In nuclear sciences and technologies, "activity" is the SI quantity related to the phenomenon of natural and artificial radioactivity. The SI unit of "activity" is becquerel (Bq) while that of "specific activity" is Bq/g. The old unit of "activity" was curie (Ci) making "specific activity", Ci/g.
Experimentally-measured specific activity can be used to calculate the half-life of a radioactive element.
Starting from the definition of half-life (T1/2) - with, N0, atoms of an element, the number of atoms, N, remaining after time, t, is given by:
The natural log of both sides
The derivative with respect to time, t
Multiplying both sides by N
dN/dt represents the decay rate of atoms. The negative sign shows that the rate is negative, so the number of atoms is decreasing with time. Rearranging terms:
Example: half-life of Rb-87
One gram of rubidium-87 and a radioactivity count rate that, after taking solid angle effects into account, is consistent with a decay rate of 3200 decays per second corresponds to a specific activity of 3.2×106 Bq/kg. Rubidium's atomic weight is 87, so one gram is one 87th of a mole, or N=6.9×1021 atoms. Plugging in the numbers:
Radioactivity is expressed as the decay rate of a particular radionuclide with decay constant λ and the number of atoms N:
Mass of the radionuclide is given by
Specific radioactivity S is defined as radioactivity per unit mass of the radionuclide:
In addition, decay constant λ is related to the half-life T1/2 by the following equation:
Thus, specific radioactivity can also be described by
This equation is simplified by
When the unit of half-life converts a year
For example, specific radioactivity of radium 226 with a half-life of 1600 years is obtained by
This value derived from radium 226 was defined as unit of radioactivity known as Curie (Ci).