Cambridge IGCSE · 0625

Physics

Radioactivity and atomic physics — Topic 6

Name: ____________________Date: October 10, 2026

1.
A nucleus of uranium-238 (₉₂²³⁸U) decays by emitting an alpha particle. Write the nuclide notation of the resulting daughter nucleus, including its mass number and atomic number.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that an alpha particle removes mass number 4 and atomic number 2. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Obtains mass number 234 and atomic number 90. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: Writes the daughter nuclide as ₉₀²³⁴Th (thorium-234). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Both mass number and atomic number must balance across the decay equation.
Marking points
- States that an alpha particle removes mass number 4 and atomic number 2.
- Obtains mass number 234 and atomic number 90.
- Writes the daughter nuclide as ₉₀²³⁴Th (thorium-234).
Examiner tip: Both mass number and atomic number must balance across the decay equation.
2.
A radioactive isotope has a half-life of 5 days. A sample starts with an activity of 800 counts per minute. Calculate the activity after 20 days.
[3 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Develop this part of the answer: Identifies that 20 days is four half-lives. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Halves the activity four times: 800 → 400 → 200 → 100 → 50. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States the final activity as 50 counts per minute. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Count how many whole half-lives have passed before halving; do not divide by the number of days directly.
Marking points
- Identifies that 20 days is four half-lives.
- Halves the activity four times: 800 → 400 → 200 → 100 → 50.
- States the final activity as 50 counts per minute.
Examiner tip: Count how many whole half-lives have passed before halving; do not divide by the number of days directly.
3.
Distinguish between alpha, beta and gamma radiation in terms of their penetrating power through materials.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that alpha radiation is stopped by a few centimetres of air or a sheet of paper (least penetrating). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that beta radiation is stopped by a few millimetres of aluminium. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that gamma radiation requires several centimetres of lead (or thick concrete) to significantly reduce it (most penetrating). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Penetrating power increases in the order alpha < beta < gamma, while ionising power decreases in the same order.
Marking points
- States that alpha radiation is stopped by a few centimetres of air or a sheet of paper (least penetrating).
- States that beta radiation is stopped by a few millimetres of aluminium.
- States that gamma radiation requires several centimetres of lead (or thick concrete) to significantly reduce it (most penetrating).
Examiner tip: Penetrating power increases in the order alpha < beta < gamma, while ionising power decreases in the same order.
4.
A radioactive source is placed near a Geiger-Müller tube connected to a counter. Describe how the count rate can be used to determine the half-life of the source, including one correction that should be applied.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the count rate is recorded at regular time intervals as the source decays. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that background radiation should be measured separately (with the source absent) and subtracted from each reading. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that a graph of corrected count rate against time is plotted. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that the half-life is read from the graph as the time taken for the corrected count rate to fall to half of any chosen value. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Background radiation correction is essential — without subtracting it, the measured count rate never falls to zero, and half-life calculations become inaccurate.
Marking points
- States that the count rate is recorded at regular time intervals as the source decays.
- States that background radiation should be measured separately (with the source absent) and subtracted from each reading.
- States that a graph of corrected count rate against time is plotted.
- States that the half-life is read from the graph as the time taken for the corrected count rate to fall to half of any chosen value.
Examiner tip: Background radiation correction is essential — without subtracting it, the measured count rate never falls to zero, and half-life calculations become inaccurate.
5.
Complete the nuclear equation for the beta decay of carbon-14: ₆¹⁴C → ₇¹⁴N + [particle]. Identify the missing particle.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that in beta decay, a neutron in the nucleus converts into a proton, emitting a beta particle (a fast-moving electron). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Identifies the emitted particle as a beta particle (electron), symbol ₋₁⁰e or ₋₁⁰β. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Work through this mathematical step: Confirms mass number is unchanged (14 = 14) and atomic number increases by 1 (6 → 7), consistent with a beta particle carrying negligible mass and charge −1. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: In beta decay, mass number stays the same (a neutron becomes a proton, both mass number 1) while atomic number increases by 1.
Marking points
- States that in beta decay, a neutron in the nucleus converts into a proton, emitting a beta particle (a fast-moving electron).
- Identifies the emitted particle as a beta particle (electron), symbol ₋₁⁰e or ₋₁⁰β.
