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A2 Daily A Level Physics question

2025-08-30 OCR A A Level Physics 6.4.3(b)(ii) 6.4.3(d)

In a school lab, a sealed beta− source gives 800 counts in 2.0 minutes on a GM tube 10 cm away with a 2 mm aluminium sheet in front. Background is negligible. A second identical source is placed next to the first at the same distance, with the same absorber still in place. Which is the best estimate for the new count in 2.0 minutes, and why?

  1. A About 800 counts; the aluminium absorber fixes the transmitted rate, so an extra source makes no difference.
  2. B About 400 counts; with twice as many nuclei the half-life doubles, so the sample decays more slowly giving a lower count.
  3. C About 1600 counts; doubling the number of undecayed nuclei doubles the activity, and the same absorber transmits the same fraction. (correct)
  4. D About 1200 counts; placing sources together increases mass defect so some energy becomes extra binding, reducing the detected decays.

Answer

The correct answer is C.

Correct: C — About 1600 counts; doubling the number of undecayed nuclei doubles the activity, and the same absorber transmits the same fraction. A Aluminium does not set a fixed count rate; for the same absorber the transmitted count is proportional to source strength. B Half-life is a property of the nuclide and does not depend on the amount present, so it does not double with twice the nuclei. C This is correct because activity is proportional to the number of undecayed nuclei, so two identical sources give twice the count through the same absorber. D Mass defect concerns binding within nuclei and is not changed by placing two sealed sources together; it does not reduce the decay rate in this way.

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