This topic is about how the activity of a radioactive sample falls with time, and how to handle real detector readings that include background radiation. You learn to correct counts, read a half-life from a graph, apply repeated halving, explain why decay is random, and keep a calculation separate from a judgement about personal risk.
Exact wording and required items change between syllabus years, so check the current Cambridge IGCSE Physics 0625 page for your exam year. The Physics learning guide shows where this topic sits in the whole subject.
What do I need to know first?
You need the basic nuclear model and the idea that radiation counts are random, from atomic and nuclear models. You also need simple division, reading values from a graph, and halving a number several times. No advanced maths is needed.
One worked example that uses the main skills
(Invented data.) With no source, a detector records 60 counts in 15 minutes. With a source, the measured rates are 84 counts/min at 0 min, 44 at 10 min and 24 at 20 min.
Background: 60 ÷ 15 = 4 counts/min.
Corrected rates: 84 − 4 = 80, 44 − 4 = 40 and 24 − 4 = 20 counts/min.
Half-life: the corrected rate halves every 10 minutes, so the half-life is 10 min.
Prediction: at 40 min there are 4 half-lives, so 80 → 40 → 20 → 10 → 5 counts/min. The detector would show 5 + 4 = 9 counts/min.
Notice the order: correct first, find the half-life second, predict third, and add background back only if the question asks for the detector reading.
In what order should I study the lessons?
- Subtract background from supplied counts. Every later step depends on clean data.
- Determine half-life from a decay graph. This uses the corrected values and turns a curve into a time.
- Apply successive halving to a stated interval. Once you have a half-life, you can predict amounts.
- Explain why one atom has no predictable decay time. This explains why the calculations describe groups, not single atoms.
- Distinguish a model calculation from personal radiation-risk advice. This teaches what the answers do not show, so it comes last.
Then test everything together with the half-life and background practice set.
What traps catch students?
- Subtracting totals that cover different times. Convert to a rate, or scale the background to the same time.
- Halving the measured rate instead of the corrected rate.
- Giving a count rate as the half-life. The half-life is a time.
- Thinking all is gone after two half-lives. One quarter remains.
- Treating half-life as a timer for one atom.
- Turning an activity into a safety claim.
How do I use the practice set well?
Attempt it after the lessons, with a pen and paper, and write units every time. Then compare your reasoning with the worked answers. The set ends with a table that sends each error type back to the right lesson.
Clear written reasoning is a habit that improves with feedback. Our teachers can work on it with you in online one-to-one Physics tuition. Next in the course comes space physics.