O Level Physics: Nuclear physics — Practice Questions (Cambridge 5054)

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — Cambridge International holds copyright in its own papers. Use these alongside the official past papers available from your board.

Most questions practise a skill tested in the June 2025 Paper 22. After each answer there is a tip and, where a paper question matches the skill, the real question to try next.

---

Questions

1. In a nuclear power station, the reactor core contains fuel rods, control rods and a moderator, and the whole core is surrounded by thick concrete. State the purpose of (a) the control rods, (b) the moderator, (c) the concrete. [3]

2. Explain how the fission of a uranium-235 nucleus is started, and describe the products of the fission. [3]

3. (a) Explain what is meant by a chain reaction in a nuclear reactor. (b) Explain how the control rods are used to keep the rate of fission steady. [3]

4. A nucleus of radium is written as ²²⁶₈₈Ra. (a) State the number of protons and the number of neutrons in this nucleus. (b) The nucleus decays by emitting an alpha particle and becomes a nucleus of radon (Rn). Write the nuclide notation for the radon nucleus. [3]

5. A radioactive sample has a count rate of 480 counts per minute. Twenty-four days later the count rate is 60 counts per minute. Both values have already been corrected for background radiation. (a) Calculate the half-life of the sample. (b) State the count rate after a further 8 days. [3]

6. A factory makes thin sheets of paper and uses a radioactive source on one side of the paper and a detector on the other to check the thickness. (a) State which type of radiation, alpha, beta or gamma, should be used, and explain why the other two are not suitable. (b) State which of the three radiations is the most strongly ionising. [4]

---

Answers

1. (a) Control rods absorb neutrons, which controls the rate of fission [1]. (b) The moderator slows down the fast neutrons so that they are more likely to be absorbed by uranium-235 nuclei and cause fission [1]. (c) The concrete absorbs radiation (especially gamma rays and neutrons) that escapes from the core, protecting workers and the public [1].

Tip: keep the jobs separate: control rods absorb neutrons, the moderator slows them down. Mixing these two up is an easy way to lose marks.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 8.

2. A slow-moving neutron is absorbed by the uranium-235 nucleus, making it unstable [1]. The nucleus splits into two smaller nuclei (daughter nuclei) of roughly similar size [1], and two or three neutrons are released along with a large amount of energy, mostly as kinetic energy of the products (gamma rays are also emitted) [1].

Tip: start with the cause (a neutron is absorbed), then describe the products; “the atom splits in half” misses both the neutron and the released neutrons.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 8.

3. (a) The neutrons released by one fission go on to cause further fissions in other uranium-235 nuclei, which release more neutrons, and so on [1]. (b) The rods are lowered further into the core to absorb more neutrons if the reaction speeds up, or raised to absorb fewer if it slows down [1], so that on average exactly one neutron from each fission causes another fission [1].

Tip: a steady reactor does not stop the chain reaction; it keeps it going at a constant rate, one new fission for each fission.

Try the real question next: Cambridge O Level Physics 5054, June 2025, Paper 22, Question 8.

4. (a) 88 protons [1] and 226 − 88 = 138 neutrons [1]. (b) An alpha particle is ⁴₂He, so the nucleon number falls by 4 and the proton number by 2: ²²²₈₆Rn [1].

Tip: the top number is the nucleon number (protons + neutrons) and the bottom number is the proton number; check that both totals balance on each side of the decay.

5. (a) 480 → 240 → 120 → 60 is three half-lives [1], so half-life = 24 ÷ 3 = 8 days [1]. (b) A further 8 days is one more half-life: 30 counts per minute [1].

Tip: count the halvings step by step; do not divide the count rate by the time.

6. (a) Beta [1]. Alpha would be completely stopped by the paper, so the reading would not change with thickness [1]; gamma would pass through almost unaffected, so small changes in thickness would hardly change the reading [1]. (b) Alpha [1].

Tip: the chosen radiation must be partly absorbed, so that a change in thickness gives a clear change in the count rate.

---

Where marks are usually lost

• Saying control rods “slow down neutrons”, which is the job of the moderator.
• Describing fission without mentioning that a neutron is absorbed first and that further neutrons are released.
• Getting proton number and nucleon number the wrong way round in nuclide notation.
• Dividing the count rate by the time instead of counting half-lives.
• Choosing gamma for thickness gauges of thin materials.