Explain why background radiation must be subtracted from measurements before the half-life of a radioactive source is determined.

Cambridge International IGCSE Physics (0625) — 5.2 Radioactivity · Explain · 4 marks · View as Markdown

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

A student uses a Geiger-Müller tube connected to a counter to measure the count rate of a radioactive source at regular time intervals in order to find its half-life.

Model answer (4 marks)

Background radiation is always present and therefore contributes to the measured count rate.
The measured count rate therefore contains two components: the source and the background.
Subtracting the background gives the count rate due only to the source.
If the background is not subtracted, the calculated half‑life will be too long because the count rate will never fall to zero and the halving point will be incorrectly estimated.

Examiner tips

State that background radiation is independent of the source. Explain that it adds to the measured count rate. Show that subtraction gives the true source rate. Mention that an uncorrected rate gives an over‑estimated half‑life.

Common mistakes

Failing to recognise that background is independent of the source. Not explaining that subtraction corrects the count rate. Assuming the background has no effect on the half‑life calculation.

Mark scheme (4 marks)

Background radiation is always present / exists independently of the source being investigated
Background radiation contributes to / adds to the measured count rate
Subtracting gives the corrected count rate / count rate due only to the source
Without subtraction the half-life calculated would be incorrect / too long, because the count rate would not fall to zero / the background count prevents the count rate from halving correctly

Key terms in this question

background radiation · half-life