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Radioactivity and the Nucleus: Question 1

Syllabus 5.2.2

Multiple choice Core 1 mark

A student is investigating three sealed radioactive sources, labelled P, Q and R, in a school physics laboratory. A Geiger–Müller tube and counter is placed the same fixed distance from each source in turn, and different absorbers are placed between the source and the tube. The student's results are:

  • Source P: detected through a sheet of paper and through 5 mm of aluminium, but not through 20 mm of lead.
  • Source Q: not detected even through a single sheet of paper.
  • Source R: detected through a sheet of paper, but not through 5 mm of aluminium.

Based on these results, which type of radiation is source R most likely emitting?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the penetrating power of each type of radiation

  • Alpha particles are stopped by a single sheet of paper (or a few centimetres of air).
  • Beta particles pass through paper but are stopped by a few millimetres of aluminium.
  • Gamma rays pass through paper and aluminium, and need many centimetres of a dense material such as lead to reduce them significantly.

Step 2: Use the results to identify each source

  • Source Q is stopped by paper alone, so Q is emitting alpha particles.
  • Source P passes through paper and 5 mm of aluminium, but is stopped by 20 mm of lead, so P is emitting gamma rays.
  • Source R passes through paper but is stopped by 5 mm of aluminium, so R is emitting beta particles.

Step 3: Match to the correct option

Source R’s radiation passes through paper but is stopped by aluminium, exactly the penetrating power of beta particles. This is option B.

Why the other options are wrong

OptionRadiationWhy it doesn’t fit source R
AalphaAlpha particles would already be stopped by the paper, but R’s radiation passes through the paper.
CgammaGamma rays would pass through 5 mm of aluminium, but R’s radiation is stopped by the aluminium.
DnoneR was detected through paper, so it is emitting ionising radiation of some kind.

Final answers

  • Source R is most likely emitting beta particles, option B.