Activity is the rate at which a source of unstable nuclei decays.

The idea

The measurement of radioactive decay focuses on how frequently unstable nuclei emit radiation, quantified in becquerels (Bq), where 1 Bq equals one decay per second. This frequency depends on the total number of unstable nuclei present and the specific isotope's half-life. Given identical starting quantities of two isotopes, the one with a shorter half-life exhibits a higher initial frequency of decay events. For example, a sample of polonium-210 (half-life 138 days) registers significantly more counts per second on a detector than an equal sample of polonium-209 (half-life 125 years). As unstable nuclei deplete over time, the frequency of decay events naturally decreases, meaning the sample becomes less radioactive.

What earns the marks

• Identifies that the activity of a radioactive source is measured in becquerels (Bq).
• States that one becquerel is equivalent to one nuclear decay per second.
• Explains that activity decreases over time as the number of unstable nuclei in the sample reduces.
• Compares two sources to conclude that for an equal number of nuclei, a shorter half-life results in a higher activity.

Common mistakes and how to put them right

• Confusing activity with the total number of undecayed nuclei remaining in a sample.

  Put it right: Activity is the rate of decay (decays per second), not the absolute number of unstable nuclei present.

• Assuming that a longer half-life means a higher activity.

  Put it right: A longer half-life means the nuclei are more stable and decay less frequently, resulting in a lower activity for a given number of nuclei.

• Believing that the activity of a specific radioactive source remains constant over time.

  Put it right: As unstable nuclei decay, there are fewer left to decay, so the activity decreases over time.

Exam tips

• When defining activity, always include the unit becquerel (Bq) and state that 1 Bq equals 1 decay per second.
• If asked to compare the activity of two samples, check both the number of unstable nuclei and their respective half-lives before concluding which is more active.