Physics 0625 · syllabus 5.2
Cambridge IGCSE Physics 0625 — Radioactivity topical past papers
Background radiation and count rate, alpha, beta and gamma emissions, decay equations, half-life with corrected count rates, the uses of radioisotopes and safe handling.
- Questions
- 781
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 307
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Radioactivity covers
An unstable nucleus can decay at any moment, at random and without any outside cause, giving out an α-particle, a β-particle or γ-radiation; after α- or β-decay it is the nucleus of a different element. An α-particle is a helium nucleus, two protons and two neutrons; a β-particle is a fast electron, made when a neutron in the nucleus changes into a proton; γ-radiation is high-frequency electromagnetic radiation. α is the most ionising and the least penetrating, stopped by a sheet of paper or a few centimetres of air; β is stopped by a few millimetres of aluminium; γ is the least ionising and the most penetrating, and only thick lead or concrete greatly reduces it.
Ionising radiation is measured with a detector, such as a Geiger–Müller tube, connected to a counter, and recorded as a count rate in counts/s or counts/min. There is always background radiation, from radon gas in the air, rocks and buildings, food and drink, and cosmic rays, and Extended candidates subtract it to find the corrected count rate. Extended candidates also explain that nuclei are unstable when they have too many neutrons or are too heavy, write decay equations in nuclide notation — α-decay lowers the nucleon number by 4 and the proton number by 2, β-decay raises the proton number by 1, and γ-emission changes neither — and describe how α- and β-particles are deflected in opposite directions by electric and magnetic fields, while γ-rays go straight on.
The half-life of an isotope is how long it takes for half of its nuclei in a sample to decay, read from a decay curve or a table as the time for the count rate to halve. The radiation and the half-life together decide each use: a smoke alarm needs an α-source with a long half-life; gauges that control the thickness of paper or thin sheets use β, whose absorption changes measurably with thickness; γ-rays sterilise food and medical equipment and are used to diagnose and treat cancer. Ionising radiation can kill cells or cause mutations and cancer, so sources are moved with tongs, stored in lead-lined boxes and used for the shortest time, at the greatest distance and behind the most shielding possible.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 5.2 Radioactivity
What you should be able to do
- Describe α, β and γ radiation: what each is, how strongly it ionises and how far it penetrates
- Name the main sources of background radiation, and find a corrected count rate (Extended)
- Write decay equations in nuclide notation and describe what each decay does to the nucleus (Extended)
- Find a half-life from a decay curve or a table, and use it to work out how much of a sample remains
- Explain which isotope suits a given use from the radiation it emits and its half-life (Extended)
- Describe how radioactive sources are moved, used and stored safely, and explain why time, distance and shielding matter
Key ideas
- α: helium nucleus ⁴₂He, most ionising, stopped by paper; β: fast electron, stopped by a few mm of aluminium; γ: electromagnetic radiation, reduced by thick lead
- α-decay: A falls by 4 and Z by 2; β-decay: A unchanged and Z up by 1, as a neutron becomes a proton and an electron; γ-emission: no change (Extended)
- Half-life: how long it takes for half the radioactive nuclei in a sample of an isotope to decay
- Corrected count rate = measured count rate − background count rate (Extended)
- Safety: less time, more distance, more shielding
How Radioactivity has been examined
From 2022 to 2026, Radioactivity was on Paper 4 · Theory (Extended) in 13 of 14 exam series, worth about 10% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 122 | 10% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 125 | 10% | 13 of 14 |
| Paper 3 · Theory (Core) | 28 | 7% | 13 of 14 |
| Paper 4 · Theory (Extended) | 32 | 10% | 13 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.
Filed in Momentum under: Atoms and Radioactivity.
A worked example
A detector placed near a radioactive sample reads 990 counts/min; with the sample taken away it reads 30 counts/min. Twelve hours later, with the sample back in place, it reads 270 counts/min. Find the half-life of the isotope.
- The 30 counts/min without the sample is the background, so the corrected count rates are 990 − 30 = 960 counts/min at the start and 270 − 30 = 240 counts/min after 12 hours
- 960 → 480 → 240 is two halvings
- Two half-lives take 12 hours, so the half-life is 6.0 hours
- Check: the raw readings, 990 and 270, are not a whole number of halvings apart, a sign that the background had to come off first
Answer: 6.0 hours
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Finding the half-life without taking off the background — halve the corrected count rate: with a background of 20 counts/min, a reading of 420 counts/min must fall to 220, not 210
- Saying α is the most penetrating because it is the largest — it is the most ionising, so it loses its energy quickly and is stopped by paper; γ is the most penetrating
- Changing the nucleon number in β-decay — a neutron becomes a proton, so A stays the same and Z rises by 1
- Saying a source is safe after one half-life — half the nuclei are still there; a quarter remain after two half-lives and an eighth after three
- Expecting to predict when one particular nucleus will decay — decay is random, and a half-life describes what happens to a large number of nuclei
Exam technique
- In half-life calculations, set out a table of time, number of half-lives and the amount or count rate remaining
- On a decay curve, draw lines across to the halved count rate and down to the time axis, and repeat from a second starting point to check
- When choosing an isotope for a use, justify it with both the radiation (how far it penetrates) and the half-life (long enough to last, short enough not to linger)
- For deflection questions, say that α and β bend in opposite directions, β much more strongly, and that γ goes straight on
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.