- Confirms mass number is unchanged (14 = 14) and atomic number increases by 1 (6 → 7), consistent with a beta particle carrying negligible mass and charge −1.
Examiner tip: In beta decay, mass number stays the same (a neutron becomes a proton, both mass number 1) while atomic number increases by 1.
6.
Explain one medical use of a gamma-emitting radioactive isotope, referring to why gamma radiation specifically is suitable.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States a valid medical use, e.g. as a tracer to monitor organ function, or in the external treatment of cancerous tumours. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that gamma radiation can penetrate the body (unlike alpha, which cannot even penetrate skin), allowing it to be detected externally or to reach internal tissue. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that gamma radiation causes relatively low ionisation damage to surrounding tissue per unit path length compared to alpha, making a tracer dose safer. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The choice of radiation type for a medical or industrial use always depends on matching its penetrating power to the specific requirement of the application.
Marking points
- States a valid medical use, e.g. as a tracer to monitor organ function, or in the external treatment of cancerous tumours.
- States that gamma radiation can penetrate the body (unlike alpha, which cannot even penetrate skin), allowing it to be detected externally or to reach internal tissue.
- States that gamma radiation causes relatively low ionisation damage to surrounding tissue per unit path length compared to alpha, making a tracer dose safer.
Examiner tip: The choice of radiation type for a medical or industrial use always depends on matching its penetrating power to the specific requirement of the application.
7.
Describe the structure of the atom in terms of the nucleus and electron arrangement, including the relative size of the nucleus compared to the whole atom.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the atom has a small, dense, positively charged nucleus at its centre, containing protons and neutrons. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that electrons orbit the nucleus at relatively large distances, occupying most of the atom's volume. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that the nucleus is extremely small compared to the overall size of the atom (most of the atom is empty space). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This nuclear model of the atom was established by the Rutherford (gold foil) alpha-scattering experiment, which showed most alpha particles passed straight through — evidence that atoms are mostly empty space.
Marking points
- States that the atom has a small, dense, positively charged nucleus at its centre, containing protons and neutrons.
- States that electrons orbit the nucleus at relatively large distances, occupying most of the atom's volume.
- States that the nucleus is extremely small compared to the overall size of the atom (most of the atom is empty space).
Examiner tip: This nuclear model of the atom was established by the Rutherford (gold foil) alpha-scattering experiment, which showed most alpha particles passed straight through — evidence that atoms are mostly empty space.
8.
Explain why alpha particles are more strongly ionising than gamma rays, and link this to their difference in penetrating power.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that alpha particles have a relatively large mass and charge, so they interact strongly and frequently with atoms they pass, knocking electrons off and causing intense ionisation over a short distance. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that gamma rays, having no mass or charge, interact much less frequently with matter, producing far less ionisation per unit path length. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: Explains that because alpha particles lose their energy so rapidly through intense ionisation, they are absorbed (stopped) over a very short distance, whereas gamma rays, interacting rarely, travel much further before being absorbed — the inverse relationship between ionising power and penetrating power. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Ionising power and penetrating power are inversely related across all three radiation types: the more strongly a radiation ionises matter, the more quickly it loses its energy and the less far it penetrates.
Marking points
- States that alpha particles have a relatively large mass and charge, so they interact strongly and frequently with atoms they pass, knocking electrons off and causing intense ionisation over a short distance.
- States that gamma rays, having no mass or charge, interact much less frequently with matter, producing far less ionisation per unit path length.
- Explains that because alpha particles lose their energy so rapidly through intense ionisation, they are absorbed (stopped) over a very short distance, whereas gamma rays, interacting rarely, travel much further before being absorbed — the inverse relationship between ionising power and penetrating power.
Examiner tip: Ionising power and penetrating power are inversely related across all three radiation types: the more strongly a radiation ionises matter, the more quickly it loses its energy and the less far it penetrates.
9.
A sample initially contains 4.0 × 10¹⁰ radioactive nuclei. After 3 half-lives, calculate the number of radioactive nuclei remaining.
[3 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Develop this part of the answer: States that after each half-life, the number of radioactive nuclei halves. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Halves the initial number three times: 4.0×10¹⁰ → 2.0×10¹⁰ → 1.0×10¹⁰ → 0.5×10¹⁰. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States the number remaining as 5.0 × 10⁹. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Equivalently, the fraction remaining after n half-lives is (1/2)ⁿ — here (1/2)³ = 1/8 of the original number.
