Part 1 of 5Medical Uses
The type of radiation and the half-life are chosen to give maximum benefit with minimum risk.
Tracers
- A gamma-emitting source with a short half-life is injected or swallowed, and a gamma camera outside the body follows where it collects.
Why a short half-life
- The tracer decays quickly, so the patient receives a small total dose and the source does not stay in the body long.
Radiotherapy
- Beams of gamma radiation are directed at a tumour from several angles to deliver a high dose to cancer cells and a low dose to healthy tissue.
Sterilisation
- Gamma rays kill bacteria on surgical instruments and dressings after they are sealed in packaging, with no heating needed.
Choosing gamma
- Gamma is chosen for medical imaging and treatment because it is penetrating enough to leave the body or reach deep tissue.
Part 2 of 5Industrial and Domestic Uses
Industrial and Domestic Uses
Thickness control
- A beta source and detector on either side of moving paper or foil measure the count;
- fewer counts mean a thicker sheet.
Why beta
- Beta is partly absorbed by thin sheets,
- so the count changes with thickness; alpha would be stopped and gamma would pass through almost unaffected.
Smoke alarms
- A weak alpha source ionises air in a small chamber;
- smoke absorbs the alpha, reducing the current and triggering the alarm.
Leak tracing
- A gamma tracer put into a pipe is detected from above ground, showing where it escapes.
Food irradiation
- Gamma radiation can kill bacteria in food to extend shelf life without making the food radioactive.
| Application | Radiation used | Reason |
|---|---|---|
| Paper thickness | Beta | Partly absorbed, count depends on thickness |
| Smoke alarm | Alpha | Strong ionisation in air, easily blocked by smoke |
| Medical tracer | Gamma | Passes out of the body to be detected |
| Sterilising | Gamma | Penetrates sealed packaging |
Part 3 of 5Choosing a Source
1
Decide what the radiation has to do: be detected through a barrier, be absorbed to change a material, or kill cells.
2
Select the type of radiation by considering its penetrating power and ionising effect.
3
Choose the half-life so that the source lasts long enough for the job but not so long that waste is a problem.
4
Check the safety rules, ensuring the source is sealed, shielded and handled for the shortest necessary time.
5
Compare the benefits with the risks and make sure the dose to people is as low as reasonably achievable.
Part 4 of 5Worked Example
Question
- A factory wants to monitor the thickness of aluminium foil using a radioactive source and detector.
- Explain which type of radiation should be used.
- The radiation must pass through the thin foil so that the detector receives a signal;
- alpha is stopped by the foil, so it is unsuitable.
- Gamma passes through the foil with almost no absorption, so changes in thickness would hardly change the count.
- Beta is partly absorbed by thin metal, so the count rate falls as the foil gets thicker.
- The detector count therefore indicates thickness, so a beta source is the best choice.
Answer A beta source, because its count rate changes measurably with the foil thickness.
Part 5 of 5Exam Checklist & Tips
6.1.9 – Key Points for the Exam
- Tracer A radioactive substance used to follow the path of a material through a system.
- Half-life The time taken for the activity of a source to fall to half its original value.
- Radiotherapy The use of radiation to treat cancer by killing tumour cells.
- Sterilisation Killing microorganisms on instruments or food.
- Sealed source A radioactive source enclosed so that its material cannot escape.
Common Mistakes
- Choosing a long half-life for a medical tracer, which leaves the patient irradiated for longer.
- Saying that irradiated food becomes radioactive.
- Suggesting alpha for thickness control through metal foil, when alpha cannot penetrate it.
- Forgetting to link the choice of source to penetrating power.