Specific activity

Specific activity is the radioactive activity of a sample per unit mass. In SI units, it is measured in becquerels per kilogram (Bq kg⁻¹).

Definition

The activity A of a sample is defined as

A = dN/dt = λN

where N is the number of radioactive atoms in the sample and λ is the decay constant. For a sample of mass m, the specific activity is simply A/m.

A specific activity can also be defined for a particular nuclide, by assuming a sample consisting solely of the nuclide in question. For one mole of atoms, N = N_A and so A_m = λN_A. One mole of atoms has a mass equal to the molar mass of the nuclide, M, so the specific activity is λN_A/M. Equivalent definitions of the specific activity are λ/m_a (where m_a is the atomic mass of the nuclide) or λ/A_rm_u (where m_u is the atomic mass constant and A_r is the relative atomic mass of the nuclide).