Marking points
- States that after each half-life, the number of radioactive nuclei halves.
- Halves the initial number three times: 4.0×10¹⁰ → 2.0×10¹⁰ → 1.0×10¹⁰ → 0.5×10¹⁰.
- States the number remaining as 5.0 × 10⁹.
Examiner tip: Equivalently, the fraction remaining after n half-lives is (1/2)ⁿ — here (1/2)³ = 1/8 of the original number.
10.
Discuss one benefit and one risk associated with the use of radioactive isotopes in industry (e.g. checking the thickness of metal sheets).
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Build a supported judgement: identify the claim, use the question's evidence, consider a relevant limitation or alternative, and make the conclusion depend on that evidence. There may be more than one defensible answer.
2. Develop this part of the answer: States a valid benefit, e.g. beta radiation passing through a metal sheet allows continuous, non-contact monitoring of its thickness during manufacture, improving quality control without stopping production. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States a valid risk, e.g. workers or the public may be exposed to harmful ionising radiation, which can damage living cells and increase cancer risk if not properly shielded and monitored. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Industrial radioactive source questions expect a balanced answer: a specific, plausible benefit tied to a property of the radiation, alongside a specific, plausible risk from exposure.
Marking points
- States a valid benefit, e.g. beta radiation passing through a metal sheet allows continuous, non-contact monitoring of its thickness during manufacture, improving quality control without stopping production.
- States a valid risk, e.g. workers or the public may be exposed to harmful ionising radiation, which can damage living cells and increase cancer risk if not properly shielded and monitored.
Examiner tip: Industrial radioactive source questions expect a balanced answer: a specific, plausible benefit tied to a property of the radiation, alongside a specific, plausible risk from exposure.
11.
Describe what happens during the nuclear fission of a uranium-235 nucleus, and explain how a chain reaction can occur.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that a slow-moving neutron is absorbed by a uranium-235 nucleus, causing it to become unstable and split into two smaller (daughter) nuclei. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that this fission releases a large amount of energy, along with two or three additional neutrons. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that these released neutrons can go on to be absorbed by other uranium-235 nuclei, causing them to also undergo fission. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that this self-sustaining sequence of fission reactions is called a chain reaction. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A chain reaction requires at least one neutron from each fission event to go on and cause another fission — otherwise the reaction dies out rather than sustaining itself.
Marking points
- States that a slow-moving neutron is absorbed by a uranium-235 nucleus, causing it to become unstable and split into two smaller (daughter) nuclei.
- States that this fission releases a large amount of energy, along with two or three additional neutrons.
- States that these released neutrons can go on to be absorbed by other uranium-235 nuclei, causing them to also undergo fission.
- States that this self-sustaining sequence of fission reactions is called a chain reaction.
Examiner tip: A chain reaction requires at least one neutron from each fission event to go on and cause another fission — otherwise the reaction dies out rather than sustaining itself.
12.
Describe what happens during nuclear fusion, giving the Sun as an example, and state one difference between fusion and fission.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that fusion is the joining (combining) of two light nuclei, e.g. hydrogen nuclei, to form a single heavier nucleus, e.g. helium, releasing a large amount of energy. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that fusion is the process that powers the Sun and other stars. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States a valid difference from fission, e.g. fusion combines light nuclei together while fission splits a heavy nucleus apart, or fusion requires extremely high temperatures and pressures to overcome the electrostatic repulsion between nuclei. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Both fission and fusion release energy because the resulting nuclei are more tightly bound (more stable) than the starting nuclei — the energy released comes from this increase in binding energy.
Marking points
- States that fusion is the joining (combining) of two light nuclei, e.g. hydrogen nuclei, to form a single heavier nucleus, e.g. helium, releasing a large amount of energy.
- States that fusion is the process that powers the Sun and other stars.
- States a valid difference from fission, e.g. fusion combines light nuclei together while fission splits a heavy nucleus apart, or fusion requires extremely high temperatures and pressures to overcome the electrostatic repulsion between nuclei.
