IGCSE Physics revision · Atomic physics
Radioactivity
The topic one of my students said she never understood until a ten-minute explanation made it perfectly clear. It really is that compressible: three radiation types with fixed properties, one decay law, and the word random used exactly where the mark scheme wants it.
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What the syllabus demands
- —Describe the nuclear atom: protons, neutrons, electrons; proton number and nucleon number; isotopes
- —Describe the nature, charge, penetration and ionising power of alpha, beta and gamma radiation
- —Know that radioactive decay is spontaneous and random
- —Define half-life and perform half-life calculations from data or graphs
- —Describe uses (medical tracers, thickness control, dating) and safety precautions
- —Write nuclide equations for alpha and beta decay (Extended)
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Isotopes
- Atoms of the same element with the same number of protons but different numbers of neutrons.
- Half-life
- The time taken for half the nuclei in a sample of a radioactive isotope to decay — or for the activity to fall to half its original value.
- Radioactive decay
- The spontaneous and random emission of radiation from an unstable nucleus. It is unaffected by temperature, pressure or chemical state.
- Background radiation
- The radiation present everywhere, from rocks, cosmic rays, food and medical sources — subtracted from measurements before analysis.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Omitting 'random and spontaneous'. Decay cannot be predicted for any individual nucleus and is unaffected by external conditions — a recurring assessment point.
- 02
Forgetting to subtract background radiation before half-life calculations from a table. If the numbers do not halve cleanly, background is hiding in them.
- 03
Mixing the penetration order: alpha is stopped by paper, beta by a few millimetres of aluminium, gamma reduced by thick lead. Ionising power runs the other way — alpha is by far the most ionising.
- 04
Choosing the wrong isotope in application questions: medical tracers need gamma emitters with short half-lives (detectable outside the body, quickly gone); paper-thickness gauges need beta (alpha would be stopped, gamma barely affected).
One worked example, done properly
Question
The activity of a sample falls from 1200 Bq to 150 Bq in 12 days. Calculate the half-life.
Method
- 1.Count the halvings: 1200 → 600 → 300 → 150. That is 3 half-lives.
- 2.3 half-lives = 12 days.
- 3.Half-life = 12 ÷ 3.
Half-life = 4 days
Test yourself
10 questions · instant marking
Question 1 of 10
Which radiation is stopped by a sheet of paper?
Question 2 of 10
A beta particle is:
Question 3 of 10
The most penetrating radiation is:
Question 4 of 10
Radioactive decay is described as:
Question 5 of 10
A sample's activity falls from 800 Bq to 100 Bq in 15 days. Its half-life is:
Question 6 of 10
After alpha decay, the nucleon number of a nucleus:
Question 7 of 10
After beta decay, the proton number of a nucleus:
Question 8 of 10
Isotopes of an element have the same number of:
Question 9 of 10
The best radiation source for a paper-thickness gauge is:
Question 10 of 10
Before analysing radioactivity data, you should subtract: