IGCSE Physics: Radioactivity Practice Questions
Radioactive decay is the random emission of alpha particles, beta particles or gamma rays from an unstable nucleus. The three types differ in what they are made of, how far they penetrate, and how strongly they ionise.
Sub-topic 5.2 of Cambridge IGCSE Physics 0625 splits neatly into three skills: describing the properties of the three radiations, balancing decay equations using nucleon and proton numbers, and half-life calculations. Half-life questions in particular are pure marks once you learn to halve repeatedly rather than reach for a formula.
What you need to know for Radioactivity
- Alpha particleA helium nucleus, two protons and two neutrons, charge 2+. Strongly ionising but stopped by a few centimetres of air or a sheet of paper.
- Beta particleA high speed electron emitted from the nucleus when a neutron changes into a proton, charge 1-. Moderately ionising and stopped by a few millimetres of aluminium.
- Gamma rayA high frequency electromagnetic wave with no charge and no mass. Weakly ionising but very penetrating, needing several centimetres of lead to reduce it significantly.
- Nuclide notationWritten with the nucleon number above and the proton number below the chemical symbol. Alpha decay reduces the nucleon number by 4 and the proton number by 2. Beta decay leaves the nucleon number unchanged and increases the proton number by 1.
- Half-lifeThe time taken for half the undecayed nuclei in a sample to decay, or equivalently for the activity to fall to half its original value.
- Background radiationRadiation always present in the environment, from sources including radon gas from rocks, cosmic rays, food and drink, building materials and medical procedures.
IGCSE Physics Radioactivity questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
A uranium-238 nucleus, proton number 92, decays by emitting an alpha particle. State the nucleon number and proton number of the nucleus produced, and explain how you worked them out.
Show the worked answer
- An alpha particle is a helium nucleus containing 2 protons and 2 neutrons, so it has a nucleon number of 4 and a proton number of 2.
- Nucleon number of the new nucleus = 238 - 4 = 234.
- Proton number of the new nucleus = 92 - 2 = 90.
- The element with proton number 90 is thorium, so the product is thorium-234.
The measured activity of a radioactive source is 800 counts per minute. After 24 days the activity has fallen to 100 counts per minute. Calculate the half-life of the source. Ignore background radiation.
Show the worked answer
- Halve the activity repeatedly and count the steps: 800 to 400 is one half-life.
- 400 to 200 is a second half-life.
- 200 to 100 is a third half-life. Three half-lives have passed in 24 days.
- Half-life = 24 divided by 3 = 8 days.
A factory monitors the thickness of aluminium foil using a radioactive source and a detector placed on opposite sides of the foil. Explain why a beta source is used rather than an alpha or a gamma source.
Show the worked answer
- Alpha particles are stopped completely by a few centimetres of air and by the foil itself, so no radiation would reach the detector and no change could be measured.
- Gamma rays pass almost entirely through thin aluminium, so the count rate would barely change as the thickness varied.
- Beta particles are partly absorbed by a few millimetres of aluminium, so the count rate reaching the detector varies noticeably with foil thickness.
- A change in count rate can therefore be used to control the rollers and keep the thickness constant.
State two natural sources of background radiation and explain why background radiation must be measured before an experiment on a radioactive source.
Show the worked answer
- Natural source 1: radon gas released from rocks and soil, particularly granite.
- Natural source 2: cosmic rays reaching the Earth from space. Other acceptable answers include radioactive isotopes in food, drink and building materials.
- A detector registers counts from background radiation whether or not the source is present.
- Measuring the background count rate first allows it to be subtracted from every reading, giving the true count rate due to the source alone.
Common mistakes in this topic
- Confusing nucleon number with proton number when balancing decay equations.
- Forgetting that beta decay increases the proton number by one while leaving the nucleon number unchanged.
- Treating half-life as the time for the source to decay completely. It is the time to halve.
- Forgetting to subtract background radiation from count rate data.
- Saying alpha is the most dangerous in all situations. Alpha is the most dangerous inside the body but the least dangerous outside it, because skin stops it.
Exam tips
- For half-life questions, write the halving sequence out in full and count the arrows. It is faster and safer than any formula at IGCSE.
- Check every decay equation by adding the top numbers and the bottom numbers separately. Both sides must match.
- Learn penetration in pairs: paper stops alpha, a few millimetres of aluminium stops beta, thick lead reduces gamma.
- When a question asks which source to use for an application, always link the penetration to what the detector would read.
- Radioactive decay is random and spontaneous. Use both words when asked to describe the nature of decay.
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Radioactivity FAQs
How do I calculate half-life from a count rate?
Halve the starting activity repeatedly until you reach the final activity, counting how many halvings that takes. Then divide the total elapsed time by that number. If activity falls from 800 to 100 counts per minute in 24 days, that is three halvings, so the half-life is 8 days.
What are the three types of nuclear radiation and what stops each one?
Alpha is a helium nucleus, stopped by a sheet of paper or a few centimetres of air. Beta is a fast electron, stopped by a few millimetres of aluminium. Gamma is a high frequency electromagnetic wave and is only reduced significantly by several centimetres of lead or thick concrete.
What happens to the nucleus in beta decay?
A neutron in the nucleus changes into a proton and an electron. The electron is emitted as the beta particle. The nucleon number is unchanged because the total number of protons and neutrons stays the same, but the proton number increases by one, so the atom becomes a different element.
Why must background radiation be subtracted from readings?
A detector records background radiation from rocks, cosmic rays, food and building materials whether or not a source is present. Measuring the background count rate first and subtracting it from each reading gives the corrected count rate caused by the source alone, which is what the experiment is actually testing.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.