Units of activity (the curie and the becquerel) also refer to a quantity of radioactive atoms. Because the probability of decay is a fixed physical quantity, for a known number of atoms of a particular
radionuclide, a predictable number will decay in a given time. The number of decays that will occur in one second in one gram of atoms of a particular radionuclide is known as the
specific activity of that radionuclide. The activity of a sample decreases with time because of decay. The rules of
radioactive decay may be used to convert activity to an actual number of atoms. They state that 1 Ci of radioactive atoms would follow the expression :
N (atoms) ×
λ (s) = 1 Ci = 3.7 × 10 Bq, and so :
N = 3.7 × 10 Bq /
λ, where
λ is the
decay constant in s−1. Here are some examples, ordered by half-life: ==Radiation related quantities==