Units (Radioactivity)
3 marks · HardA class is designing an experiment to compare the ranges of different types of ionising radiation in air. They list the following possible measurements for their experiment: - The distance from the radioactive source to the detector - The thickness of aluminium sheets placed between the source and detector - The number of decays per unit time measured at the detector For each measurement above, identify and justify the most appropriate unit to use according to the syllabus requirements.
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Common mistakes: using mm or m instead of cm for such measurements; confusing counts per second with becquerel; writing 'seconds' for distance
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Marking points
B1 · 1 mark · Correct unit for distance from source to detector (centimetre, cm), justified by syllabus/content.
B1 · 1 mark · Correct unit for thickness of absorber (centimetre, cm), justified by syllabus/content.
B1 · 1 mark · Correct unit for number of decays per unit time (becquerel, Bq or counts per second), justified by linking to activity; or mention of appropriate time unit such as second (s) or minute (min).
Worked solution
1. Distance from the radioactive source to the detector: The most appropriate unit is the centimetre (cm). According to the syllabus, distances in radioactivity experiments—such as the range of radiation in air—should be measured in cm, not in m or mm. This is because the distances involved are typically small.
2. Thickness of aluminium sheets: Again, the centimetre (cm) is the suitable unit. The experimental procedure and content specify using cm as the unit to measure thicknesses of absorbent materials that block radiation. Using cm ensures results are consistent with the syllabus.
3. Number of decays per unit time: The appropriate unit is the becquerel (Bq), which is defined as one decay per second. Alternatively, counts per second (cps) is sometimes used during measurement, but Bq is the standard SI unit required by the syllabus. The time unit used should be second (s) or minute (min), both listed in the content.