OxfordAQA International GCSE Physics: Half-life, contamination, nuclear fission and fusion (9203) – Practice Questions

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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions follow the OxfordAQA International GCSE Physics (9203) specification, for exams May/June 2018 onwards, sections 3.7.2 h to j, 3.7.3 and 3.7.4. Links: study guide, revision notes, course hub, printable checklist and the 10-minute diagnostics.

Questions

1. Give the two forms of the definition of the half-life of a radioactive isotope: one in terms of the nuclei in a sample and one in terms of the count rate. [2]

2. A sample contains 9.6 × 10⁹ nuclei of an isotope with a half-life of 15 hours. Calculate the number of undecayed nuclei of this isotope after 60 hours. [3]

3. A detector placed near a source reads 410 counts per minute. With the source removed it reads 26 counts per minute. The half-life of the source is 8 minutes. Calculate the count rate the detector will read 32 minutes later. [4]

4. A detector gives these readings near a source. The background count rate is 12 counts per minute.

Time / days	0	3	6	9	12
Reading / counts per minute	252	132	72	42	27

(a) Calculate the count rate due to the source at each time. [1] (b) Determine the half-life of the source. [2] (c) Calculate the reading you would expect at 15 days. [2]

5. Radioactive decay is a random process, yet a half-life can be used to make predictions. Explain how both statements can be true. [3]

6. Food packages are passed through a beam of gamma rays from a sealed cobalt-60 source. In another laboratory, a radioactive solution is spilled on a bench. (a) State whether the food packages become radioactive. [1] (b) Give two ways in which the hazard from the spilt solution differs from the hazard to the packages. [2] (c) State one precaution that protects a worker near the gamma source. [1]

7. A hospital needs an isotope to be injected into a patient and detected from outside the body. Isotope J emits alpha particles and has a half-life of 2 hours. Isotope K emits gamma rays and has a half-life of 5 hours. Isotope L emits gamma rays and has a half-life of 30 years. Evaluate which isotope is most suitable. [4]

8. Two waste samples contain the same number of radioactive nuclei. Sample P has a half-life of 5 minutes. Sample R has a half-life of 20 000 years. (a) State which sample emits more radiation each second at the start. [1] (b) Identify the sample that remains a hazard for the longer time, giving a reason. [2]

9. A reactor uses uranium-235 as fuel. Describe the sequence of events in one fission of a nucleus of this fuel. [4]

10. In one process each fission releases 2 neutrons, and every neutron goes on to cause a new fission. One fission starts the process. (a) Calculate the number of fissions in the eighth generation and the total number of fissions in the first eight generations. [3] (b) Explain how control rods make a reactor operate at a steady rate. [2] (c) State what happens to the rate of fission if, on average, fewer than one neutron per fission causes another fission. [1]

11. Explain why the waste from nuclear reactors needs careful management, and why this may influence a country’s decision to generate electricity from nuclear power. [4]

12. This question is about nuclear fusion. (a) State what is meant by nuclear fusion. [1] (b) State what happens to some of the mass of the smaller nuclei. [1] (c) Explain why fusion needs very high temperatures and pressures. [3] (d) State where energy is released by fusion in nature. [1] (e) State one difference between fission and fusion in what happens to the nuclei. [1]

Answers

1. The average time for the number of nuclei of the isotope in a sample to halve [1]; the time for the count rate from a sample containing the isotope to fall to half its initial level [1]. [2] Examiner insight: “Half the time” does not say what is halving. Name the quantity, as in “number of nuclei” or “count rate”.

2. 60 ÷ 15 = 4 half-lives [1]; 9.6 × 10⁹ ÷ 2⁴ = 9.6 × 10⁹ ÷ 16 [1]; 6.0 × 10⁸ nuclei [1]. [3] Examiner insight: Showing the number of half-lives separately lets a slip in the later division be traced. A bare final value gives no working to assess.

3. Corrected start = 410 − 26 = 384 [1]; 32 ÷ 8 = 4 half-lives [1]; 384 ÷ 16 = 24 [1]; add background: 24 + 26 = 50 counts per minute [1]. [4] Examiner insight: The question asks for the detector reading, so the background goes back on at the end. Stopping at 24 answers a different question.

4. (a) 240, 120, 60, 30, 15 counts per minute [1]. (b) 240 falls to 120 in 3 days [1]; check: 120 falls to 60 between 3 and 6 days. Half-life = 3 days [1]. (c) 15 ÷ 3 = 5 half-lives, so 240 ÷ 32 = 7.5 [1]; reading = 7.5 + 12 = 19.5 counts per minute [1]. [5] Examiner insight: Halving the raw readings gives 126 then 63, which is not the half-life. Subtract background from every reading first.

