Explain why the half-life of a radioactive isotope is an important factor when choosing a suitable isotope for use as a medical tracer inside the human body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The half‑life must be short enough that the isotope does not remain in the body for a long time, minimising the radiation dose received by the patient.
The half‑life must also be long enough for the tracer to travel through the body and be detected before it decays away.
During the procedure the activity (count‑rate) must remain high enough for a detector (e.g. Geiger‑Müller tube) to measure the radiation and locate the tracer.
After the procedure a short half‑life means the isotope quickly becomes stable or decays to a safe level, reducing long‑term risk to the patient.
The half‑life must also be long enough for the tracer to travel through the body and be detected before it decays away.
During the procedure the activity (count‑rate) must remain high enough for a detector (e.g. Geiger‑Müller tube) to measure the radiation and locate the tracer.
After the procedure a short half‑life means the isotope quickly becomes stable or decays to a safe level, reducing long‑term risk to the patient.
Examiner tips
- Use the word "must" to show the requirement; link half‑life to dose, detection time and safety.
- Mention both short and long limits – a balanced explanation shows full understanding.
Common mistakes
- Confusing half‑life with overall activity; forgetting that a very short half‑life gives low count‑rate.
- Failing to mention the need for the tracer to be detectable during the scan.
Mark scheme (4 marks)
- The half-life must be short enough that the isotope does not remain in the body for a long time / to minimise the radiation dose received by the patient
- The half-life must be long enough for the tracer to travel through the body and be detected before it decays away
- Activity / count-rate must remain high enough during the procedure for a detector (e.g. Geiger-Muller tube) to measure the radiation / locate the tracer
- After the procedure, a short half-life means the isotope quickly becomes stable / decays to a safe level, reducing long-term risk to the patient
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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