Mean Life
The average expected lifetime of a radioactive nucleus before undergoing radioactive decay.
Governing formula
τ = 1 / λ = t_½ / ln(2) ≈ 1.443 t_½
SI unit
Seconds or Years
In depth
Mean life τ is the arithmetic average lifetime of all individual radioactive atoms in a sample. It equals the time required for a sample's activity to decay to 1/e (~36.8%) of its initial value.
Examples in the real world
Free neutrons having a mean lifetime τ ≈ 879 seconds (~14.6 minutes) before decaying via beta-minus decay.