# Formula Radioactive Decay Half-life    Decay constant

## Half-life

Unit
Half-life is the period of time after which the initial amount of atomic nuclei has decayed to half.

## Decay constant

Unit
Decay constant specifies the probability of decay per unit of time. In other words: The decay constant determines how fast a nuclide (radioactive substance) decays. Different nuclide decay at different rates.

The reciprocal of the decay constant is the lifetime: $$\tau = \frac{1}{\lambda}$$.