Activity and Half-Life

This lesson covers activity and half-life as required by the AQA GCSE Physics specification (4.4.2). You need to understand how the activity of a radioactive source changes over time, how to define and calculate half-life, and how to interpret decay curves.

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Activity

The activity of a radioactive source is the rate at which nuclei decay — in other words, the number of nuclear decays per second. Activity is measured in becquerels (Bq).

1 Bq = 1 decay per second

Larger units are also commonly used:

Unit	Symbol	Value
Becquerel	Bq	1 decay per second
Kilobecquerel	kBq	1,000 decays per second
Megabecquerel	MBq	1,000,000 decays per second
Gigabecquerel	GBq	1,000,000,000 decays per second

As time passes, the number of undecayed nuclei in a sample decreases. Since there are fewer unstable nuclei available to decay, the activity decreases over time. This means that radioactive sources become less active as they get older.