An Etymological Dictionary of Astronomy and Astrophysics

A constant of proportionality occurring in the formula expressing spontaneous → decay of → radionuclides. The number of atoms decaying is given by N = N₀e^-kt, where N₀ is the number of nuclei in the given volume of the substance at instant t = 0, N is the number of nuclei at t, and k is decay constant. Decay constant is related to → half-life by τ = ln2/k, roughly 0.693/k.

See also: → decay; → constant.