A geologist discovers a rock sample that contains a radioactive isotope. She uses a Geiger-Muller tube to measure the activity of the sample and records a value of 2400 Bq. State what is meant by the unit becquerel and explain why the geologist must account for background radiation before using this reading to determine the half-life of the isotope.

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

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

Model answer (4 marks)

A becquerel (Bq) is the SI unit of radioactivity and equals one disintegration per second.

Background radiation is always present and is detected by the Geiger‑Muller tube. The measured activity therefore contains both the sample and this background.

To obtain the true activity of the rock sample, the background count rate must be subtracted from the 2400 Bq reading. If this correction is not made the activity will be over‑estimated, leading to an incorrect calculation of the isotope’s half‑life.

Examiner tips

  • Use the exact definition of Bq (1 disintegration s⁻¹).
  • Explain that background is always present and must be subtracted to get the sample’s true activity.
  • Show that an uncorrected reading gives a too‑high activity and wrong half‑life.

Common mistakes

  • Saying Bq is just a unit of radiation without linking it to disintegrations per second.
  • Forgetting to mention that background radiation is always present and must be subtracted.
  • Claiming the background has no effect on the half‑life calculation.

Mark scheme (4 marks)

  1. One becquerel is equal to one decay (disintegration) per second
  2. Background radiation is present all around us / is always being detected by the Geiger-Muller tube
  3. The background count rate must be subtracted from the measured activity to give the true activity of the sample alone
  4. If background radiation is not subtracted, the activity reading will be too high / the half-life calculated will be incorrect

Key terms in this question

becquerel · activity · background radiation · half-life · Geiger-Muller tube

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