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S6 Physics: Nuclear Radiation – Practice Paper 3 (Challenge, 10 Questions)
S6 Physics: Nuclear Radiation – Practice Paper 3 (Challenge, 10 Questions)
Paper three focuses on reasoning: why a nucleus can still be unstable after gamma emission, the internal hazard of alpha sources, activity against half-life, carbon-14 dating, lead against paper as shielding, conservation of both numbers in nuclear reactions, beta-plus against beta-minus, and finding a half-life and a percentage from activity changes. The last item targets the idea that decay pauses.
Quick answer Gamma only removes energy and leaves the composition unchanged; alpha cannot cross the dead skin layer outside the body but is damaging inside; the shorter the half-life the higher the activity; 17190 years is three half-lives leaving 12.5%; lead works because it is dense with a high atomic number; 238 equals 234 plus 4 and 92 equals 90 plus 2; beta-plus lowers the proton number by one; 9600 to 1200 Bq is three half-lives of 9 hours; 0.6 over 2.4 gives 25%; decay is continuous exponential decay with no pause.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 questions |
| Suggested time | about 30 minutes |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S6 Physics | Nuclear Radiation: Alpha, Beta, Gamma, Decay Equations and Half-life |
| Level | Challenge |
02 Questions (10 items)
- FoundationWhy can a nucleus still emit an alpha particle after it has emitted a gamma ray?
- FoundationWhy is the danger of alpha radiation mainly after inhalation or ingestion?
- FoundationThree samples of equal mass have half-lives of 2 hours, 2 days and 2 years. Which has the highest activity?
- FoundationCarbon-14 has a half-life of 5730 years. What fraction of the original carbon-14 remains in a sample that is 17190 years old?
- IntermediateWhy is lead rather than paper used to shield gamma rays?
- IntermediateWhy must mass number and proton number be conserved in a nuclear reaction? Explain with the alpha decay of uranium-238.
- IntermediateWhat is the difference between beta-minus and beta-plus decay?
- ChallengeThe activity of a sample falls from 9600 Bq to 1200 Bq in 27 hours. Find its half-life.
- ChallengeA person receives an extra dose of 2.4 mSv in a year, of which 0.6 mSv comes from medical examinations. What percentage of the extra dose is medical?
- Error redoA student says decay stops for a while at the halfway point and then continues. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | A gamma ray only removes surplus energy from an excited state, so the nucleus may still be unstable; if the proton and neutron numbers are still not stable it can later decay by alpha or beta emission | Gamma emission does not change the composition of the nucleus, so an unstable nuclide stays unstable. |
| 2 | Outside the body an alpha particle cannot even cross the dead layer of skin, but once inside it deposits all its energy in a tiny volume of tissue and does great biological damage | The point of protection is to keep alpha sources out of the body, not to fear them crossing the skin. |
| 3 | The sample with a half-life of 2 hours; the shorter the half-life, the more nuclei decay in a given time | Activity is proportional to the number of nuclei and inversely proportional to the half-life. |
| 4 | 17190 divided by 5730 is three half-lives, so one eighth remains, that is 12.5% | Radiometric dating works backwards from the remaining fraction to the number of half-lives. |
| 5 | Gamma rays have no mass and no charge and only weaken after repeated interactions with the nuclei and electrons of a dense, high atomic number material; paper is far too thin and has too few electrons per unit volume | Shielding depends on density and thickness of the material, not on colour or hardness. |
| 6 | The total number of nucleons and of protons is the same before and after: 238 equals 234 plus 4 and 92 equals 90 plus 2 | A decay equation is correct as soon as both numbers balance on each side. |
| 7 | In beta-minus decay a neutron becomes a proton and an electron plus an antineutrino are emitted, raising the proton number by one; in beta-plus decay a proton becomes a neutron and a positron plus a neutrino are emitted, lowering the proton number by one | Both keep the mass number unchanged, and they differ only in the direction of the proton number change. |
| 8 | 9600 halves to 4800, then 2400, then 1200, which is three half-lives; 27 divided by 3 gives 9 hours | Count the half-lives by repeated halving first, then divide the total time by that number. |
| 9 | 0.6 divided by 2.4 gives 0.25, that is 25% | Take the extra dose as the denominator, not the total dose. |
| 10 | Decay is continuous exponential decay: the rate is proportional to the number of nuclei left, and the half-life is only the time for half of them to go, so the process never pauses | The decay curve is smooth, with no steps or pauses anywhere on it. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
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