A very significant quantity that can be measured directly for small numbers of atoms is the mean lifetime. If there are n active nuclei, (atoms) (of the same type, of course), the mean life is

8. Where |dN| is the number of nuclei decaying between t, t + dt; the modulus sign is required to ensure that it is positive.

Illustration 1: Cascade Decays: Frequently, a nucleus A decays into another nucleus B, which again decays into C, and so on until the series ends in a stable nuclide. These are known as cascade decays. We will consider a simple example of a cascade decay where a nuclide a decays into B, and then, B decays into C:

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