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Formula Radioactive Decay Activity    Number of atomic nuclei    Decay constant

Formula
Formula: Radioactive Decay
Radioactive decay law - Graph

Activity

Unit
Activity of a radioactive sample indicates how fast the atoms of the radioactive sample decay.

Number of atomic nuclei

Unit
Number of atomic nuclei of a radioactive sample that have not yet decayed. The number is given by:$$ N(t) ~=~ N_0 \, \mathrm{e}^{-\lambda \, t} $$

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 nuclides decay at different rates.

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