Technetium-99m is a radioactive isotope widely used in hospitals as a medical tracer. It has a half-life of 6 hours and is a gamma emitter. Explain why technetium-99m is well suited for use as a medical tracer, rather than an alpha or beta emitter with a half-life of several years.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Technetium-99m is injected into a patient's bloodstream. Detectors outside the body monitor the gamma radiation emitted as the technetium travels through tissues and organs.
Model answer (5 marks)
1. Gamma rays are highly penetrating; they can pass through body tissues and reach external detectors.
2. Alpha or beta particles are absorbed by tissue and would not reach detectors.
3. A 6‑hour half‑life means the isotope decays quickly, limiting patient exposure.
4. The 6‑hour half‑life is long enough for the tracer to circulate and be imaged before it decays.
5. An isotope with a multi‑year half‑life would remain radioactive in the body for a long time, increasing the risk of harm.
2. Alpha or beta particles are absorbed by tissue and would not reach detectors.
3. A 6‑hour half‑life means the isotope decays quickly, limiting patient exposure.
4. The 6‑hour half‑life is long enough for the tracer to circulate and be imaged before it decays.
5. An isotope with a multi‑year half‑life would remain radioactive in the body for a long time, increasing the risk of harm.
Examiner tips
- Use the exact terminology (gamma, alpha, beta, half‑life).
- Show the logical sequence: penetration, detection, safety, timing.
- Mention both advantages and disadvantages of a long half‑life.
Common mistakes
- Confusing gamma with beta radiation.
- Forgetting to explain why a short half‑life is beneficial.
Mark scheme (5 marks)
- Gamma radiation is the most penetrating type of radiation, so it can pass through body tissue without being absorbed and reach the external detectors.
- Alpha and beta radiation would be absorbed by the body tissue before reaching external detectors, so they would not be suitable.
- A short half-life of 6 hours means the technetium decays away quickly, so the patient is not exposed to radiation for a long time after the procedure.
- The 6-hour half-life is long enough for the technetium to circulate through the body and be detected before it decays away.
- An isotope with a half-life of several years would remain radioactive inside the patient for a very long time, increasing the risk of harm from prolonged irradiation.
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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