Radioactive decay is largely insensitive to conditions outside the nucleus (although not always so), and the resulting behaviour can be characterised by fixed modes of decay, transition energies and probabilities. The number of atoms decaying per unit time is the activity (A) given by the equation:

A = NX =

where N is the number of atoms at time t, X is the decay constant (probability that an atom will decay in unit time), and dN is the number of spontaneous nuclear transitions from that energy state in time interval dt. Activity is expressed in Becquerels (Bq) in which 1 Bq is one disintegration per second (dps). Integration of the above equation and substitution results in the expression:

N = N0e-Xt

where N0 is the number of atoms at time t = 0. Each radionuclide has a characteristic decay constant that is related to the half-life (t/2), the time taken for the number of original radionuclides to reduce by a factor of two:

ln2

The half-life of a radionuclide is a primary parameter in any radioactive decay process.