IGCSE Physics / Radiation and half-life
Activity and half-life revision notes
Edexcel International GCSE Physics (4PH1), chapter 23: Radiation and half-life, specification point 7.11, 7.12. Free lesson.
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Activity is the rate at which unstable nuclei decay, measured in becquerels. Half-life describes how long it takes for activity to halve, so it helps explain how quickly a radioactive source changes over time.
What activity measures
The activity of a radioactive sample is the number of unstable nuclei that decay each second. Its unit is the becquerel (Bq). As time passes, activity decreases. 1 Bq means one decay per second.
Why activity falls
Radioactive nuclei decay at random. Each unstable nucleus can decay only once. After it decays, fewer unstable nuclei remain, so fewer decays happen each second. This is why activity falls over time.
How activity changes with time
An activity-time graph is a curve, not a straight line. It falls quickly at first and then more slowly. For example, a sample starting at 800 Bq falls to 400 Bq after 6 hours, then to 200 Bq after another 6 hours. The curve gets closer to zero but never quite reaches it. The activity halves in equal time intervals. The length of each interval is the half-life.
What half-life means
Half-life is the time taken for the activity of a sample to halve. It is also the time taken for the number of unstable nuclei in the sample to halve. Use the precise wording: the time taken for the activity to halve. A source never decays completely. Its activity keeps halving, so there is no complete time to take half of.
How half-lives differ between isotopes
Each isotope has its own half-life. A short half-life means activity falls very quickly. A long half-life means it falls extremely slowly. Half-lives range from seconds to billions of years. Technetium-99m: about 6 hours, so activity falls very quickly. Carbon-14: about 5700 years, so activity falls slowly. Uranium-238: about 4.5 billion years, so activity falls extremely slowly.