A student measures the activity of a radioactive sample using a Geiger-Muller tube and records a value of 340 becquerels. Explain what is meant by the term 'activity', state what unit it is measured in, and describe how the activity of the sample will change over time.

Pearson Edexcel International GCSE Physics (4PH1) — 7.1 Units · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Activity is the number of radioactive decays that occur per unit time, i.e. the rate of decay of unstable nuclei.
The unit of activity is the becquerel (Bq), where 1 Bq = 1 decay s⁻¹.
Over time the activity of the sample will decrease because the number of remaining unstable nuclei falls as they decay, so fewer nuclei are available to decay each second.

Examiner tips

  • Define activity as a decay rate, not a quantity of material.
  • State the unit Bq and its definition.
  • Explain the decrease in terms of loss of undecayed nuclei.
  • Use the word ‘decreases’ or ‘reduces’ to match the mark scheme.

Common mistakes

  • Confusing activity with the amount of material (e.g. mass or moles).
  • Writing the unit as ‘Bq’ without the definition of 1 decay s⁻¹.
  • Failing to mention that the decrease is due to the reduction in undecayed nuclei.

Mark scheme (4 marks)

  1. Activity is the number of (radioactive/nuclear) decays per unit time / the rate at which unstable nuclei decay
  2. Activity is measured in becquerels (Bq)
  3. The activity decreases / reduces over time
  4. This is because unstable nuclei are used up / the number of unstable nuclei remaining decreases as decay occurs, so fewer decays happen each second

Key terms in this question

activity · becquerel

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