E.3.11—Activity, count rate and half-life

Define activity

Activity is the number of nuclear decays per unit time, measured in becquerels: one Bq is one decay per second. As the number of undecayed nuclei falls, activity falls.

Use half-life steps

After each half-life, half of the remaining nuclei survive: $N=N_0(1/2)^n$, where $n=t/T_{1/2}$ is the number of half-lives elapsed.

Track count rate

If detector efficiency and background are unchanged, count rate is proportional to activity. Apply the same half-life scaling to the net count rate.

Common trap

Do not halve the original amount repeatedly without using the remaining amount, and do not confuse count rate with the number of nuclei when background is present.