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The activity of a radioactive sample is the number of decays per unit time.
Decay constant
The decay constant, $\lambda$, measures how quickly a radioactive substance decays. It is defined as the probability of decay per unit time for a single nucleus.
Radioactive decay
Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation.
The half-life
The half-life $T_{1/2}$ is the time it takes for half of the radioactive nuclei in a sample to decay.