of Units” (SI) was very simple yet lovely; defining new units in such a way that equation for every unit has the same form of expression in terms of 1’s. For instance, “1 newton” is the force required to accelerate a mass of “1 kilogram” at “1 meter per second squared”. “1 joule” is the energy transferred to an object when a force of “1 newton” is applied on it for a distance of “1 meter”. To achieve this for every unit, a consensus was needed on 7 base units for 7 main measurements, so that they could be used to generate new “derived units”. These base units were chosen to be;
“Meter” for length,
“Kilogram” for mass,
“Second” for time,
“Ampere” for electric current,
“Kelvin” for temperature,
“Mole” for amount of substance,
“Candela” for luminous intensity.
But do we have a substantive definition for these units? How do you define a “meter” to an alien? To build a solid ground, each of these base units was attributed to a permanent definition of its own. These definitions rely on some specific, unchanging events in the nature. Meter is defined as the distance travelled by light in vacuum in 1 / 299792458 second. “Second” is defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. Definition of kilogram is even nicer; mass of a small squat cylinder of ~47 cubic centimetres of platinum-iridium alloy kept in the Pavillon de Breteuil, France.
Later on, these units were used to generate new units for measures of radioactivity as well. “1 becqeurel” is defined as “1 radioactive decay” per “1 second”. “1 gray” is the absorption of “1 joule” of radiation energy per “1 kilogram”. “1 sievert” is biological equivalent of radiation energy absorbed per “1 kilogram”.
Precise measurements revealed that, 37 billion atoms will decay in one second in a 1-gram sample of radium-226. So this is how we come up to say today that 1 Ci is equal to 37 billion Bq. Or, as the common usage goes; 1 milli-curie is equal to 37 mega-becqeurel.
The “NIST Guide to the SI” notes that, traditional radioactivity units (curie, roentgen, rad, and rem) are accepted for use along with the SI, since they are still in wide use. Nevertheless, the guide “strongly discourages” the continued use of these units except when absolutely necessary. The guide also suggests that, if these traditional units are to be used, the values of relevant quantities shall be given in terms of SI units first, followed by these outdated non-SI units in parentheses. 2
A Thompson, BN Taylor. Guide for the Use of the International System of Units (SI). NIST Special Publication 811. 2008 Edition. Retrieved 24 July 2018. ↩