In radioactivity, half life is the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay.

Understanding the concept of half-life is useful for determining excretion rates as well as steady-state concentrations for any specific drug.

**Formula to calculate half life.**

In (2) is the natural logarithm of 2 which is 0.693.

*λ* is a positive number called the decay constant of the decaying quantity.

**Example:**

Calculate the half life of plutonium 239 whose decay constant is 2.88 x 10^-5 .

Therefore, plutonium 239 half life is 24,062.5 years.