Ionising Radiation
Quantum and Nuclear

What did they learn about the topic of radioactivity?

Teaching Guidance for 14-16 Supporting Physics Teaching

Students' understandings of aspects of radioactivity

The following questions were given to 144 school students aged 15–16 years and drawn from four schools. The students had all previously studied the topic of radioactivity.

The first question focuses on a familiar school demonstration with a radioactive source and Geiger–Müller tube, which students are likely to have seen in their lessons and involves radiation travelling a short distance. The second question focuses on a familiar example of contamination, over large distances, from the Chernobyl accident. The questions were developed together to probe students' explanations of effects at large and small distances. Both questions were answered by all of the students.

Students' views about why the count rate falls with distance

In a demonstration experiment, a teacher places a Geiger counter near a radioactive source. The counter counts quite rapidly. The teacher then moves the Geiger counter away from the source. As she does this, the count rate gradually goes down. By the time she has moved the counter right across the room, the Geiger counter has almost stopped recording any counts. Explain why the Geiger counter counts less and less as it is moved away from the source.

About 5 % of students gave complete responses using correct scientific ideas, while about 40 % of students gave incomplete responses using correct ideas.

About 40 % of students gave responses including incorrect scientific ideas.

The correct scientific ideas were as follows:

  • Emitted radiation diverges from source.
  • Radiation causes ionisation of the air.
  • Intensity (count rate) decreases with distance.

The incorrect ideas included:

  • The source emits radioactive material into the atmosphere (see Challenge: Sorting out words and ideas.
  • Radioactive particles reach the tube (see Challenge: Are alpha particles radioactive?).
  • Radioactivity is emitted (see Challenge: Sorting out words and ideas).
  • Radioactive source becomes weaker.

Students' views about radioactive fallout

In April 1986, a serious accident occurred at the nuclear power station at Chernobyl in Russia. A week later, radiation detectors (Geiger counters) in Britain recorded higher than usual levels of radiation. Britain is more than 1000 miles from Chernobyl! Explain what reached the Geiger counters in Britain to make them record extra counts.

About 12 % of students gave complete responses using correct scientific ideas, while about 24 % of students gave incomplete responses using correct ideas.

About 50 % of students gave responses including incorrect scientific ideas.

The correct scientific ideas used were:

  • Radioactive material reached the UK.
  • Radioactive material was transferred by the wind.
  • Radioactive material emits radiation.

The incorrect ideas included:

  • Radiation was carried by the wind (see Challenges: Sorting out words and ideas and The difference between radiation contamination and irradiation?).
  • Radiation polluted the atmosphere in UK (see Challenges: Sorting out words and ideas and The difference between radiation contamination and irradiation?).
  • Gamma radiation reached the UK (see Challenges: Sorting out words and ideas and The difference between radiation contamination and irradiation?).

Thinking about the teaching

The numbers of students giving complete correct responses to these two questions is strikingly low at 5 % and 12 %, respectively. Viewed the other way round, 40 % and 50 % of students gave responses including incorrect scientific ideas. What are the messages for teaching and learning? The key point is that the incorrect responses all map onto the learning challenges identified in this episode. An optimistic point of view is that if these challenges are addressed directly through teaching then there is a greater chance of more positive learning outcomes.

Ionising Radiation
is used in analyses relating to Radioactive dating
can be analysed using the quantity Half-Life Decay Constant Activity
features in Medical Physics

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