A research team is investigating the suitability of two radioactive isotopes, Isotope P and Isotope Q, for use in treating cancerous tumours using chemotherapy. Isotope P is a gamma emitter with a half-life of 8 hours. Isotope Q is an alpha emitter with a half-life of 200 years. Explain why Isotope P is more suitable than Isotope Q for use in cancer treatment by chemotherapy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Chemotherapy using radioactive materials involves directing radiation onto cancerous cells so they absorb the energy and are destroyed. The radiation source may be placed near the tumour inside the body or directed at the body from outside.
Model answer (5 marks)
Isotope P is a gamma emitter with a short half‑life of 8 h, whereas Isotope Q is an alpha emitter with a half‑life of 200 y.
1. Gamma rays are highly penetrating; they can travel through body tissue and reach an internal tumour.
2. Alpha particles have very low penetration – a few centimetres of air or skin stops them – so they would not reach an internal tumour.
3. The 8‑h half‑life of Isotope P means the activity falls rapidly, reducing the long‑term radiation dose to the patient.
4. The 200‑year half‑life of Isotope Q keeps it strongly radioactive for a very long time, posing a long‑term hazard to the patient and the environment.
5. Gamma radiation is less ionising than alpha radiation; it therefore causes less collateral damage to healthy cells while still delivering dose to the tumour.
1. Gamma rays are highly penetrating; they can travel through body tissue and reach an internal tumour.
2. Alpha particles have very low penetration – a few centimetres of air or skin stops them – so they would not reach an internal tumour.
3. The 8‑h half‑life of Isotope P means the activity falls rapidly, reducing the long‑term radiation dose to the patient.
4. The 200‑year half‑life of Isotope Q keeps it strongly radioactive for a very long time, posing a long‑term hazard to the patient and the environment.
5. Gamma radiation is less ionising than alpha radiation; it therefore causes less collateral damage to healthy cells while still delivering dose to the tumour.
Examiner tips
- Use the exact terms ‘gamma emitter’, ‘alpha emitter’, ‘penetrating power’, ‘half‑life’, ‘long‑term hazard’.
- Show the causal chain: penetration → tumour reach, half‑life → patient safety.
- Keep each point brief and directly linked to a mark‑scheme point.
Common mistakes
- Confusing alpha with beta radiation; alpha is the least penetrating.
- Failing to mention the short half‑life as a safety advantage.
- Using vague phrases like ‘more suitable’ without explaining why.
Mark scheme (5 marks)
- Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue to reach the tumour
- Alpha radiation has the least penetrating power / is stopped by a few centimetres of air or skin, so it would not reach an internal tumour
- A short half-life means the source does not remain strongly radioactive for a long time, reducing long-term risk to the patient
- A half-life of 200 years means Isotope Q would remain radioactive for a very long time, posing a long-term hazard to the patient
- Gamma radiation is less ionising than alpha, so it causes less damage to surrounding healthy cells as it passes through the body
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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