If a nucleus decays a new nucleus is born. The new one may also be radioactive and decay—until a stable nucleus is reached. The decay probability of an atomic nucleus depends on the number of still existing nuclei and not on the age of a particular nucleus for example. The measure for the decay probability of isotopes is expressed by its half-life. The half-life is the time span, after which half of the original present nuclei of a certain kind have decayed. Half-lives of naturally radioactive nuclei vary between 10−7 s and more than 1014 years.