Examiner tip: Both fission and fusion release energy because the resulting nuclei are more tightly bound (more stable) than the starting nuclei — the energy released comes from this increase in binding energy.
13.
Discuss one advantage and one disadvantage of generating electricity using nuclear fission in a power station, compared with burning fossil fuels.
[2 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Build a supported judgement: identify the claim, use the question's evidence, consider a relevant limitation or alternative, and make the conclusion depend on that evidence. There may be more than one defensible answer.
2. Develop this part of the answer: States a valid advantage, e.g. nuclear power stations do not release carbon dioxide or other greenhouse gases during operation, unlike fossil fuel power stations. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States a valid disadvantage, e.g. nuclear power stations produce radioactive waste that remains hazardous for a very long time and is difficult to safely store or dispose of. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Balanced-argument questions like this expect a specific, genuine advantage and a specific, genuine disadvantage — vague or one-sided answers rarely gain full credit.
Marking points
- States a valid advantage, e.g. nuclear power stations do not release carbon dioxide or other greenhouse gases during operation, unlike fossil fuel power stations.
- States a valid disadvantage, e.g. nuclear power stations produce radioactive waste that remains hazardous for a very long time and is difficult to safely store or dispose of.
Examiner tip: Balanced-argument questions like this expect a specific, genuine advantage and a specific, genuine disadvantage — vague or one-sided answers rarely gain full credit.
14.
State three sources of background radiation, and identify which of these is typically the largest contributor.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States radon gas from rocks and soil as a source of background radiation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States cosmic rays from space as a source of background radiation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States medical procedures, e.g. X-rays, as a source of background radiation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States that radon gas from rocks and soil is typically the largest single contributor to background radiation for most people. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Background radiation comes from a mix of natural sources (radon, cosmic rays, rocks) and artificial sources (medical, nuclear industry) — natural sources make up the large majority for most people.
Marking points
- States radon gas from rocks and soil as a source of background radiation.
- States cosmic rays from space as a source of background radiation.
- States medical procedures, e.g. X-rays, as a source of background radiation.
- States that radon gas from rocks and soil is typically the largest single contributor to background radiation for most people.
Examiner tip: Background radiation comes from a mix of natural sources (radon, cosmic rays, rocks) and artificial sources (medical, nuclear industry) — natural sources make up the large majority for most people.
15.
State three safety precautions that should be taken when handling a radioactive source in a school laboratory.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States using tongs (forceps) to handle the source, keeping it as far from the body as reasonably possible. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States minimising the time of exposure to the source. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States storing the source in a shielded (lead-lined) container when not in use. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The three key principles for radiation safety are: maximise distance, minimise time of exposure, and use shielding — remembering these covers most practical safety questions.
Marking points
- States using tongs (forceps) to handle the source, keeping it as far from the body as reasonably possible.
- States minimising the time of exposure to the source.
- States storing the source in a shielded (lead-lined) container when not in use.
Examiner tip: The three key principles for radiation safety are: maximise distance, minimise time of exposure, and use shielding — remembering these covers most practical safety questions.
16.
A nucleus of radium-226 (₈₈²²⁶Ra) decays by emitting an alpha particle. Write the nuclide notation of the resulting daughter nucleus.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that an alpha particle removes mass number 4 and atomic number 2. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Obtains mass number 222 and atomic number 86. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: Writes the daughter nuclide as ₈₆²²²Rn (radon-222). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Radium-226 decaying into radon-222 is a real step in the naturally occurring uranium-238 decay chain, and is the source of most radon gas found in homes.
Marking points
- States that an alpha particle removes mass number 4 and atomic number 2.
- Obtains mass number 222 and atomic number 86.
- Writes the daughter nuclide as ₈₆²²²Rn (radon-222).
Examiner tip: Radium-226 decaying into radon-222 is a real step in the naturally occurring uranium-238 decay chain, and is the source of most radon gas found in homes.
17.
After emitting an alpha or beta particle, a nucleus is often left in an excited (higher-energy) state. Explain what happens when the nucleus subsequently emits a gamma ray, and state what happens to its mass number and atomic number.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that the nucleus loses its excess energy by emitting a gamma ray (electromagnetic radiation). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that the mass number and atomic number remain unchanged during gamma emission. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States that this is because a gamma ray has no mass and no charge, so it carries away energy only, without changing the composition of the nucleus. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Gamma emission almost always follows alpha or beta decay, as the nucleus settles from an excited state to its stable ground state — it very rarely occurs on its own.
