A hospital medical physicist states that a sample of technetium-99m has an activity of 800 000 Bq. State what is meant by the unit becquerel and explain what happens to the activity of this sample over a period of several days.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Technetium-99m is a radioactive isotope widely used in medical diagnosis. It has a half-life of approximately 6 hours.
Model answer (4 marks)
A becquerel (Bq) is the SI unit of radioactivity and equals one decay per second.
The activity of a radioactive sample is the number of decays that occur per unit time – it is the rate at which the nuclei in the sample are undergoing fission or alpha/beta decay.
Because technetium‑99m decays, the number of radioactive nuclei in the sample falls, so the activity decreases with time.
The decrease follows the half‑life rule: after each 6‑hour period the activity is halved, so over several days the activity will have dropped to a very small fraction of its initial 800 000 Bq value.
The activity of a radioactive sample is the number of decays that occur per unit time – it is the rate at which the nuclei in the sample are undergoing fission or alpha/beta decay.
Because technetium‑99m decays, the number of radioactive nuclei in the sample falls, so the activity decreases with time.
The decrease follows the half‑life rule: after each 6‑hour period the activity is halved, so over several days the activity will have dropped to a very small fraction of its initial 800 000 Bq value.
Examiner tips
- State the definition of Bq first, then explain activity, then describe the decay trend and mention the half‑life rule.
- Use the exact phrase "one decay per second" and "halves every half‑life" to match the scheme.
- Keep the answer concise – 4 marks allow a short paragraph, not a long essay.
Common mistakes
- Confusing becquerel with curie or using "decays per minute" instead of per second.
- Failing to mention the half‑life or the exponential nature of the decay.
- Writing a long explanation that repeats the definition of activity without linking it to the decrease in activity.
Mark scheme (4 marks)
- One becquerel is one (radioactive) decay per second / the unit of activity equal to one nucleus decaying per second
- Activity is the number of decays per unit time / the rate at which unstable nuclei decay
- The activity decreases / reduces over time
- This is because the number of unstable nuclei in the sample decreases (as they decay), so there are fewer nuclei available to decay / the activity halves every half-life
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "State and Explain" question →
- Decode the mark scheme abbreviations →
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