2.2 Radioactive decay

The atomic nuclei of some isotopes have a surplus of energy, are unstable, and disintegrate to form more stable nuclei of a different isotope. This process is accompanied by the emission of particles or energy, termed nuclear radiation. Nuclides with this feature are called radionuclides, and the process is called nuclear decay or disintegration. The radioactivity decay law expresses the decrease in the number of atoms of a radionuclide with time:

N_t = N₀e^-λt

where:
N_t = the number of atoms present after time t(s)
N₀ = the number of atoms present at time t = 0
λ = the decay constant of a radionuclide (s^-1)

A related constant, the half-life T_½(s), is the time taken for half the radionuclides to decay:

T_½= 0.693 / λ

The product λN gives the current activity (Bq) of the radionuclide. Radioactive decay is independent of other physical conditions.