About Radioactivity in O Level Physics
Radioactivity is one of the 14 main topics in our O Level Physics (Cambridge (CAIE) 5054) question bank, and about 4% of its real questions come from it. Tayyari Karo has 29 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2; 18 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 17 medium, 7 easy, 5 hard.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
Try 8 real questions
These are real past-paper questions. Think of your answer first, then open the explanation.
May/June 2026 · Paper 1 · Radioactivity
A student uses a radiation detector to measure the count from a radioactive source. Without any source present, the count he obtains in 2.0 min is C1. With the counter next to the source, the count he obtains in 2.0 min is C2.
What is the count rate from the source alone, measured in counts / min?
- C2 − C1
- C2 + C1
- (C2 − C1) ÷ 2.0
- (C2 + C1) ÷ 2.0
Show answer and explanation
Answer: C. (C2 − C1) ÷ 2.0
Subtract the background count to get the count from the source alone, then divide by the 2.0 min counting time.
May/June 2024 · Paper 1 · Radioactivity
A radioactive source emitting only gamma radiation is placed in front of a radiation detector which measures the count rate. What must cause the count rate to increase?
- a thick sheet of lead placed between the source and the detector
- a strong magnetic field placed between the source and the detector
- the radioactive source moved closer to the detector
- the detector moved further away from the radioactive source
Show answer and explanation
Answer: C. the radioactive source moved closer to the detector
Moving the source closer to the detector increases the count rate because the gamma radiation spreads out less before reaching the detector; lead would absorb gamma rays and a magnetic field has no effect on uncharged gamma radiation.
May/June 2023 · Paper 1 · Radioactivity
During paper manufacture the thickness of the sheet is monitored by placing a source of beta-particles on one side of the paper and a detector on the other. If the sheet is too thick, fewer beta-particles reach the detector.
Why are beta-particles used for this purpose?
- Alpha-particles are all stopped by the paper.
- Alpha-particles make the paper radioactive.
- Gamma radiation is all stopped by the paper.
- Gamma radiation is only produced by sources with a short half-life.
Show answer and explanation
Answer: A. Alpha-particles are all stopped by the paper.
Beta radiation is partly absorbed by paper, so the count rate responds to thickness; alpha would be stopped completely and gamma would pass through almost unchanged.
Oct/Nov 2023 · Paper 1 · Radioactivity
Alpha and beta particles are emitted during radioactive decay. Which particle has the weaker ionising effect and which has the weaker penetrating power?
- weaker ionising: α, weaker penetrating: α
- weaker ionising: α, weaker penetrating: β
- weaker ionising: β, weaker penetrating: α
- weaker ionising: β, weaker penetrating: β
Show answer and explanation
Answer: C. weaker ionising: β, weaker penetrating: α
Beta particles ionise less strongly than alpha particles (weaker ionising effect), while alpha particles are stopped very easily by a sheet of paper (weaker penetrating power) compared with beta.
Oct/Nov 2022 · Paper 1 · Radioactivity
Which statement about all three types of radioactive emission is correct?
- They are all completely absorbed by a thin aluminium sheet.
- They are all deflected by electric fields.
- They are all deflected by magnetic fields.
- They all cause ionisation of gases.
Show answer and explanation
Answer: D. They all cause ionisation of gases.
All three emissions knock electrons off atoms they pass, but only the charged alpha and beta particles are deflected by fields, and gamma is not stopped by thin aluminium.
Oct/Nov 2022 · Paper 1 · Radioactivity
Radioactive isotopes that emit alpha-particles, beta-particles and gamma rays are stored in wooden boxes that are lined with a material that prevents most radiation escaping from the box.
Which material is used for the lining?
- copper
- glass
- lead
- plastic
Show answer and explanation
Answer: C. lead
Lead has a very high density, so it absorbs the penetrating gamma rays as well as the alpha and beta particles.
May/June 2024 · Paper 2 · Radioactivity
A Geiger-Müller tube and counter record 20 counts per minute with no source present, and 420 counts per minute with a radioactive source in front of the tube. State what is meant by 'background radiation'.
Show model answer
Background radiation is the (low-level) radiation that is present in the environment all the time, from natural sources (e.g. rocks, cosmic rays, the air) and some man-made sources, and is not produced by the source under investigation.
Background radiation is the ambient radiation always present, unrelated to the source being tested.
May/June 2024 · Paper 2 · Radioactivity
The radioactive source has a half-life of 45 minutes. Determine the reading on the counter 90 minutes later.
Show model answer
reading on counter = 120 counts per minute
90 minutes = 2 half-lives, so the corrected count rate falls from 400 to 400 ÷ 2 = 200, then to 200 ÷ 2 = 100 counts per minute; the counter reading also includes background, so reading = 100 + 20 = 120 counts per minute.
