A scientist is investigating the half-life of a radioactive isotope in a laboratory. Explain why the scientist must measure the background radiation before recording the count rate of the sample, and describe how the half-life of the isotope can be determined from the corrected count rate readings taken over time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A Geiger-Muller tube connected to a counter is used to measure the count rate of a radioactive sample at regular time intervals over several hours.
Model answer (4 marks)
Background radiation must be subtracted from the measured count rate to give an accurate count rate for the sample alone.
Background radiation comes from natural sources such as rocks, cosmic rays and man‑made sources such as nuclear accidents. If it is not removed the count rate will be artificially high.
The scientist records the corrected count rate at regular time intervals, noting the initial corrected count rate. The half‑life is found by identifying the time at which the corrected count rate has fallen to half the initial value. The same interval should be observed again to confirm that the half‑life is constant.
Background radiation comes from natural sources such as rocks, cosmic rays and man‑made sources such as nuclear accidents. If it is not removed the count rate will be artificially high.
The scientist records the corrected count rate at regular time intervals, noting the initial corrected count rate. The half‑life is found by identifying the time at which the corrected count rate has fallen to half the initial value. The same interval should be observed again to confirm that the half‑life is constant.
Examiner tips
- State that background must be subtracted to obtain the true sample rate. Explain that background comes from natural and man‑made sources. Show how to find the time when the corrected rate is half the initial value. Confirm the half‑life by checking a second half‑decay interval.
Common mistakes
- Failing to subtract background radiation. Confusing the total count rate with the corrected rate. Not recognising that the half‑life must be the same for each interval.
Mark scheme (4 marks)
- Background radiation must be subtracted from the measured count rate to give an accurate/corrected count rate for the sample alone.
- Background radiation is present from natural and/or man-made sources (e.g. rocks, cosmic rays, nuclear accidents) and would cause the count rate to appear artificially high.
- Record the corrected count rate at regular time intervals and identify the initial corrected count rate; note the time at which the count rate has fallen to half the initial value.
- The half-life can be confirmed/is constant by checking that the count rate halves again in the same time interval (i.e. the half-life is the same regardless of when it is measured).
Key terms in this question
background radiation · half-life · count rate
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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