5. For a single nucleus, there is no way to predict whether or when it will decay [1]; in a sample containing very many nuclei, the proportion that decays in a set time can be predicted [1]; the half-life is the average time for the number of nuclei (or the count rate) to halve, so it describes the sample, not one nucleus [1]. [3] Examiner insight: Keep the single nucleus and the large sample as two separate statements. Answers that run them together often leave neither point clear.

6. (a) No, irradiation does not make the object radioactive [1]. (b) The solution contains radioactive atoms that can stick to skin or be taken into the body [1]; the hazard continues until the atoms are removed or have decayed, whereas irradiation stops when the exposure stops [1]. (c) Any one of: keep the exposure time short, stay far from the source, use shielding, handle with remote tools [1]. [4] Examiner insight: “Contamination is worse” is a judgement. Give the reason, such as atoms entering the body or the exposure continuing.

7. Gamma rays can pass out of the body to reach a detector, but alpha particles would be absorbed in the tissue, so J cannot be detected [1] and would damage nearby cells [1]; L stays active for decades, so the patient would be exposed to radiation for a very long time [1]; K’s half-life is long enough to complete the scan, yet the activity soon falls, so K is the most suitable [1]. [4] Examiner insight: On an evaluate question, reach a clear choice and reject each alternative for a stated reason.

8. (a) P [1]. (b) R [1]; its half-life is very long, so it takes a very long time for the activity to fall, even though it emits little radiation each second [1]. [3] Examiner insight: A short half-life means more radiation in a short time. A long half-life means less each second but for far longer. Keep these linked to the right sample.

9. The nucleus first absorbs a neutron [1]; this makes it unstable, and it splits into two smaller nuclei [1]; two or three neutrons are released [1]; energy is released, much more than in a chemical reaction involving a similar mass [1]. [4] Examiner insight: The neutron absorbed first is easy to leave out. Use the order of events: absorb, split, release.

10. (a) Generations double: 1, 2, 4, 8, 16, 32, 64, 128 [1]; eighth generation = 2⁷ = 128 fissions [1]; total = 1 + 2 + 4 + 8 + 16 + 32 + 64 + 128 = 255 fissions [1]. (b) Control rods absorb fission neutrons [1]; so that on average only one neutron per fission goes on to cause another fission, keeping the rate steady [1]. (c) The rate of fission falls and the reaction dies away [1]. [6] Examiner insight: The eighth generation is 2⁷, not 2⁸. Counting the starting fission as generation 1 is the usual convention here, and listing the terms shows it.

11. The waste is radioactive, so it emits ionizing radiation that is harmful [1]; it may remain radioactive for a long time, depending on its half-life and products [1]; so its disposal must be managed with care [1]; this is a factor that may influence whether nuclear power is used to generate electricity [1]. [4] Examiner insight: A strong answer links waste to half-life and then to the decision about power, rather than saying “nuclear is dangerous”.

12. (a) Two light nuclei join to make one heavier nucleus [1]. (b) It is converted into energy [1]. (c) The nuclei are both positive, so they repel each other [1]; high temperature gives them enough speed (kinetic energy) to get close despite the repulsion [1]; high pressure holds them close together so they approach each other often [1]. (d) In stars [1]. (e) In fission one large nucleus splits into smaller nuclei; in fusion light nuclei join to form a heavier one [1]. [7] Examiner insight: Mention the repulsion in an explanation like (c). Temperature and pressure alone do not give a reason.

Where marks are usually lost

• Using half the elapsed time as the number of half-lives.
• Forgetting to subtract background before halving, or forgetting to add it back when the detector reading is asked for.
• Defining half-life as “half the time” without saying what is halving.
• Saying irradiated objects become radioactive.
• Describing a long half-life as a high activity.
• Judging alpha emitters without saying whether the source is inside or outside the body.
• Missing the neutron absorbed before fission starts.
• Saying control rods stop the reaction, rather than keeping it at one neutron per fission.
• Giving temperature and pressure for fusion without the repulsion between positive nuclei.

Next steps

• Re-read the revision notes and the study guide.
• Tick off the topics on the printable checklist and visit the course hub.
• Try the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

OxfordAQA International GCSE Physics (9203), published by OxfordAQA, for exams May/June 2018 onwards: sections 3.7.2 h to j, 3.7.3 and 3.7.4.