Marking points
- States that the nucleus loses its excess energy by emitting a gamma ray (electromagnetic radiation).
- States that the mass number and atomic number remain unchanged during gamma emission.
- States that this is because a gamma ray has no mass and no charge, so it carries away energy only, without changing the composition of the nucleus.
Examiner tip: Gamma emission almost always follows alpha or beta decay, as the nucleus settles from an excited state to its stable ground state — it very rarely occurs on its own.
18.
A student records the count rate of a radioactive source, corrected for background radiation, over time: 400 counts/min at 0 minutes, 200 counts/min at 8 minutes, 100 counts/min at 16 minutes. Determine the half-life of the source.
[3 marks]
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
2. Develop this part of the answer: Identifies that the count rate halves from 400 to 200 counts/min in the first 8 minutes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Identifies that it halves again, from 200 to 100 counts/min, in a further 8 minutes, confirming a consistent pattern. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States the half-life of the source as 8 minutes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Checking that the count rate halves over the same time interval more than once confirms the half-life is constant, since half-life does not depend on the amount of the sample remaining.
Marking points
- Identifies that the count rate halves from 400 to 200 counts/min in the first 8 minutes.
- Identifies that it halves again, from 200 to 100 counts/min, in a further 8 minutes, confirming a consistent pattern.
- States the half-life of the source as 8 minutes.
Examiner tip: Checking that the count rate halves over the same time interval more than once confirms the half-life is constant, since half-life does not depend on the amount of the sample remaining.
19.
Explain why exposure to ionising radiation can be harmful to living cells, and state one possible long-term effect on the body.
[3 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that ionising radiation can knock electrons out of atoms and molecules within living cells, damaging or altering their structure. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: States that this ionisation can damage the DNA within cells. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States a valid long-term effect, e.g. this DNA damage can cause mutations that may lead to cancer. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The harm from ionising radiation comes specifically from its ability to ionise (remove electrons from) atoms within cells — this is what distinguishes it from non-ionising radiation such as radio waves.
Marking points
- States that ionising radiation can knock electrons out of atoms and molecules within living cells, damaging or altering their structure.
- States that this ionisation can damage the DNA within cells.
- States a valid long-term effect, e.g. this DNA damage can cause mutations that may lead to cancer.
Examiner tip: The harm from ionising radiation comes specifically from its ability to ionise (remove electrons from) atoms within cells — this is what distinguishes it from non-ionising radiation such as radio waves.
20.
Define the term isotope, and give an example of two isotopes of the same element, stating one way in which their properties differ and one way in which they are the same.
[4 marks] · no calculator
Answer explanation
Draft walkthroughs are based on marking guidance, not independently verified derivations.
1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
2. Develop this part of the answer: States that isotopes are atoms of the same element (same number of protons/same atomic number) that have different numbers of neutrons, and therefore different mass numbers. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
3. Develop this part of the answer: Gives a valid example, e.g. carbon-12 and carbon-14. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
4. Develop this part of the answer: States a way their properties differ, e.g. carbon-14 is radioactive (unstable) while carbon-12 is stable, or they have different masses. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
5. Develop this part of the answer: States a way they are the same, e.g. they have identical chemical properties, since chemical behaviour depends on the number of protons/electrons, not neutrons. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Isotopes of the same element always behave identically in chemical reactions, since chemistry is governed by electron arrangement (which depends on proton number) — only their nuclear (physical) properties differ.
Marking points
- States that isotopes are atoms of the same element (same number of protons/same atomic number) that have different numbers of neutrons, and therefore different mass numbers.
- Gives a valid example, e.g. carbon-12 and carbon-14.
- States a way their properties differ, e.g. carbon-14 is radioactive (unstable) while carbon-12 is stable, or they have different masses.
- States a way they are the same, e.g. they have identical chemical properties, since chemical behaviour depends on the number of protons/electrons, not neutrons.
Examiner tip: Isotopes of the same element always behave identically in chemical reactions, since chemistry is governed by electron arrangement (which depends on proton number) — only their nuclear (physical) properties differ.