Formula Radioactive Decay Half-life Decay constant
$$t_{\text h} ~=~ \ln(2) \, \lambda$$ $$t_{\text h} ~=~ \ln(2) \, \lambda$$
Half-life
$$ t_{\text h} $$ Unit $$ \mathrm{s} $$ Half-life is the period of time after which the initial amount of atomic nuclei has decayed to half.
Decay constant
$$ \lambda $$ Unit $$ \frac{1}{\mathrm s} $$ 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: \( \tau = \frac{1}{\lambda} \).