Students should be able to apply their knowledge to the uses of radiation…

The idea

Choosing a radioactive source for a job means matching its penetration, range and ionising power to the task while keeping exposure as low as reasonably practicable. For thickness monitoring of thin metal foil, a beta source such as strontium-90 is suitable because some beta passes through the foil and the count rate changes as thickness varies; alpha would be stopped by the foil and gamma would pass through almost unchanged. For smoke detectors, an alpha source such as americium-241 ionises air between electrodes; the smoke absorbs the ions and the current falls, triggering the alarm. For sterilising medical equipment or treating tumours, gamma from cobalt-60 penetrates packaging or tissue. Evaluation means weighing benefits against the hazard, choosing the least penetrating radiation that still does the job, and using shielding, distance and short exposure time.

What earns the marks

• Identify the property needed by the task, such as penetration through foil, ionisation of air or penetration through tissue or packaging.
• Select a source whose radiation has that property, for example beta for thickness monitoring, alpha for a smoke detector and gamma for sterilisation.
• Explain why the chosen radiation works, referring to what happens to the radiation in the material, such as partial absorption by the foil.
• Evaluate alternatives by explaining why another radiation would fail, such as alpha being stopped by foil or gamma passing through it almost unchanged.
• Consider safety and practicality, including shielding, distance, exposure time and the risk from ionising radiation.
• Reach a justified conclusion that names the best source for the stated situation.

Common mistakes and how to put them right

• Choosing gamma for thickness monitoring because it is penetrating.

  Put it right: Gamma passes through the foil almost unchanged, so the count rate barely varies and beta is better.

• Choosing alpha for sterilising sealed equipment.

  Put it right: Alpha is stopped by packaging and has a very short range, so gamma is needed.

• Treating all sources as equally safe or ignoring the hazard.

  Put it right: Compare the risk and explain how shielding, distance and time reduce exposure.

Exam tips

• Start by stating the property the situation requires, then name the radiation that has it, then justify the choice.
• Include one sentence explaining why a rejected radiation would not work, as evaluation requires comparison.
• Link safety advice to the specific source, for example lead shielding for gamma rather than a general warning.